Samacheer Kalvi Class 9 Science Solutions Chapter 13 Chemical Bonding

I. Choose The Correct Answer:

 

Question 1. Number of valence electrons in carbon is
(a) 2
(b) 4
(d) 5
Answer: (b) 4
In simple words: Carbon has 6 electrons in total. The first two electrons fill the innermost shell, and the remaining four electrons are in its outer shell. These four outer electrons are called valence electrons.

🎯 Exam Tip: Remember that valence electrons are the ones in the outermost shell, which determine how an atom will react with others.

 

Question 2. Sodium having atomic number 11, is ready to ________ electron/electrons to attain the nearest noble gas electronic configuration.
(a) gain one
(b) gain two
(c) lose one
(d) lose two
Answer: (c) lose one
In simple words: Sodium has one electron in its outer shell. To become stable like a noble gas, it prefers to get rid of this single electron rather than gaining seven more.

🎯 Exam Tip: Atoms with 1, 2, or 3 valence electrons usually lose them to achieve a stable configuration, forming positive ions.

 

Question 3. The element that would form anion by gaining electrons in a chemical reaction is ...............
(a) potassium
(b) calcium
(e) fluorine
(d) iron
Answer: (c) fluorine
In simple words: Fluorine needs only one more electron to fill its outermost shell and become stable. When it gains an electron, it forms a negatively charged ion called an anion.

🎯 Exam Tip: Elements that need only a few electrons (usually 5, 6, or 7 valence electrons) to complete their outer shell tend to gain electrons and form anions.

 

Question 4. Bond formed between a metal and non metal atom is usually .............
(a) ionic bond
(b) covalent bond
(e) co-ordinate bond
Answer: (a) ionic bond
In simple words: Metals tend to lose electrons, while non-metals tend to gain electrons. This transfer of electrons creates positive and negative ions, which are then strongly attracted to each other, forming an ionic bond.

🎯 Exam Tip: Look for a large difference in electronegativity between the two bonding atoms; this is a strong indicator of an ionic bond.

 

Question 5. ............... compounds have high melting and boiling points.
(a) Covalent
(b) Coordinate
(e) Ionic
Answer: (c) Ionic
In simple words: Ionic compounds are made of charged particles (ions) held together very strongly. Because of these strong forces, a lot of heat energy is needed to break them apart and make them melt or boil.

🎯 Exam Tip: The strong electrostatic attraction between oppositely charged ions in an ionic lattice requires significant energy to overcome, leading to high melting and boiling points.

 

Question 6. Covalent bond is formed by .............
(a) transfer of electrons
(b) sharing of electrons
(c) sharing a pair of electrons
Answer: (b) sharing of electrons
In simple words: A covalent bond happens when two atoms both need electrons to become stable. Instead of one atom giving electrons and the other taking them, they decide to share electrons so both can have a full outer shell.

🎯 Exam Tip: Covalent bonds are common between non-metal atoms where the electronegativity difference is small, leading to electron sharing.

 

Question 7. Oxidising agents are also called as ............... because they remove eletrons form other substances.
(a) electron donors
(b) electron acceptors
Answer: (b) electron acceptors
In simple words: An oxidizing agent takes electrons from another substance. By accepting electrons, it causes the other substance to lose electrons (get oxidized).

🎯 Exam Tip: Remember, oxidation is loss of electrons, and reduction is gain of electrons. An oxidizing agent itself gets reduced by gaining electrons.

 

Question 8. Elements with stable electronic configurations have eight electrons in their valence shell. They are ............
(a) halogens
(b) metals
(c) nobel gases
(d) non metals
Answer: (c) noble gases
In simple words: Noble gases like Neon and Argon already have a full outer shell of eight electrons (or two for Helium), making them very stable and unreactive. They do not easily form chemical bonds.

🎯 Exam Tip: The complete octet (eight valence electrons) in noble gases explains their chemical inertness and stable configuration.

II. Answer Briefly:

 

Question 1. How do atoms attain Noble gas electronic configuration?
Answer: Atoms of all elements, except for inert gases, bond together to form molecules. They do this because their outermost electron shell is not complete. They try to get a stable electron setup that is like the noble gases, which means having eight electrons in their outer shell. Atoms can achieve this stability in two ways: either by moving valence electrons from one atom to another, or by sharing valence electrons to complete their outer shell with eight electrons. This desire for a stable outer shell drives chemical reactions.
In simple words: Atoms either gain, lose, or share electrons to get a full outer shell, just like stable noble gases. This makes them chemically stable.

🎯 Exam Tip: The 'Octet Rule' (or 'Duplet Rule' for very light elements like Hydrogen and Helium) is the key concept explaining how atoms form bonds to achieve stability.

 

Question 2. NaCl is insoluble in carbon tetrachloride but soluble in water. Give reason.
Answer: Sodium chloride (NaCl) is an ionic compound. It dissolves well in polar solvents like water. Water molecules are polar, meaning they have a slight positive and a slight negative end, which helps them pull apart the charged ions of NaCl. On the other hand, carbon tetrachloride (CCl4) is a nonpolar compound. This means its molecules do not have distinct positive and negative ends. Because CCl4 is nonpolar, it cannot interact strongly enough with the charged NaCl ions to pull them apart, so NaCl is not soluble in it. The rule "like dissolves like" applies here, where polar dissolves polar and nonpolar dissolves nonpolar.
In simple words: NaCl dissolves in water because both are polar, meaning they have electrical charges that attract each other. It does not dissolve in carbon tetrachloride because CCl4 is nonpolar and cannot attract the charged NaCl parts.

🎯 Exam Tip: When discussing solubility, always refer to the "like dissolves like" principle: polar solvents dissolve polar solutes (and ionic solutes), and nonpolar solvents dissolve nonpolar solutes.

 

Question 3. Explain the Octet rule with an example.
Answer: The Octet rule, also called the "Rule of eight," states that atoms tend to have eight electrons in their outermost shell. This makes them stable, like noble gases. For example, sodium (atomic number 11) has an electron configuration of 2, 8, 1. It readily gives up its single outer electron to become stable like neon (2, 8). This forms a positive sodium ion \( \text{Na}^+ \). Similarly, chlorine (atomic number 17) has an electron configuration of 2, 8, 7. It needs one more electron to achieve a stable configuration like argon (2, 8, 8). So, chlorine readily gains one electron from another atom to form a stable negative chloride ion \( \text{Cl}^- \). In summary, atoms achieve a stable outer shell of eight electrons by either losing or gaining electrons. The electron transfer diagrams (like those showing sodium losing an electron and chlorine gaining one) clearly illustrate this process.
In simple words: The Octet rule says atoms want 8 electrons in their outer shell to be stable. Sodium loses 1 electron, and chlorine gains 1 electron to follow this rule.

🎯 Exam Tip: Clearly state the Octet rule and use an example that shows both electron loss (for metals) and electron gain (for non-metals) to illustrate its application fully.

 

Question 4. Write a note on different types of bonds.
Answer: Chemical bonds are forces that hold atoms together to form molecules and compounds. The type of bond formed depends on how atoms interact with their valence electrons. Here are the main types:
1. **Ionic Bond:** This bond forms when electrons are completely transferred from one atom to another, typically between a metal and a non-metal. The atoms become charged ions (cations and anions) which are then held together by strong electrostatic forces. Example: NaCl.
2. **Covalent Bond:** This bond forms when atoms share electrons to achieve a stable electron configuration, usually between two non-metals. The shared electrons are attracted to both nuclei. Covalent bonds can be single, double, or triple. Example: \( \text{H}_2\text{O} \), \( \text{O}_2 \).
3. **Coordinate Covalent Bond (Dative Bond):** A special type of covalent bond where one atom donates both electrons for the shared pair. The other atom accepts this pair of electrons. Example: \( \text{NH}_4^+ \).
4. **Metallic Bond:** Found in metals, this bond involves a "sea" of delocalized electrons shared among a lattice of positive metal ions. This allows metals to conduct electricity and heat well. Example: Copper, Iron.
5. **Hydrogen Bond:** A weak intermolecular force between a hydrogen atom (that is bonded to a highly electronegative atom like N, O, or F) and another electronegative atom in a different molecule. These bonds are important for properties of water and biological structures. Example: Water.
6. **Van der Waals Forces:** These are very weak intermolecular forces of attraction, including dispersion forces and dipole-dipole interactions, that exist between all molecules. They are responsible for the physical properties of many substances, especially nonpolar ones. Example: Noble gases, liquid nitrogen.
In simple words: Chemical bonds are like "glue" holding atoms together. Ionic bonds happen when electrons are moved from one atom to another. Covalent bonds happen when atoms share electrons. Coordinate bonds are when one atom gives both shared electrons. Metallic bonds are a "sea" of shared electrons in metals. Hydrogen bonds and Van der Waals forces are weaker attractions between molecules.

🎯 Exam Tip: When listing types of bonds, remember to briefly explain the nature of electron interaction (transfer, sharing, or delocalized) for each, as this is the core distinguishing factor.

 

Question 5. Correct the wrong statements.
(a) Ionic compounds dissolve in non-polar solvents.
(b) Covalent compounds conduct electricity in molten or solution state.
Answer:
(a) The incorrect statement is "Ionic compounds dissolve in non-polar solvents." The corrected statement is: **Ionic compounds dissolve in polar solvents.** (For example, water).
(b) The incorrect statement is "Covalent compounds conduct electricity in molten or solution state." The corrected statement is: **Ionic compounds conduct electricity in molten or solution state.** (Covalent compounds generally do not conduct electricity because they lack free ions or electrons).
In simple words: Ionic compounds dissolve in water, which is polar, not in non-polar liquids. Also, ionic compounds can carry electricity when melted or dissolved, but covalent compounds usually cannot.

🎯 Exam Tip: For correction questions, clearly identify the incorrect part and then provide the accurate scientific principle or fact as the correction.

 

Question 6. Complete the table given below.

Answer: The table shows the electron arrangement and Lewis dot structure for Lithium, Boron, and Oxygen. For Lithium, with atomic number 3, the electron distribution is 2,1, meaning it has 1 valence electron, represented by a single dot. Boron, with atomic number 5, has a distribution of 2,3, giving it 3 valence electrons, shown as three dots. Oxygen, with atomic number 8, has a distribution of 2,6, resulting in 6 valence electrons, depicted with six dots around its symbol. The Lewis dot structure helps visualize the number of valence electrons an atom has, which are crucial for chemical bonding.
In simple words: The table lists atoms, how many electrons they have, and how those electrons are arranged in shells. The "valence electrons" are the ones in the outermost shell, and the Lewis dot structure shows these outer electrons as dots around the atom's symbol.

🎯 Exam Tip: Practice drawing Lewis dot structures for common elements, as they are fundamental for understanding covalent bonding and molecular geometry.

 

Question 7. Draw the electron distribution diagram for the formation of Carbon dioxide (CO2) molecule.
Answer: In the formation of a carbon dioxide \( (\text{CO}_2) \) molecule, one carbon atom shares electrons with two oxygen atoms. Carbon has 4 valence electrons (electronic configuration 2, 4), and each oxygen atom has 6 valence electrons (electronic configuration 2, 6). To achieve stability, the carbon atom shares two pairs of electrons with each oxygen atom, forming two double covalent bonds. This results in the carbon atom and both oxygen atoms achieving a stable octet (8 electrons) in their outermost shells. The diagram would show the central carbon atom with two double bonds, one to each oxygen atom, and each oxygen atom also having two lone pairs of electrons. The electron distribution diagram clearly illustrates these shared and unshared electron pairs, leading to the stable \( \text{O=C=O} \) structure.
In simple words: For \( \text{CO}_2 \), carbon shares two pairs of electrons with one oxygen and two pairs with another oxygen. This makes everyone's outer shell full and stable.

🎯 Exam Tip: When drawing electron distribution diagrams, ensure that all atoms involved achieve a stable electron configuration (usually an octet) and that all valence electrons are accounted for in either bonding or lone pairs.

 

Question 8. Fill in the following table according to the type of bonds formed in the given molecule.

Answer: The table categorizes common chemical compounds based on their primary type of bond. Ionic bonds, which involve the transfer of electrons, are found in \( \text{CaCl}_2 \), \( \text{CaO} \), and \( \text{KBr} \). These are typically formed between metals and non-metals. Covalent bonds, where electrons are shared, are present in \( \text{H}_2\text{O} \), \( \text{CCl}_4 \), \( \text{CO}_2 \), \( \text{HCl} \), and \( \text{HF} \). These bonds usually occur between non-metals. Coordinate covalent bonds, a special type of sharing where one atom provides both electrons, are seen in \( \text{CO} \) (carbon monoxide) and \( \text{Al}_2\text{Cl}_6 \) (aluminum chloride dimer), though \( \text{CO} \) also has regular covalent bonds. Understanding these bond types helps predict the properties of different substances.
In simple words: This table groups chemicals by the kind of bond holding their atoms together. \( \text{CaCl}_2 \) uses ionic bonds (electrons transferred). \( \text{H}_2\text{O} \) uses covalent bonds (electrons shared). \( \text{CO} \) uses coordinate covalent bonds (one atom gives both shared electrons).

🎯 Exam Tip: To identify bond types, first consider the types of elements (metal/non-metal) and then look at the electronegativity difference. A coordinate bond is a special case of a covalent bond where one atom donates the entire electron pair.

 

Question 9. The property which is characteristic of an Ionic compound is that
(a) it often exists as a gas at room temperature.
(b) it is hard and brittle.
(c) it undergoes molecular reactions
(d) it has a low melting point.
Answer: (b) it is hard and brittle
In simple words: Ionic compounds form strong crystal structures, making them hard. But if you hit them, the organized layers of ions can shift, causing like charges to meet and repel, which makes them break easily, or brittle.

🎯 Exam Tip: Ionic compounds typically form crystalline solids due to strong electrostatic forces, leading to hardness, but their lattice structure makes them brittle when external force is applied.

 

Question 10. Identify the following reactions as oxidation or reduction.
(a) \( \text{Na} \rightarrow \text{Na}^+ + \text{e}^- \)
(b) \( \text{Fe}^{3+} + 2 \text{e}^- \rightarrow \text{Fe}^+ \)
Answer:
(a) \( \text{Na} \rightarrow \text{Na}^+ + \text{e}^- \) is an **oxidation** reaction.
(b) \( \text{Fe}^{3+} + 2 \text{e}^- \rightarrow \text{Fe}^+ \) is a **reduction** reaction.
Oxidation involves the loss of electrons, resulting in an increase in the oxidation state. Conversely, reduction involves the gain of electrons, leading to a decrease in the oxidation state. The concept of oxidation and reduction is fundamental to understanding many chemical reactions, especially in electrochemistry.
In simple words: When an atom loses electrons, it's called oxidation, like sodium losing an electron. When an atom gains electrons, it's called reduction, like iron gaining electrons.

🎯 Exam Tip: Remember the mnemonic "OIL RIG": Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). This helps quickly identify whether a species is being oxidized or reduced.

 

Question 11. Identify the compounds as Ionic/Covalent/Coordinate based on the given characteristics.
(a) Soluble in non-polar solvents
(b) Undergoes faster/instantaneous reactions
(d) Solids at room temperature
Answer:
(a) Compounds soluble in non-polar solvents are typically **Covalent compounds**.
(b) Compounds that undergo faster or instantaneous reactions are typically **Ionic compounds**.
(d) Compounds that exist as solids at room temperature can be **Ionic compounds** (many are) or some **Covalent compounds** (e.g., sugar, diamond), but generally ionic compounds are associated with this characteristic due to their strong lattice structure.
These characteristics help distinguish between the different types of chemical bonds and the resulting properties of substances.
In simple words: (a) Non-polar solvents dissolve covalent compounds. (b) Ionic compounds react very fast. (d) Both ionic and some covalent compounds can be solid at normal room temperature.

🎯 Exam Tip: Link characteristic properties to bond types: solubility (polar/nonpolar), reaction rates (ion interactions vs. bond breaking/forming), and physical state are key indicators.

 

Question 12. An atom X with atomic number 20 combines with atom Y with atomic number 8. Draw the dot structure for the formation of the molecule XY
Answer: Atom X has atomic number 20, which is Calcium (Ca). Its electron configuration is 2, 8, 8, 2, meaning it has 2 valence electrons. Atom Y has atomic number 8, which is Oxygen (O). Its electron configuration is 2, 6, meaning it has 6 valence electrons. To form a stable molecule XY, Calcium (X) will lose its 2 valence electrons to become \( \text{Ca}^{2+} \) (a calcium cation), resembling the noble gas Argon (2, 8, 8). Oxygen (Y) will gain these 2 electrons to become \( \text{O}^{2-} \) (an oxide anion), resembling the noble gas Neon (2, 8). The transfer of electrons results in an ionic bond between \( \text{Ca}^{2+} \) and \( \text{O}^{2-} \), forming the compound \( \text{CaO} \) (Calcium Oxide). The electron dot structure would show two electrons moving from Calcium's outer shell to Oxygen's outer shell. This electron transfer helps both atoms achieve a stable octet configuration.
In simple words: Atom X (Calcium) gives away its 2 outer electrons, becoming a positive ion. Atom Y (Oxygen) takes these 2 electrons, becoming a negative ion. This transfer forms an ionic bond, and the molecule is \( \text{CaO} \).

🎯 Exam Tip: When dealing with electron dot structures for ionic compounds, clearly show the transfer of electrons and the resulting charges on the ions to demonstrate how stable configurations are achieved.

 

Question 13. Considering MgCl2 as ionic compound and CH4 as covalent compound give any two differences between these two compounds.
Answer:

These differences highlight the fundamental distinctions between ionic and covalent bonding, which in turn dictate their physical and chemical properties. For instance, ionic compounds generally have high melting points and conduct electricity when molten or dissolved, while covalent compounds often have lower melting points and are poor conductors.
In simple words: \( \text{MgCl}_2 \) is ionic because electrons are moved from magnesium to chlorine, creating strong attractions between charged particles. \( \text{CH}_4 \) is covalent because carbon and hydrogen share electrons, leading to weaker forces between the molecules.

🎯 Exam Tip: When comparing compounds, focus on the nature of the bond (electron transfer vs. sharing) and the resulting forces (electrostatic vs. intermolecular) as primary differentiating points.

 

Question 14. Why are Noble gases inert in nature?
Answer: Noble gases are naturally inert, meaning they are not very reactive. This is because their electron shells are completely filled, especially their outermost shells. Having full subshells gives them a very stable electronic structure that is extremely hard to change. Because they already have this ideal electron configuration, they have no tendency to gain, lose, or share electrons with other atoms. Examples of noble gases include Helium, Neon, Argon, Krypton, Xenon, and Radon, all found in Group 18 of the periodic table. Their filled electron shells prevent them from easily forming chemical bonds.
In simple words: Noble gases are stable and don't react much because their outer electron shells are already completely full. They do not need to gain or lose electrons.

🎯 Exam Tip: The key reason for the inertness of noble gases is their fully occupied valence electron shells, which means they do not seek to achieve an octet through bonding.

III. Spot The Error / Correct The Wrong Statement:

 

Question 1. In the formation of compounds, the inner shell electrons of an atom involved in bonding.
Answer: The original statement is incorrect.
Corrected statement: **In the formation of compounds, the valence electrons (outermost shell electrons) of an atom are involved in bonding.** Inner shell electrons are generally not involved in forming chemical bonds.
In simple words: Only the electrons in the outermost shell, called valence electrons, take part in forming chemical bonds. Inner electrons stay put.

🎯 Exam Tip: Always specify "valence electrons" when discussing chemical bonding, as these are the only electrons actively participating in bond formation.

 

Question 2. The atom that gains electrons will form a cation.
Answer: The original statement is incorrect.
Corrected statement: **The atom that gains electrons will form an anion.** An atom that gains electrons becomes negatively charged and is called an anion, while an atom that loses electrons becomes positively charged and is called a cation.
In simple words: When an atom gains electrons, it becomes negative, and that's called an anion. A cation is formed when an atom loses electrons.

🎯 Exam Tip: Clearly differentiate between cations (positive ions, formed by losing electrons) and anions (negative ions, formed by gaining electrons) to avoid common mistakes.

 

Question 3. Ionic compounds have low melting and boiling point.
Answer: The original statement is incorrect.
Corrected statement: **Ionic compounds have high melting and boiling points.** This is because ionic compounds consist of a strong crystal lattice structure, held together by powerful electrostatic forces between oppositely charged ions, which requires a lot of energy to break.
Alternatively, the statement could be corrected by changing the compound type: **Covalent compounds have low melting and boiling points.** Covalent compounds are held together by weaker intermolecular forces, requiring less energy to melt or boil.
In simple words: Ionic compounds have very strong bonds, so they need a lot of heat to melt or boil. Covalent compounds, however, usually have lower melting and boiling points.

🎯 Exam Tip: Remember that the strength of the forces holding particles together (intermolecular vs. electrostatic) directly influences melting and boiling points.

Question 3. Ionic compounds are highly brittle.
(a) Ionic
(b) Covalent
(c) Co-ordinate covalent
Answer: (a) Ionic
In simple words: Ionic compounds break easily, like glass. This is because when you try to change their shape, the positive and negative ions are forced close together, which causes them to repel and break apart.

🎯 Exam Tip: Remember that ionic compounds have a strong, rigid crystal lattice structure, which makes them hard but also brittle when force is applied.

 

Question 4. The bond which is formed by mutual sharing of electrons is called ................ bond.
(a) ionic bond
(b) covalent bond
(c) co-ordinate covalent bond
(d) all of the options
Answer: (b) covalent bond
In simple words: A covalent bond forms when two atoms share their electrons with each other. It's like two friends sharing a toy so both can play.

🎯 Exam Tip: Covalent bonds are common between non-metal atoms, allowing them to achieve a stable electron configuration without transferring electrons completely.

 

Question 5. Which of the following compounds has a high melting point?
(a) Magnesium oxide
Answer: (a) Magnesium oxide
In simple words: Magnesium oxide is an ionic compound, which means it has very strong forces holding its particles together. Because of these strong forces, a lot of heat energy is needed to break them apart and melt the compound.

🎯 Exam Tip: Ionic compounds generally have high melting and boiling points due to the strong electrostatic forces of attraction between their positive and negative ions.

 

Question 6. Which of the following compound(s) possesses a high melting point?
(a) NaCl
(b) MgCl2
(c) CCl4
(d) Both (a) and (b)
Answer: (d) Both (a) and (b)
In simple words: Both NaCl (sodium chloride) and MgCl2 (magnesium chloride) are ionic compounds. Ionic compounds have strong bonds that need a lot of heat to break, so they melt at high temperatures. CCl4 (carbon tetrachloride) is a covalent compound, which has weaker bonds and a lower melting point.

🎯 Exam Tip: Remember the general rule: ionic compounds have high melting points, while covalent compounds typically have lower melting points.

 

Question 7. The element that would form cation due to the loss of electron during the chemical reaction is ................
(a) calcium
(b) Fluorine
(c) Chlorine
(d) all of the options
Answer: (a) calcium
In simple words: Calcium is a metal. Metals tend to lose electrons to become positively charged ions called cations. Fluorine and Chlorine are non-metals, so they tend to gain electrons to become negatively charged ions.

🎯 Exam Tip: A simple way to remember is that metals generally form cations (positive ions) by losing electrons, while non-metals generally form anions (negative ions) by gaining electrons.

 

Question 8. Fajan's rule is formulated by considering the ............... and ................ of the cation and anion.
(a) charge
(b) size
(c) charge & size
(d) none
Answer: (c) charge & size
In simple words: Fajan's rule helps us understand how much an ionic bond has some covalent character. It looks at how big or small the ions are, and how much charge they carry. These two factors influence the bond's nature.

🎯 Exam Tip: Higher charge density (high charge, small size) of the cation and larger size of the anion tend to increase the covalent character of an ionic bond, according to Fajan's rule.

 

Question 9. The formation of brown colour on the freshly cut surface of vegetables and fruits is because ............ of organic compounds present in them.
(a) oxidation
(b) reduction
(c) both (a) and (b)
(d) none
Answer: (a) oxidation
In simple words: When you cut fruits or vegetables, the inside parts touch the air. The oxygen in the air reacts with special compounds inside the food, causing a chemical change that makes them turn brown. This process is called oxidation.

🎯 Exam Tip: To prevent browning, you can reduce the contact with oxygen, for example, by adding lemon juice (which contains antioxidants) or by covering the cut surface.

 

Question 10. Which of the following compounds has melting and boiling points higher than covalent compounds but lower than ionic compounds?
(a) NaCl
(b) MgCl2
(c) H2O
(d) NH3→BF3
Answer: (d) NH3→BF3
In simple words: The compound formed from \( \text{NH}_3 \) and \( \text{BF}_3 \) has a coordinate covalent bond. These bonds are stronger than typical covalent bonds but not as strong as ionic bonds, so their melting and boiling points fall in the middle. Compounds like NaCl and MgCl2 are ionic and have very high melting points. H2O is a covalent compound with hydrogen bonding, leading to a relatively higher boiling point for a covalent compound, but still lower than many ionic compounds or those with coordinate bonds.

🎯 Exam Tip: Coordinate covalent compounds often exhibit properties that are intermediate between purely ionic and purely covalent compounds, including their melting and boiling points.

 

Question 11. Atoms having 1, 2 or 3 electrons in their valence shell will readily form ................
(a) cation
(b) anion
Answer: (a) cation
In simple words: Atoms that have only a few electrons in their outermost shell find it easier to lose those electrons to become stable. When an atom loses electrons, it becomes positively charged and is called a cation. This is common for metals.

🎯 Exam Tip: Metals (like those in Group 1, 2, and 13) typically have 1, 2, or 3 valence electrons and readily form cations to achieve a stable noble gas configuration.

 

II. Fill in the blanks:

 

Question 1. Kossel-Lewis theory explains the formation of molecules.
Answer: Kossel-Lewis theory
In simple words: This theory helps us understand how atoms join together to make molecules by either sharing or transferring electrons. It focuses on atoms trying to get a stable number of electrons, like noble gases.

🎯 Exam Tip: Remember that the Kossel-Lewis approach emphasizes the octet rule, where atoms strive to achieve eight electrons in their outermost shell for stability.

 

Question 2. The valency of noble gases is ................
Answer: zero
In simple words: Noble gases already have a full outer shell of electrons, which makes them very stable and unreactive. Because they do not easily gain, lose, or share electrons, their combining power, or valency, is zero.

🎯 Exam Tip: Noble gases are often used as a reference point for stability in chemical bonding because their electron configuration (except Helium) has a complete octet.

 

Question 3. Helium is the only noble gas which does not have eight electrons in their valence shell.
Answer: Helium
In simple words: All noble gases aim for eight electrons in their outermost shell for stability, which is called an octet. But Helium is a tiny gas that only needs two electrons to fill its first shell, so it's stable with just two, not eight.

🎯 Exam Tip: Helium's stability with two valence electrons is an exception to the octet rule, known as the 'duplet rule', applicable only to the first shell.

 

Question 4. The atom that loses electrons will form a cation.
Answer: cation
In simple words: When an atom gives away one or more of its electrons, it becomes positively charged. This positive ion is called a cation. Metals often form cations.

🎯 Exam Tip: An easy way to remember is that "cat-ions" are "positive" like cats and "an-ions" are "negative" like ants, or that cations are formed by metals.

 

Question 5. Ionic compounds have high density.
Answer: Ionic
In simple words: Ionic compounds are usually tightly packed into a crystal structure. This close arrangement of many ions in a small space gives them a high density.

🎯 Exam Tip: High density is a characteristic property of ionic compounds, usually related to their strong intermolecular forces and solid state at room temperature.

 

Question 6. In covalent bond formation, the sharing of unpaired electrons takes place in their outermost shell.
Answer: unpaired
In simple words: For atoms to form a covalent bond, they need electrons that are not already part of a pair. These 'unpaired' electrons from their outer shells are then shared between the atoms to create a stable bond.

🎯 Exam Tip: The number of unpaired electrons an atom has often determines how many covalent bonds it can form, striving to achieve a stable electron configuration.

 

Question 7. Polar solvents contain bonds between atoms with different electronegativities.
Answer: different electronegativities
In simple words: In polar solvents, some atoms pull electrons more strongly than others. This creates a small positive end and a small negative end in the bond, making the molecule "polar" like a tiny magnet.

🎯 Exam Tip: The difference in electronegativity between bonded atoms is key to determining if a bond is polar or non-polar. A larger difference leads to a more polar bond.

 

Question 8. Carbon & hydrogen atoms have similar electronegativities.
Answer: Carbon & hydrogen
In simple words: Carbon and hydrogen atoms have a similar ability to attract electrons. This means that when they bond together, the electrons are shared almost equally, resulting in bonds that are mostly non-polar.

🎯 Exam Tip: Due to their similar electronegativity, C-H bonds are considered non-polar, which is why organic compounds with many C-H bonds are generally non-polar.

 

Question 9. Molecular reactions are slow in the covalent compound.
Answer: slow
In simple words: Covalent compounds react slowly because their bonds are strong and must first be broken before new bonds can form. This takes more energy and time compared to ionic reactions which often involve simpler rearrangement of existing ions.

🎯 Exam Tip: The energy required to break and form covalent bonds directly influences the reaction rate; higher activation energy often translates to slower reactions.

 

Question 10. Ionic compounds are solid in nature.
Answer: solid
In simple words: Ionic compounds are made of charged particles (ions) held together very strongly in a repeating pattern. These strong forces make them solids at room temperature and give them a structured crystal shape.

🎯 Exam Tip: The strong electrostatic forces between ions in an ionic compound lead to a crystal lattice structure, which explains their solid state and high melting points.

 

Question 11. The tendency of atoms to have eight electrons in the outer shell is known as Octet rule.
Answer: Octet rule
In simple words: The Octet rule says that atoms like to have eight electrons in their outermost shell to be stable, just like the noble gases. They achieve this stability by gaining, losing, or sharing electrons with other atoms.

🎯 Exam Tip: While the octet rule is a useful guideline, remember there are exceptions, especially for elements in the first two periods and elements that can have expanded octets.

 

Question 12. As per Fajan's rule, AlCl3 is covalent.
Answer: covalent
In simple words: Even though aluminum is a metal, Fajan's rule helps explain why aluminum chloride acts more like a covalent compound. This is because the aluminum ion is small and has a high positive charge, which pulls electrons strongly from the larger chloride ions, giving the bond significant covalent character.

🎯 Exam Tip: Fajan's rule helps predict the degree of covalent character in an ionic bond, explaining why some compounds with metal-nonmetal bonds might not behave as purely ionic.

 

Question 13. Oxidising agents are otherwise called as electron acceptors.
Answer: electron acceptors
In simple words: An oxidizing agent is a substance that causes another substance to get oxidized. It does this by taking electrons from the other substance, so we call it an "electron acceptor." When it accepts electrons, the oxidizing agent itself gets reduced.

🎯 Exam Tip: Remember that in a redox reaction, the oxidizing agent is the one that gets reduced, and the reducing agent is the one that gets oxidized.

 

Question 14. The tarnishing of metals is due to the formation of metal oxide.
Answer: metal oxide
In simple words: Tarnishing happens when a metal reacts with oxygen in the air, creating a thin layer of metal oxide on its surface. This layer often looks dull or discolored compared to the shiny original metal.

🎯 Exam Tip: Tarnishing is a type of corrosion, a natural process that converts a refined metal into a more stable form, such as its oxide, hydroxide, or sulfide.

 

Question 15. The tarnishing of metals is an example of oxidation reaction.
Answer: oxidation
In simple words: When a metal tarnishes, it loses electrons to oxygen. This loss of electrons is called oxidation. It's a chemical reaction where the metal changes its chemical state.

🎯 Exam Tip: Recognize oxidation as the loss of electrons or gain of oxygen, and reduction as the gain of electrons or loss of oxygen.

 

Question 16. The sum of oxidation number of all atoms in a compound is Zero.
Answer: Zero
In simple words: In any neutral chemical compound, the positive and negative charges from all the atoms must balance out perfectly. This means that if you add up the oxidation numbers of every atom, the total will always be zero.

🎯 Exam Tip: This rule is fundamental for calculating the oxidation number of an unknown element within a compound, as the overall charge must always be zero for neutral compounds and equal to the ion's charge for polyatomic ions.

 

Question 17. Gold is a metal that has a high resistance to corrosion.
Answer: Gold
In simple words: Gold is known as a noble metal because it does not easily react with air or water. This makes it very resistant to corrosion and tarnishing, allowing it to keep its shine for a very long time.

🎯 Exam Tip: Gold's exceptional resistance to corrosion makes it valuable for jewelry, dentistry, and electronic components where durability and stability are crucial.

 

III. Spot the error / correct the wrong statement:

 

Question 1. Correct the statement: In the formation of compounds, the inner shell electrons of an atom are involved in bonding.
Answer: In the formation of compounds, the **valence electrons** (outermost shell electrons) of an atom are involved in bonding.
In simple words: Only the electrons in the atom's very outermost shell, called valence electrons, participate in forming chemical bonds. The inner electrons are too close to the nucleus and do not take part in bonding.

🎯 Exam Tip: Emphasize that valence electrons are the key players in chemical reactions as they are the ones available to be lost, gained, or shared.

 

Question 2. Correct the statement: The atom that gains electrons will form a cation.
Answer: The atom that gains electrons will form an **anion**.
In simple words: When an atom gains extra electrons, it gets more negative charges than positive charges. This makes it a negatively charged ion, which is called an anion.

🎯 Exam Tip: Remember that anions are non-metal ions that have gained electrons, while cations are metal ions that have lost electrons.

 

Question 3. Correct the statement: Ionic compounds have low melting and boiling points.
Answer: Ionic compounds have **high** melting and boiling points.
In simple words: Ionic compounds have strong forces between their positive and negative ions. A lot of energy is needed to break these strong forces, so they have high melting and boiling points.

🎯 Exam Tip: Always associate strong electrostatic forces in ionic compounds with high melting and boiling points, and weaker intermolecular forces in covalent compounds with lower melting and boiling points.

 

Question 4. Correct the statement: Non-polar solvents contain bonds between atoms with different electronegativities.
Answer: Non-polar solvents contain bonds between atoms with **similar** electronegativities.
In simple words: In non-polar solvents, the atoms in the bonds pull on the electrons almost equally. This means there's no big difference in charge across the bond, making the molecule non-polar.

🎯 Exam Tip: The principle of "like dissolves like" is important here: non-polar solvents dissolve non-polar solutes, and polar solvents dissolve polar solutes.

 

Question 5. Correct the statement: Covalent compounds are soluble in polar solvents.
Answer: Covalent compounds are readily soluble in **non-polar** solvents.
In simple words: Covalent compounds usually dissolve well in solvents that also have non-polar bonds, following the "like dissolves like" rule. They do not mix well with polar solvents like water.

🎯 Exam Tip: While most covalent compounds dissolve in non-polar solvents, some polar covalent compounds (like sugar) can dissolve in polar solvents (like water) due to hydrogen bonding.

 

Question 6. Correct the statement: Greater the charge of the cation, greater will be the ionic character.
Answer: Greater the charge of the cation, greater will be the **covalent** character.
In simple words: According to Fajan's rule, a cation with a high positive charge can pull electron clouds from anions more strongly. This pulling makes the electron sharing more significant, leading to a greater covalent character in the bond.

🎯 Exam Tip: Remember Fajan's rule: smaller cation, larger anion, and higher charges on either ion all lead to increased covalent character in an ionic bond.

 

IV. Match the following:

 

Question 1. Match the following.
1) Monoatomic gaseous atom
2) Octet rule
3) Ionic bond
4) Non-polar solvent
5) Polar solvent
(a) Electrovalent bond
(b) Benzene
(c) Water
(d) Electronic theory of valence
(e) Noble gases
Answer:
1. - (e) Noble gases
2. - (d) Electronic theory of valence
3. - (a) Electrovalent bond
4. - (b) Benzene
5. - (c) Water
In simple words: This match connects different chemistry terms with their correct descriptions or examples. Monoatomic gaseous atoms are like noble gases, the Octet rule is a part of the electronic theory of valence, an ionic bond is also called an electrovalent bond, benzene is a non-polar solvent, and water is a polar solvent.

🎯 Exam Tip: When matching, look for direct definitions, common examples, or alternative names for the given terms. If unsure, eliminate options you know are incorrect first.

 

Question 2. Match the following.
1) Atomic bond
2) Atoms with different electronegativities
3) Atom which accepts electron pair
4) Rusting of iron
5) Atom which provides electron pair
(a) Oxygen and hydrogen
(b) Acceptor bond
(c) Covalent bond
(d) Donor atom
(e) Oxidation
(f) Ionic bond
Answer:
1. - (c) Covalent bond
2. - (a) Oxygen and hydrogen
3. - (b) Acceptor bond
4. - (e) Oxidation
5. - (d) Donor atom
In simple words: This set of matches links chemical concepts like types of bonds, electron behavior, and reactions with their corresponding definitions or examples. For example, atoms with different pull for electrons (electronegativities) are found in oxygen and hydrogen, and rusting is a form of oxidation.

🎯 Exam Tip: Pay close attention to the definition of each term. For instance, an atom that provides an electron pair acts as a donor, and an atom that accepts an electron pair acts as an acceptor, which is crucial for coordinate covalent bonds.

 

V. Find the odd one out and write the reason:

 

Question 1. Find the odd one out: Water, acetone, benzene, toluene, turpentine.
Answer: Water
**Reason:** Water is a polar solvent, whereas acetone, benzene, toluene, and turpentine are all non-polar solvents.
In simple words: Water is different because it has positive and negative poles, which allows it to dissolve certain substances. The other liquids do not have these poles and are good for dissolving different types of substances.

🎯 Exam Tip: To identify odd ones out, classify each item based on a key chemical property such as polarity, type of bond, or reactivity, and then find the one that doesn't fit the category of the others.

 

Question 2. Find the odd one out: Addition of oxygen, removal of hydrogen, loss of an electron, gain of electron.
Answer: Gain of electron
**Reason:** Gaining an electron is a reduction reaction, whereas addition of oxygen, removal of hydrogen, and loss of an electron are all oxidation reactions.
In simple words: When a substance gains an electron, it is called reduction. All the other actions (adding oxygen, taking away hydrogen, or losing electrons) are different ways to describe oxidation.

🎯 Exam Tip: Remember the mnemonic "OIL RIG": Oxidation Is Loss (of electrons) and Reduction Is Gain (of electrons). This helps differentiate between the two processes.

 

Question 3. Find the odd one out: Platinum, palladium, NaBH4, CrO3.
Answer: CrO3
**Reason:** CrO3 (Chromium(VI) oxide) is an oxidising agent, whereas platinum, palladium, and NaBH4 (sodium borohydride) are reducing agents.
In simple words: CrO3 is the one that makes other things lose electrons. The rest (platinum, palladium, and NaBH4) are substances that help other things gain electrons.

🎯 Exam Tip: Identifying whether a substance acts as an oxidizing or reducing agent often depends on its tendency to gain or lose electrons in a reaction; look for common examples of each.

 

Question 4. Find the odd one out: Ionic bond, metallic bond, Coordinate covalent bond, Hydrogen bond.
Answer: Hydrogen bond
**Reason:** Hydrogen bond is a weak intermolecular force, whereas ionic bond, metallic bond, and coordinate covalent bond are strong chemical bonds.
In simple words: A hydrogen bond is a weaker force that holds molecules together. But the other three (ionic, metallic, and coordinate covalent bonds) are much stronger forces that hold atoms together within a molecule or crystal.

🎯 Exam Tip: Distinguish between strong intramolecular bonds (like ionic, covalent, metallic) that hold atoms together within a molecule, and weaker intermolecular forces (like hydrogen bonds, van der Waals forces) that hold molecules together.

 

Question 5. Find the odd one out: Soft & waxy, a bad conductor of electricity, low boiling point, solid at room temperature.
Answer: Solid at room temperature
**Reason:** "Solid at room temperature" is a characteristic property of ionic compounds (due to strong forces), while "soft & waxy," "bad conductor of electricity," and "low boiling point" are characteristic properties of covalent compounds (due to weak intermolecular forces).
In simple words: Most of these describe covalent compounds, which are often soft and have low melting points. But "solid at room temperature" points to ionic compounds, which are hard solids with high melting points.

🎯 Exam Tip: Understand the general properties of ionic versus covalent compounds. Ionic compounds are typically hard, brittle solids with high melting points, while covalent compounds can be gases, liquids, or soft solids with lower melting points.

 

VI. Answer in brief:

 

Question 1. What is a chemical bond?
Answer: A chemical bond is a strong force of attraction that holds two or more atoms together. This force causes the atoms to link up, forming a stable unit known as a molecule or a crystal structure.
In simple words: A chemical bond is like a strong glue that holds atoms together to make bigger substances.

🎯 Exam Tip: When defining a chemical bond, clearly state that it involves forces of attraction and results in a stable entity (molecule or compound).

 

Question 2. Write the basic concept of Kossel-Lewis theory.
Answer: The basic idea of the Kossel-Lewis theory is that atoms try to achieve a stable electron configuration, similar to noble gases. They do this by either giving away, taking, or sharing electrons to fill their outermost shell, typically aiming for eight electrons (the octet rule).
In simple words: Kossel-Lewis theory says atoms bond to become stable, usually by getting eight electrons in their outer shell, like noble gases.

🎯 Exam Tip: Focus on the "octet rule" and "noble gas configuration" as the core principles of the Kossel-Lewis theory when explaining its basic concept.

 

Question 3. Define the ionic bond.
Answer: An ionic bond is a strong chemical bond formed by the complete transfer of one or more electrons from one atom (usually a metal) to another atom (usually a non-metal). This transfer creates oppositely charged ions-a positive ion (cation) and a negative ion (anion)-which are then held together by strong electrostatic forces of attraction.
In simple words: An ionic bond forms when one atom gives electrons to another, making positive and negative ions that strongly stick together.

🎯 Exam Tip: Highlight "complete transfer of electrons" and "electrostatic attraction between oppositely charged ions" as the defining characteristics of an ionic bond.

 

Question 4. The following shows the electronic distribution diagram for the formation of MgCl2 molecule. Based on this, answer the following questions.

(a) Which of the above atom loses electrons to form a cation?
(b) Which of the above atom gains electrons to form an anion?
(c) How many electrons are transferred from Mg to Cl?
(d) Write the name of the anion formed.
(e) Which noble gas configuration do these ions resemble? (Specifically, \( \text{Mg}^{2+} \) and \( \text{Cl}^- \))
Answer:
(a) Magnesium atom loses electrons to form a cation.
(b) Chlorine atom gains 1 electron to form an anion.
(c) Two electrons are transferred from the Mg atom to two Cl atoms (each Cl atom gains 1 electron from the Mg atom).
(d) The anion formed is the Chloride anion (\( \text{Cl}^- \)).
(e) The \( \text{Mg}^{2+} \) ion resembles the noble gas configuration of Neon (2, 8). The \( \text{Cl}^- \) ion resembles the noble gas configuration of Argon (2, 8, 8). Additionally, the electronic configuration of \( \text{Mg}^{2+} \) is 2, 8, and the electronic configuration of \( \text{Cl}^- \) is 2, 8, 8.
In simple words: Magnesium (Mg) gives away two electrons, becoming a positive ion, like Neon. Each Chlorine (Cl) takes one electron from Magnesium, becoming a negative ion, like Argon. Two electrons move in total, one to each chlorine, forming chloride ions.

🎯 Exam Tip: When drawing electron dot structures or diagrams for ionic compounds, ensure that electron transfer is clearly shown, and the resulting ions correctly reflect the noble gas configuration they attain.

 

Question 5. What is a covalent bond?
Answer: A covalent bond is a type of chemical bond where two atoms share one or more pairs of electrons. This sharing happens to achieve a stable electron configuration, typically an octet (eight electrons) in their outermost shell, similar to noble gases. This bond usually forms between non-metal atoms.
In simple words: A covalent bond is when atoms share electrons to become stable, usually between non-metals.

🎯 Exam Tip: Emphasize "sharing of electrons" as the defining characteristic of a covalent bond, in contrast to the "transfer of electrons" in an ionic bond.

 

Question 6. Name the following:
(a) An element which obtains the noble gas configuration of neon by losing three electrons.
(b) An element which gains two electrons to obtain the noble gas configuration of Neon.
Answer:
(a) The element is Aluminium (\( \text{Al} \)). It loses three electrons to become \( \text{Al}^{3+} \) (\( \text{Al} \rightarrow \text{Al}^{3+} + 3\text{e}^- \)).
(b) The element is Oxygen (\( \text{O} \)). It gains two electrons to become \( \text{O}^{2-} \) (\( \text{O} + 2\text{e}^- \rightarrow \text{O}^{2-} \)).
In simple words: Aluminium gives away three electrons to become like Neon. Oxygen takes two electrons to also become like Neon.

🎯 Exam Tip: To identify elements based on electron gain or loss, determine the resulting electron configuration and match it to a noble gas. Then, work backward to find the original element's atomic number.

 

Question 7. Identify the following reactions as oxidation/reduction/redox reaction.
(a) \( \text{Zn} + \text{CuSO}_4 \rightarrow \text{Cu} + \text{ZnSO}_4 \)
(b) \( \text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O} \)
(c) \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \)
Answer:
(a) This is a Redox reaction.
(b) This is a Reduction reaction.
(c) This is an Oxidation reaction.
In simple words: In (a), electrons are both lost and gained, so it's a redox reaction. In (b), copper loses oxygen, which is reduction. In (c), magnesium gains oxygen, which is oxidation.

🎯 Exam Tip: Remember the different definitions of oxidation (gain of oxygen, loss of hydrogen, loss of electrons, increase in oxidation state) and reduction (loss of oxygen, gain of hydrogen, gain of electrons, decrease in oxidation state). A redox reaction involves both simultaneously.

 

Question 8. What are oxidising agents? Give an example.
Answer: Oxidizing agents are substances that have the ability to cause other substances to get oxidized. They achieve this by accepting electrons from other substances. In this process, the oxidizing agent itself gets reduced. These are also known as electron acceptors.
**Examples:**
\( \text{H}_2\text{O}_2 \)
\( \text{MnO}_4^- \)
\( \text{CrO}_3 \)
\( \text{Cr}_2\text{O}_7^{2-} \)
In simple words: Oxidizing agents are chemicals that take electrons from other chemicals. When they do this, they make the other chemical "oxidize" and they themselves "reduce."

🎯 Exam Tip: A common way to identify an oxidizing agent is to see if an element within it decreases its oxidation state during the reaction, indicating it has gained electrons.

 

Question 9. What are reducing agents? Give examples.
Answer: Reducing agents are substances that have the ability to reduce other substances. They achieve this by donating electrons to other substances. In this process, the reducing agent itself gets oxidized. These are also known as electron donors.
**Examples:**
\( \text{NaBH}_4 \)
\( \text{LiAlH}_4 \)
Metals like Palladium and Platinum
In simple words: Reducing agents are chemicals that give electrons to other chemicals. When they do this, they make the other chemical "reduce" and they themselves "oxidize."

🎯 Exam Tip: To spot a reducing agent, look for an element within it that increases its oxidation state during the reaction, indicating it has lost electrons.

 

Question 10. What are redox reactions? Give examples.
Answer: Redox reactions, short for oxidation-reduction reactions, are chemical reactions where both oxidation and reduction occur simultaneously. This means that one reactant loses electrons (gets oxidized) while another reactant gains those electrons (gets reduced).
**Examples:**
1. \( 2\text{PbO} + \text{C} \rightarrow 2\text{Pb} + \text{CO}_2 \)
2. \( \text{Zn} + \text{CuSO}_4 \rightarrow \text{Cu} + \text{ZnSO}_4 \)
In simple words: A redox reaction is a chemical dance where one substance gives away electrons (gets oxidized) and another substance takes those electrons (gets reduced) at the same time.

🎯 Exam Tip: Every time you see a transfer of electrons in a chemical equation, you're looking at a redox reaction. You can identify which element is oxidized and which is reduced by tracking their oxidation states.

 

Question 11. Define (a) oxidation (b) reduction reactions: Give examples.
Answer:
(a) **Oxidation:** This is a chemical reaction that involves one of the following:
* Addition of oxygen: \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \)
* Removal of hydrogen: \( \text{CaH}_2 \rightarrow \text{Ca} + \text{H}_2 \)
* Loss of electrons: \( \text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^- \)
(b) **Reduction:** This is a chemical reaction that involves one of the following:
* Addition of hydrogen: \( 2\text{Na} + \text{H}_2 \rightarrow 2\text{NaH} \)
* Removal of oxygen: \( \text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O} \)
* Gain of electrons: \( \text{Fe}^{3+} + \text{e}^- \rightarrow \text{Fe}^{2+} \)
In simple words: Oxidation is when something gains oxygen, loses hydrogen, or loses electrons. Reduction is the opposite: when something loses oxygen, gains hydrogen, or gains electrons. These two often happen together.

🎯 Exam Tip: To remember oxidation and reduction definitions, use the mnemonic "LEO the lion says GER": Loss of Electrons is Oxidation, Gain of Electrons is Reduction. Also, consider the addition/removal of oxygen and hydrogen for a broader understanding.

 

Question 12. What is rancidity?
Answer: Rancidity refers to the unpleasant smell and taste that develops in food materials, especially those containing fats and oils, when they are exposed to air for a long time. This spoilage is primarily caused by oxidation reactions where oxygen in the air reacts with the fats and oils.
In simple words: Rancidity is when oily or fatty foods go bad, smelling and tasting sour, because the fats react with oxygen in the air.

🎯 Exam Tip: To prevent rancidity, food items are often stored in airtight containers, refrigerated, or packed with inert gases like nitrogen to minimize exposure to oxygen.

 

Question 14. Identify the type of bond in \( NH_4^+ \).
Answer: The diagram shows both covalent bonds and a coordinate covalent bond in the ammonium ion (\( NH_4^+ \)). The nitrogen atom shares electrons with three hydrogen atoms through covalent bonds. It also donates a lone pair of electrons to form a coordinate covalent bond with a fourth hydrogen ion (\( H^+ \)).
In simple words: The \( NH_4^+ \) ion has regular sharing bonds (covalent) and one special sharing bond where one atom gives both electrons (coordinate covalent).

🎯 Exam Tip: When drawing Lewis structures for ions like \( NH_4^+ \), remember to show the overall charge outside a bracket and correctly represent the electron pairs, especially for coordinate bonds where one atom donates both electrons.

 

VII. To interpret:

 

Question 1. Ionic bond is also called electrostatic bond.
Answer: Yes, an ionic bond is also called an electrostatic bond. This is because it forms due to the strong attraction between positive ions (cations) and negative ions (anions). These oppositely charged ions come together because of the electrostatic force. This force keeps them tightly bound, forming the ionic bond.
In simple words: An ionic bond is a strong pull between positive and negative parts, like tiny magnets. This pull is called electrostatic force, so the bond is also named after it.

🎯 Exam Tip: Understanding that ionic bonds are fundamentally electrostatic attractions between charged particles helps explain their properties, such as high melting points.

 

Question 2. Ionic compounds are crystalline solids at room temperature.
Answer: Yes, ionic compounds are crystalline solids when at room temperature. This happens because the strong electrostatic forces between the positive and negative ions pull them into a very organized, repeating three-dimensional pattern. This arrangement forms a crystal lattice, giving them their solid, crystalline structure.
In simple words: Ionic compounds are solid and crystal-like at normal temperatures because their charged parts are strongly held together in a neat pattern.

🎯 Exam Tip: The regular, orderly arrangement of ions in an ionic compound contributes to its crystalline structure and hardness.

 

Question 3. Covalent compounds have a low melting point.
Answer: Covalent compounds typically have low melting points because the forces holding their individual molecules together are quite weak. When heat is added, these weak intermolecular forces are easily overcome, allowing the molecules to move freely and change from solid to liquid or gas at lower temperatures. This is different from the strong bonds within the molecule itself.
In simple words: Covalent compounds melt easily because the pull between their molecules is not very strong. A little bit of heat is enough to make them separate and melt.

🎯 Exam Tip: Remember to distinguish between the strong covalent bonds *within* a molecule and the weak intermolecular forces *between* molecules when discussing melting points.

 

VIII. Assertion and Reason type questions :

 

Question. 1. Statement (A) : Ionic compounds do not conduct electricity in a solid-state. Reason (B) : The ions in ionic compounds are tightly held together by a strong electrostatic force of attraction and they can not move freely.
(a) B explains A
(b) B does not explain A
(c) B is wrong A
(d) A is right B is wrong
Answer: (a) B explains A
In simple words: Statement A says ionic solids don't conduct electricity. Reason B explains this by saying the charged parts (ions) are stuck in place and cannot move. Since movement of charged particles is needed for electricity, Reason B correctly explains why Statement A is true.

🎯 Exam Tip: For assertion-reason questions, first check if both statements are true. If so, then determine if the reason directly explains the assertion.

 

Question. 2. Statement (A) : Covalent compounds are bad conductors of electricity. Reason (B) : Covalent compounds contain charged particles (ions)
(a) B explains A
(b) B does not explain A
(c) Both A & B are right
(d) Both A & B are wrong
Answer: (b) B does not explain A
In simple words: Statement A is true: covalent compounds do not conduct electricity well. However, Reason B is false because covalent compounds do not have charged particles (ions). Electricity needs free-moving charged particles, which covalent compounds lack, but not because they contain ions. So, the reason given is incorrect.

🎯 Exam Tip: Be careful to identify if the reason itself is a correct statement, even if it doesn't explain the assertion. Here, Reason (B) is incorrect because covalent compounds *do not* contain ions.

 

IX. Find the oxidation number of the elements in the following compounds.

 

Question. (1) Zn in ZnSO4
Answer: To find the oxidation number of Zn in \( ZnSO_4 \):
Let the oxidation number of Zn be x.
The oxidation number of S (in sulfate) is +6.
The oxidation number of O is -2.
The sum of oxidation numbers in a neutral compound is 0.
So, \( 1(Zn) + 1(S) + 4(O) = 0 \)
\( x + 1(+6) + 4(-2) = 0 \)
\( x + 6 - 8 = 0 \)
\( x - 2 = 0 \)
\( \implies x = +2 \)
The oxidation number of Zn in \( ZnSO_4 \) is +2.
In simple words: We find the oxidation number for zinc by setting the total charge of the compound to zero. Zinc comes out to be +2 after considering sulfur's +6 and oxygen's -2 charges.

🎯 Exam Tip: Remember the common oxidation states for elements in compounds, especially oxygen (-2) and hydrogen (+1), unless specified otherwise.

 

Question. (2) Ca in CaH2
Answer: To find the oxidation number of Ca in \( CaH_2 \):
Let the oxidation number of Ca be x.
In metal hydrides like \( CaH_2 \), hydrogen typically has an oxidation number of -1.
The sum of oxidation numbers in a neutral compound is 0.
So, \( 1(Ca) + 2(H) = 0 \)
\( x + 2(-1) = 0 \)
\( x - 2 = 0 \)
\( \implies x = +2 \)
The oxidation number of Ca in \( CaH_2 \) is +2.
In simple words: We calculate calcium's oxidation number in \( CaH_2 \) by knowing hydrogen acts as -1 when with a metal. This makes calcium +2.

🎯 Exam Tip: Always pay attention to the context for hydrogen; it's usually +1, but in metal hydrides, it's -1.

 

Question. (3) Mg in MgO
Answer: To find the oxidation number of Mg in \( MgO \):
Let the oxidation number of Mg be x.
The oxidation number of O is -2.
The sum of oxidation numbers in a neutral compound is 0.
So, \( 1(Mg) + 1(O) = 0 \)
\( x + 1(-2) = 0 \)
\( x - 2 = 0 \)
\( \implies x = +2 \)
The oxidation number of Mg in \( MgO \) is +2.
In simple words: In magnesium oxide, since oxygen is -2, magnesium must be +2 to balance the charge in the neutral compound.

🎯 Exam Tip: For binary compounds, often one element's oxidation state is fixed (like oxygen, usually -2), making it easier to determine the other's.

 

Question. (4) N in NH3
Answer: To find the oxidation number of N in \( NH_3 \):
Let the oxidation number of N be x.
The oxidation number of H is +1.
The sum of oxidation numbers in a neutral compound is 0.
So, \( 1(N) + 3(H) = 0 \)
\( x + 3(+1) = 0 \)
\( x + 3 = 0 \)
\( \implies x = -3 \)
The oxidation number of N in \( NH_3 \) is -3.
In simple words: In ammonia, with each hydrogen being +1, the nitrogen atom must have an oxidation number of -3 to make the molecule neutral.

🎯 Exam Tip: Nitrogen can have various oxidation states. Always calculate based on the other known elements in the compound.

 

Question. (5) Al in AlCl3
Answer: To find the oxidation number of Al in \( AlCl_3 \):
Let the oxidation number of Al be x.
The oxidation number of Cl is -1.
The sum of oxidation numbers in a neutral compound is 0.
So, \( 1(Al) + 3(Cl) = 0 \)
\( x + 3(-1) = 0 \)
\( x - 3 = 0 \)
\( \implies x = +3 \)
The oxidation number of Al in \( AlCl_3 \) is +3.
In simple words: For aluminum chloride, since chlorine always has a -1 charge, the aluminum must be +3 to balance the three chlorines and make the compound neutral.

🎯 Exam Tip: Halogens (like Cl) generally have an oxidation state of -1 in compounds, especially with less electronegative elements like metals.

 

X. Complete the following table:

 

Question 1.

Answer: The table above shows the atomic number and electron distribution for Oxygen, Nitrogen, Chlorine, and Magnesium. The electron distribution follows the shells, with the first shell holding up to 2, the second up to 8, and so on. This arrangement helps us understand how atoms bond. For example, oxygen has 6 valence electrons, meaning it needs 2 more to complete its outer shell.
In simple words: We filled in the table by finding how many electrons are in each shell for oxygen, nitrogen, chlorine, and magnesium, based on their atomic number.

🎯 Exam Tip: Remember that the electron distribution for an atom starts filling from the innermost shell outwards (2, 8, 18, etc.) and the outermost shell electrons are called valence electrons.

 

Question 2.

Answer: The table categorizes chemical concepts. A dative bond is another name for a co-ordinate covalent bond, where one atom donates both electrons. The reaction where \( CaH_2 \) loses hydrogen and calcium's oxidation state changes from +2 to 0 for H and +2 for Ca in the product, or if we consider H moving from -1 to 0, this is an oxidation process. The reaction \( Fe^{3+} + e^- \rightarrow Fe^{2+} \) is a reduction because an electron is gained, lowering the oxidation state. Finally, a redox reaction involves both oxidation and reduction happening at the same time.
In simple words: This table explains chemical terms: dative is coordinate covalent, \( CaH_2 \) changing is oxidation, gaining an electron is reduction, and both happening at once is a redox reaction.

🎯 Exam Tip: Clearly distinguish between oxidation (loss of electrons, gain of oxygen, loss of hydrogen) and reduction (gain of electrons, loss of oxygen, gain of hydrogen).

 

XI. To Match:

 

Question 1.

Answer: The table matches compounds to their bond types. Sodium chloride (NaCl) is an ionic compound because it's formed by the transfer of electrons between a metal (Na) and a non-metal (Cl). The \( NH_3 \rightarrow BF_3 \) adduct is a co-ordinate covalent compound as nitrogen donates a lone pair to boron. Methane (\( CH_4 \)) and fluorine (\( F_2 \)) are covalent compounds because atoms share electrons to achieve stability. The sharing of electrons forms these bonds.
In simple words: This table connects each substance to its bond type: sodium chloride is ionic, \( NH_3 \rightarrow BF_3 \) is coordinate covalent, and methane and fluorine are covalent because of electron sharing.

🎯 Exam Tip: To classify compounds, remember that ionic bonds usually involve a metal and a non-metal, while covalent bonds involve two non-metals. Coordinate covalent bonds are a special type of covalent bond.

 

XII. Answer in detail :

 

Question 1. Explain the ionic bond formation in sodium chloride with electron distribution diagram.
Answer: The formation of an ionic bond in sodium chloride (NaCl) involves the transfer of electrons from a sodium atom to a chlorine atom to achieve stable electron configurations.
(i) A sodium atom (Na) has an atomic number of 11, with an electron configuration of 2, 8, 1. It has one electron in its outermost shell, which it tends to lose to become stable.
(ii) By losing this one electron, sodium forms a positive ion, called a sodium cation (\( Na^+ \)), with a stable configuration of 2, 8, which is similar to that of the noble gas Neon.
(iii) A chlorine atom (Cl) has an atomic number of 17, with an electron configuration of 2, 8, 7. It needs one electron to complete its outermost shell and become stable.
(iv) By gaining one electron, chlorine forms a negative ion, called a chloride anion (\( Cl^- \)), with a stable configuration of 2, 8, 8, which resembles the noble gas Argon.
(v) When a sodium atom and a chlorine atom react, the single valence electron from the sodium atom is transferred to the chlorine atom. This transfer creates oppositely charged ions, \( Na^+ \) and \( Cl^- \). These ions are then strongly attracted to each other by electrostatic forces, forming the ionic bond in the sodium chloride molecule. Both atoms achieve a stable octet (eight electrons) in their outermost shells, making the compound very stable.
In simple words: Sodium gives its extra electron to chlorine. Sodium becomes positive, chlorine becomes negative. Then, these opposite charges pull them together strongly, forming salt. Both atoms become stable like noble gases.

🎯 Exam Tip: When explaining ionic bond formation, clearly describe the electron configuration of each atom before and after electron transfer, showing how they achieve a stable octet (or duplet for hydrogen/helium).

 

Question 2. Explain the covalent bond formation in the following molecules.
(a) Chlorine
(b) Nitrogen
(c) Hydrogen
(d) Oxygen
Answer: Covalent bonds form when atoms share electrons to achieve a stable electron configuration, typically an octet (eight valence electrons) or a duplet (two valence electrons for hydrogen).
(a) Chlorine (\( Cl_2 \)): A chlorine molecule forms from two chlorine atoms. Each chlorine atom has seven valence electrons (configuration 2, 8, 7). To achieve a stable octet, each chlorine atom shares one electron with the other chlorine atom. This forms one shared pair of electrons, resulting in a single covalent bond. Both atoms now effectively have eight electrons in their outermost shell.
(b) Nitrogen (\( N_2 \)): A nitrogen molecule forms from two nitrogen atoms. Each nitrogen atom has five valence electrons (configuration 2, 5). To achieve a stable octet, each nitrogen atom shares three electrons with the other nitrogen atom. This creates three shared pairs of electrons, forming a triple covalent bond. This strong bond makes nitrogen gas very stable.
(c) Hydrogen (\( H_2 \)): A hydrogen molecule forms from two hydrogen atoms. Each hydrogen atom has one valence electron. To achieve a stable duplet (like helium), each hydrogen atom shares its single electron with the other hydrogen atom. This forms one shared pair of electrons, resulting in a single covalent bond. Both hydrogen atoms now have two electrons in their outer shell.
(d) Oxygen (\( O_2 \)): An oxygen molecule forms from two oxygen atoms. Each oxygen atom has six valence electrons (configuration 2, 6). To achieve a stable octet, each oxygen atom shares two electrons with the other oxygen atom. This forms two shared pairs of electrons, resulting in a double covalent bond. This sharing completes the outer shell for both oxygen atoms.
In simple words: In covalent bonds, atoms share electrons to become stable. Chlorine shares one pair (single bond), nitrogen shares three pairs (triple bond), hydrogen shares one pair (single bond), and oxygen shares two pairs (double bond). This makes each atom's outer shell full.

🎯 Exam Tip: When explaining covalent bonds, always specify the number of shared electron pairs (single, double, or triple bond) and relate it to achieving the stable octet or duplet configuration.

 

Question 3. Explain the co-ordinate covalent bond formation in between \( NH_3 \rightarrow BF_3 \) molecules.
Answer: A co-ordinate covalent bond (also called a dative bond) forms when one atom donates both electrons for a shared pair. This is different from a regular covalent bond where each atom contributes one electron.
In the reaction between ammonia (\( NH_3 \)) and boron trifluoride (\( BF_3 \)):
(i) Ammonia (\( NH_3 \)) has a nitrogen atom with a lone pair of electrons (two unshared electrons). This makes nitrogen an electron-rich species and capable of donating electrons.
(ii) Boron trifluoride (\( BF_3 \)) has a boron atom that is electron-deficient. Boron has only six electrons in its outermost shell after bonding with three fluorine atoms, so it needs two more electrons to complete its octet.
(iii) The nitrogen atom in \( NH_3 \) donates its lone pair of electrons to the electron-deficient boron atom in \( BF_3 \). This donation forms a co-ordinate covalent bond between nitrogen and boron. The bond is often shown with an arrow pointing from the donor (N) to the acceptor (B). This results in an adduct \( F_3B \leftarrow NH_3 \). This type of bond allows both atoms to achieve a stable electron configuration.
In simple words: In this bond, nitrogen from ammonia gives its two extra electrons to boron from boron trifluoride. The boron needs these electrons to be stable. So, nitrogen acts as a giver and boron as a taker, creating a special kind of shared bond.

🎯 Exam Tip: Identify the electron donor (usually with a lone pair) and the electron acceptor (usually electron-deficient) to correctly show the direction of the arrow in a co-ordinate covalent bond.

 

Question 4. Write notes on the characteristics of covalent compounds.
Answer: Covalent compounds form when atoms share electrons. Their properties are generally influenced by the weak intermolecular forces between molecules, not the strong covalent bonds within them. Here are their main characteristics:
(a) Physical state: Covalent compounds can exist as gases, liquids, or solids at room temperature. This depends on the strength of the intermolecular forces. For example, oxygen is a gas, water is a liquid, and diamond (a giant covalent structure) is a solid.
(b) Electrical conductivity: Covalent compounds do not contain charged particles (ions) that can move freely. Therefore, they are generally bad conductors of electricity in all states.
(c) Melting point: Most covalent compounds have relatively low melting and boiling points compared to ionic compounds. This is because little energy is needed to overcome the weak forces between the molecules.
(d) Solubility: Covalent compounds are typically soluble in non-polar solvents like benzene (\( C_6H_6 \)) or carbon tetrachloride (\( CCl_4 \)). They are generally insoluble in polar solvents like water, unless they can form hydrogen bonds with water molecules.
(e) Hardness and brittleness: Unlike ionic compounds, covalent compounds are generally soft and waxy, not hard and brittle. This again relates to the weaker forces between their molecules.
(f) Reactions: Chemical reactions involving covalent compounds usually involve breaking and forming new covalent bonds. These reactions tend to be slower compared to ionic reactions, which are often instantaneous.
In simple words: Covalent compounds can be gas, liquid, or solid. They don't carry electricity because they have no free charged parts. They melt and boil easily. They mix well with non-polar liquids but not usually with water. They are soft and react slowly.

🎯 Exam Tip: Remember that the properties of covalent compounds (like melting point and conductivity) are largely determined by the *intermolecular forces* between molecules, not the *intramolecular covalent bonds* within them, except for giant covalent structures.

 

Question 5. Write notes on the characteristics of ionic compounds.
Answer: Ionic compounds are formed by the transfer of electrons between atoms, creating positive and negative ions. These ions are held together by strong electrostatic forces, leading to a distinct set of properties:
(a) Physical state: Due to the strong electrostatic forces, ionic compounds form a crystal lattice structure where ions are arranged in a well-defined geometrical pattern. This makes them crystalline solids at room temperature.
(b) Electrical conductivity: In their solid state, ionic compounds do not conduct electricity because their ions are tightly held in the crystal lattice and cannot move. However, in a molten (liquid) state or when dissolved in water, the ions become mobile and can conduct electricity efficiently.
(c) Melting point: Ionic compounds have very high melting and boiling points. A large amount of energy is required to overcome the strong electrostatic forces holding the ions together in the crystal lattice.
(d) Solubility: Ionic compounds are generally soluble in polar solvents, such as water. This is because the polar water molecules can effectively surround and separate the positive and negative ions. They are typically insoluble in non-polar solvents like benzene or carbon tetrachloride.
(e) Density, hardness, and brittleness: Ionic compounds tend to have high densities. They are also quite hard due to the strong electrostatic forces. However, they are brittle, meaning they shatter when struck, because a shift in the lattice can bring like-charged ions close together, causing repulsion.
(f) Reactions: Ionic compounds typically undergo ionic reactions, which are very fast and often instantaneous. This is because the reactions involve the interaction of already formed ions.
In simple words: Ionic compounds are solid crystals at room temperature. They don't conduct electricity as solids, but they do when melted or in water. They have very high melting points and are hard but can break easily. They dissolve in water but not in oil-like liquids, and their reactions are very quick.

🎯 Exam Tip: Always remember that the conductivity of ionic compounds depends on the mobility of their ions, which is why they conduct only in molten or aqueous states, not as solids.