Selina Concise Solutions for ICSE Class 10 Chemistry Chapter 2 Chemical Bonding

Intext - Question- 1

Question 1. How do atoms attain noble gas configuration?
Answer: Atoms lose, gain or share electrons to attain noble gas configuration.
In simple words: Atoms want to become stable like noble gases, so they either give away electrons, take electrons from others, or share electrons with other atoms.

📝 Teacher's Note: Use the analogy of people wanting to feel complete or secure — atoms "feel secure" when they have the same electron arrangement as noble gases. Show students the periodic table to identify noble gases first.

🎯 Exam Tip: Always mention all three methods (lose, gain, share) to get full marks. Don't just write "transfer electrons" — be specific.

Question. Define:
(a) a chemical bond
(b) an electrovalent bond
(c) a covalent bond
Answer:
(a) A chemical bond may be defined as the force of attraction between any two atoms, in a molecule, to maintain stability.
(b) The chemical bond formed between two atoms by transfer of one or more electrons from the atom of a metallic electropositive element to an atom of a non-metallic electronegative element.
(c) The chemical bond formed due to mutual sharing of electrons between the given pairs of atoms of non-metallic elements.
In simple words: Chemical bonds are like invisible glue that holds atoms together. Electrovalent bonds form when metals give electrons to non-metals, while covalent bonds form when non-metals share electrons equally.

📝 Teacher's Note: Use the analogy of friendship — sharing toys (covalent) vs giving gifts (ionic). Draw simple diagrams showing electron transfer vs sharing to make concepts visual.

🎯 Exam Tip: For definitions, always mention "force of attraction" and "stability" for chemical bonds. Include "transfer" for ionic and "sharing" for covalent bonds.

Question. What are the conditions for the formation of an electrovalent bond?
Answer: Conditions for formation of Ionic bond are:
(i) The atom which changes into cation should possess 1, 2 or 3 valency electrons. The other atom which changes into anion should possess 5, 6 or 7 electrons in the valence shell.
(ii) A high difference of electronegativity of the two atoms is necessary for the formation of an Ionic bond.
(iii) There must be an overall decrease in energy i.e., energy must be released. For this an atom should have low value of Ionisation potential and the other atom should have high value of electron affinity.
(iv) Higher the lattice energy, greater will be the case of forming an ionic compound.
In simple words: For ionic bonds to form, one atom should easily give electrons (metals with 1-3 outer electrons) and another should easily take electrons (non-metals with 5-7 outer electrons). The process should release energy to be stable.

📝 Teacher's Note: Emphasize the "give and take" nature — metals are generous (low ionization energy) and non-metals are greedy (high electron affinity). Use examples like Na and Cl to illustrate each condition.

🎯 Exam Tip: Remember the magic numbers: 1-3 electrons for cation formation, 5-7 for anion formation. Always mention energy decrease for stability.

Question. An atom X has three electrons more than the noble gas configuration. What type of ion will it form? Write the formula of its
(i) sulphate
(ii) nitrate
(iii) phosphate
(iv) carbonate
(v) hydroxide
Answer: It will form a cation: \( M^{3+} \)
\( M_2(SO_4)_3 \)
\( M(NO_3)_3 \)
\( M_3(PO_4) \)
\( M_2(CO_3)_3 \)
\( M(OH)_3 \)
In simple words: Since X has 3 extra electrons beyond noble gas configuration, it will lose these 3 electrons to become stable, forming a +3 charged ion.

📝 Teacher's Note: Explain that "three electrons more" means it's easier to lose 3 than gain 5. Practice criss-cross method for writing compound formulas with different charges.

🎯 Exam Tip: When writing formulas, use the criss-cross method — the charge of one ion becomes the subscript of the other. Check that charges balance to zero.

Question. Mention the basic tendency of an atom which makes it to combine with other atoms.
Answer: Atoms combine with other atoms to attain stable octet or noble gas configuration.
In simple words: Every atom wants to have 8 electrons in its outer shell (or 2 for hydrogen) because this makes them very stable and happy, just like noble gases.

📝 Teacher's Note: Introduce the octet rule early and relate it to noble gases being unreactive. Use helium (2 electrons) and neon (8 electrons) as examples of stable configurations.

🎯 Exam Tip: Always mention "octet configuration" or "noble gas configuration" — both terms are acceptable and score marks.

Question. What type of compounds are usually formed between metals and non-metals and why?
Answer: Ionic compounds are generally formed between metals and non-metals as metals always lose electrons to form cations while non-metals gain electrons forming anions to complete their octet. These oppositely charged ions are held together by electrostatic force of attraction and hence results in an ionic compound.
In simple words: Metals and non-metals form ionic compounds because metals easily give electrons and non-metals easily take them, creating opposite charges that attract each other strongly.

📝 Teacher's Note: Use the analogy of magnets — opposite charges attract just like opposite poles of magnets. Demonstrate with examples like table salt (NaCl).

🎯 Exam Tip: Always mention "electrostatic force of attraction" between oppositely charged ions — this phrase commonly appears in marking schemes.

Question. In the formation of the compound \( XY_2 \), an atom X gives one electron to each Y atom. What is the nature of bond in \( XY_2 \)? Draw the electron dot structure of this compound.
Answer: X and Y form an ionic bond in \( XY_2 \).
In simple words: Since X is giving electrons to Y atoms, this is electron transfer which creates ionic bonds between the atoms.

📝 Teacher's Note: Emphasize that "giving electrons" always indicates ionic bonding. Practice drawing electron dot diagrams showing the transfer process with arrows.

🎯 Exam Tip: When asked about bond nature, look for keywords like "gives," "transfers," or "shares" to identify ionic vs covalent bonding.

Question. An atom X has 2, 8, 7 electrons in its shell. It combines with Y having 1 electron in its outermost shell.
(a) What type of bond will be formed between X and Y?
(b) Write the formula of the compound formed.
Answer:
(a) X has 7 electrons in its outermost shell and Y has only one electron in its outermost shell so Y loses its one electron and X gains that electron to form an ionic bond.
(b) The formula of the compound would be XY.
In simple words: X needs 1 more electron to complete its octet, and Y wants to lose its 1 electron. So Y gives its electron to X, forming an ionic bond.

📝 Teacher's Note: Point out that X is like chlorine (needs 1 electron) and Y is like sodium (wants to lose 1 electron). This makes the perfect 1:1 ratio in the formula.

🎯 Exam Tip: Always analyze electron configurations first — count how many electrons each atom needs to gain or lose to reach noble gas configuration.

Question. Draw orbit structure and electron dot diagram of NaCl, \( MgCl_2 \) and CaO.
Answer:
Orbit structure and electron dot diagram of NaCl:
Orbit structure and electron dot diagram of \( MgCl_2 \):
Orbit structure and electron dot diagram of CaO:
In simple words: These diagrams show how electrons are arranged in shells and how they transfer between atoms to form ionic compounds.

📝 Teacher's Note: Practice drawing these diagrams step by step — first show individual atoms, then the electron transfer, and finally the resulting ions. Use different colors for different atoms.

🎯 Exam Tip: Always show electron transfer with arrows and clearly mark the charges on the resulting ions in your diagrams.

Question. Compare:
(a) sodium atom and sodium ion
(b) chlorine atom and chloride ion, with respect to
(i) atomic structure
(ii) electrical state
(iii) chemical action
(iv) toxicity
Answer:
(a) Sodium atom and sodium ion
(i) Sodium atom has one electron in M shell while sodium ion has 8 electrons in L shell.
(ii) Sodium atom is neutral while sodium ion is positively charged.
(iii) Sodium atom is highly reactive while its ion is inert.
(iv) Sodium atom is poisonous while sodium ion is non-poisonous.
(b) Chlorine atom and chlorine ion
(i) Chlorine atom has 7 electrons in its M shell while Chloride ion has 8 electrons in the same shell.
(ii) Chlorine atom is neutral while chloride ion is negatively charged.
(iii) Chlorine atom is highly reactive while its ion is inert.
(iv) Chlorine gas is poisonous while chloride ion is non-poisonous.
In simple words: Atoms and their ions are completely different — atoms are reactive and often dangerous, while ions are stable and safe, like the difference between a wild animal and a tame pet.

📝 Teacher's Note: Use everyday examples — sodium metal explodes in water but sodium ions are safe in table salt. Chlorine gas is toxic but chloride ions are harmless in salt.

🎯 Exam Tip: Remember the key contrasts: atoms are neutral and reactive, ions are charged and inert. This pattern applies to most atom-ion comparisons.

Question. The electronic configuration of fluoride ion is the same as that of a neon atom. What is the difference between the two?
Answer: Fluoride ion is negatively charged while neon atom is neutral.
In simple words: Even though fluoride ion and neon atom have the same electron arrangement, fluoride ion has an extra negative charge because it gained an electron.

📝 Teacher's Note: Explain that same electron configuration doesn't mean same identity — the nuclear charge (number of protons) determines the element's identity.

🎯 Exam Tip: When comparing isoelectronic species, always mention the difference in charge while noting the same electron configuration.

Intext - Question- 2

Question 1. What are the conditions necessary for the formation of covalent molecules?
Answer:
(i) Both atoms should have four or more electrons in their outermost shells, i.e., non-metals.
(ii) Both the atoms should have high electronegativity.
(iii) Both the atoms should have high electron affinity and high ionisation potential.
(iv) Electronegativity difference between the two atoms should be zero or negligible.
(v) The approach of the atoms towards one another should be accompanied by decrease of energy.
In simple words: For covalent bonds, both atoms should be non-metals with similar tendency to attract electrons, so they decide to share electrons rather than one taking from the other.

📝 Teacher's Note: Contrast these conditions with ionic bonding — covalent needs similar atoms (both non-metals) while ionic needs different atoms (metal + non-metal).

🎯 Exam Tip: Key phrase: "negligible electronegativity difference" for covalent vs "high electronegativity difference" for ionic bonds.

Question. Elements A, B and C have atomic number 17, 19 and 10 respectively.
(a) State which one is: (i) a non-metal (ii) a metal, (iii) chemically inert?
(b) write down the formula of the compound formed by two of the above elements.
Answer:
(a) A is a non-metal; B is a metal while C is a chemically inert element.
(b) BA
In simple words: A (atomic number 17) is chlorine, B (atomic number 19) is potassium, and C (atomic number 10) is neon. Potassium and chlorine combine to form KCl.

📝 Teacher's Note: Help students identify elements from atomic numbers — 17 is Cl, 19 is K, 10 is Ne. Practice writing formulas using charges: K⁺ and Cl⁻ give KCl.

🎯 Exam Tip: When given atomic numbers, first identify the elements from the periodic table, then predict their properties based on their position.

Question. Five atoms are labelled from A to E.

(a) Which one of these atoms:
(i) contains 7 protons,
(ii) has an electronic configuration 2, 7?
(b) Write down the formula of the compound formed between C and D.
(c) predict which are: (i) metals, (ii) non-metals?
Answer:
(a) (i) E contains 7 protons
(ii) B has electronic configuration 2, 7
(b) \( C_2D \)
(c) (i) A and C are metals (ii) B, D and E are non-metals
In simple words: Number of protons equals atomic number. Electronic configuration depends on atomic number. C (Li) and D (O) form lithium oxide with formula \( Li_2O \).

📝 Teacher's Note: Teach students that atomic number = number of protons = number of electrons. Practice determining electron configuration from atomic number using 2, 8, 8 rule.

🎯 Exam Tip: For compound formulas, use the criss-cross method with ion charges. Li⁺ and O²⁻ give Li₂O when charges are balanced.

Question. What is the difference between:
(a) ionic compounds and polar covalent compounds,
(b) ionic compounds and covalent compounds,
(c) a polar covalent compound and a non-polar covalent compound?
Answer: (a) Ionic compounds are formed as a result of transfer of one or more electrons from the atom of a metallic electropositive element to an atom of a non-metallic electronegative element. A polar covalent compound is the one in which there is an unequal distribution of electrons between the two atoms.
(b) Ionic compounds, made up of ions, are generally crystalline solids with high melting and boiling points. They are soluble in water and good conductors of electricity in aqueous solution and molten state. Covalent compounds, made up of molecules, can exist as soft solids or liquids or gases with low melting and boiling points. They are generally insoluble in water and poor conductors of electricity.
(c) Polar covalent compounds are formed between 2 non-metal atoms that have different electro negativities and therefore have unequal sharing of the bonded electron pair. Nonpolar compounds are formed when two identical non-metals equally share electrons between them.
In simple words: Ionic compounds are made when atoms completely give or take electrons, polar covalent compounds share electrons unequally, and non-polar covalent compounds share electrons equally.

📝 Teacher's Note: Use salt (NaCl) vs water (H₂O) vs hydrogen gas (H₂) as examples to help students visualize the difference between ionic, polar covalent, and non-polar covalent bonding.

🎯 Exam Tip: Remember the key words: "transfer" for ionic, "unequal sharing" for polar covalent, and "equal sharing" for non-polar covalent.

Question. The element X has the electronic configuration 2, 8, 18, 8, 1. Without identifying X,
(a) predict the sign and charge on a simple ion of X.
(b) write if X be an oxidizing agent or reducing agent and why.
Answer: (a) \( X^+ \)
(b) X will be a strong reducing agent as it will have the tendency to donate its valence electron.
In simple words: Element X has only 1 electron in its outermost shell, so it easily gives it away to become positively charged and helps other substances get reduced.

📝 Teacher's Note: Help students recognize that elements with 1 valence electron (like alkali metals) always form +1 ions and act as reducing agents by giving away their electron easily.

🎯 Exam Tip: For electronic configuration questions, focus on the outermost shell electrons to predict charge and behavior.

Question. What do you understand by polar covalent compounds? Explain it by taking hydrogen chloride as an example.
Answer: Covalent compounds are said to be polar when shared pair of electrons are unequally distributed between the two atoms. For example in HCl, the high electronegativity of the chlorine atom attracts the shared electron pair towards itself. As a result, it develops a slight negative charge and hydrogen atom develops a slight positive charge. Hence, a polar covalent bond is formed.
In simple words: In polar covalent compounds, one atom is more greedy for electrons than the other, creating slight positive and negative ends like a magnet.

📝 Teacher's Note: Use the analogy of a tug-of-war where one player (chlorine) is stronger and pulls the rope (electrons) more towards their side.

🎯 Exam Tip: Always mention "unequal sharing of electrons" and "partial charges" when explaining polar covalent bonds.

Question. Methane molecule is non-polar molecule. Explain.
Answer: During the formation of a non-polar covalent bond between two similar atoms or dissimilar atoms, the atoms involved in sharing share the electrons equally. The molecule of methane has four carbon-hydrogen single covalent bonds. It is a non-polar covalent compound as the electrons are shared by the carbon and hydrogen atoms equally and hence the shared pair lies between the atoms at an equal distance from both carbon and hydrogen atom.
In simple words: In methane, carbon and hydrogen share electrons fairly equally, so there are no positive or negative ends in the molecule.

📝 Teacher's Note: Emphasize that even though C-H bonds are slightly polar individually, the tetrahedral shape of methane makes the overall molecule non-polar due to symmetry.

🎯 Exam Tip: For non-polar molecules, mention "equal sharing of electrons" and "no partial charges" as key points.

Question. Give the characteristic properties of:
(a) electrovalent compounds,
(b) covalent compounds.
Answer: (a) Properties of Ionic Compounds:
1. Ionic compounds usually exist in the form of crystalline solids.
2. Ionic compounds have high melting and boiling points.
3. Ionic compounds are generally soluble in water but insoluble in organic solvents.
4. They are good conductors of electricity in the fused or in aqueous solution state.
(b) Properties of Covalent Compounds:
1. The covalent compounds exist as gases or liquids or soft solids.
2. The melting and boiling points of covalent compounds are generally low.
3. Covalent compounds are insoluble in water but dissolve in organic solvents.
4. They are non-conductors of electricity in solid, molten or aqueous state.
In simple words: Ionic compounds are hard crystals that melt at high temperatures and conduct electricity, while covalent compounds are softer, melt easily, and don't conduct electricity.

📝 Teacher's Note: Create a comparison chart on the board and use everyday examples like salt (ionic) vs sugar (covalent) to demonstrate these properties.

🎯 Exam Tip: Remember the pattern: ionic compounds have "high" values (melting point, boiling point) while covalent compounds have "low" values.

Question. What do you understand by redox reactions? Explain oxidation and reduction in terms of loss or gain of electrons.
(b) Divide the following redox reactions into oxidation and reduction half reactions.
(i) \( Zn + Pb^{2+} \rightarrow Zn^{2+} + Pb \)
(ii) \( Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu \)
(iii) \( Cl_2 + 2Br^- \rightarrow Br_2 + 2Cl^- \)
(iv) \( Sn^{2+} + 2Hg^{2+} \rightarrow Sn^{4+} + Hg_2^{2+} \)
(v) \( 2Cu^+ \rightarrow Cu + Cu^{2+} \)
(c) Potassium (at No. 19) and chlorine (at No. 17) react to form a compound. Explain on the basis of electronic concept.
(i) oxidation
(ii) reduction
(iii) oxidizing agent
(iv) reducing agent
Answer: (a) A reaction in which oxidation and reduction occur simultaneously is called an oxidation-reduction, or simply, a redox reaction. Redox reactions involve the transfer of electrons between two chemical species. The reaction in which electron is gained is called a reduction reaction and the reaction in which electron is lost is called oxidation reaction. The compound that loses an electron is said to be oxidized, the one that gains an electron is said to be reduced.
(b)
(i) Oxidation: \( Zn \rightarrow Zn^{2+} \)
Reduction: \( Pb^{2+} \rightarrow Pb \)
(ii) Oxidation: \( Zn \rightarrow Zn^{2+} \)
Reduction: \( Cu^{2+} \rightarrow Cu \)
(iii) Oxidation: \( Br^- \rightarrow Br \)
Reduction: \( Cl_2 \rightarrow 2Cl^- \)
(iv) Oxidation: \( Sn^{2+} \rightarrow Sn^{4+} \)
Reduction: \( 2Hg^{2+} \rightarrow Hg_2 \)
(v) Oxidation: \( Cu^+ \rightarrow Cu^{2+} \)
Reduction: \( Cu^+ \rightarrow Cu \)
(c)
(i) Potassium undergoes oxidation as it loses an electron and forms a cation.
(ii) Chlorine undergoes reduction as it gains an electron and forms chloride anion.
(iii) Potassium acts a reducing agent and gets oxidised.
(iv) Chlorine acts an oxidizing agent and gets reduced.
In simple words: Redox reactions are like electron trading - one substance gives electrons (gets oxidized) while another takes electrons (gets reduced).

📝 Teacher's Note: Use the mnemonic "OIL RIG" (Oxidation Is Loss, Reduction Is Gain) to help students remember electron movement in redox reactions.

🎯 Exam Tip: Always identify which species loses electrons (oxidized) and which gains electrons (reduced) before writing half-reactions.

Question. What do you understand by a dipole molecule? Give one example.
Answer: Dipole molecule is a molecule that has both, slight positive and slight negative charge. For example, in HCl hydrogen has a slight positive charge and chlorine has a slight negative charge. The dipole moment of HCl molecule is 1.03 D and may be represented as: H \( \rightarrow \) Cl
In simple words: A dipole molecule is like a tiny magnet with a positive end and a negative end due to unequal sharing of electrons.

📝 Teacher's Note: Use a bar magnet analogy to explain dipoles - just like magnets have north and south poles, dipole molecules have positive and negative ends.

🎯 Exam Tip: Always mention "partial positive and negative charges" and give HCl or H₂O as examples of dipole molecules.

Exercise Intext - Question - 3

Question. Explain the following:
(a) Electrovalent compounds conduct electricity.
(b) Electrovalent compounds have a high melting point and boiling point while covalent compounds have low melting and boiling points.
(c) Electrovalent compounds dissolve in water whereas covalent compounds do not.
(d) Electrovalent compounds are usually hard crystals yet brittle.
(e) polar covalent compounds electricity.
Answer: (a) Electrovalent compounds in the solid state do not conduct electricity because movement of ions in the solid state is not possible due to their rigid structure. But these compounds conduct electricity in the molten state. This is possible in the molten state since the electrostatic forces of attraction between the oppositely charged ions become weak. Thus, the ions move freely and conduct electricity.
(b) The atoms of covalent compounds are bound tightly to each other in stable molecules, but the molecules are generally not very strongly attracted to other molecules in the compound. On the other hand, the atoms (ions) in electrovalent compounds show strong attractions to other ions in their vicinity. This generally leads to low melting points for covalent solids, and high melting points for electrovalent solids.
(c) Electrovalent compounds dissolve in polar solvents like water because the forces of attraction between positive and negative charges become weak in water. But since covalent compound are made up of molecules, they do not ionize in water and hence do not dissolve in water.
(d) Since it takes a lot of energy to break the positive and negative charges apart from each other, the ionic compounds are so hard. But on applying stress, Ions of the same charge are brought side-by-side and so the opposite ions repel each other and crystal breaks into pieces.
(e) Since polar covalent compounds are made up of charged particles, they conduct electricity in aqueous solution.
In simple words: Ionic compounds behave differently because they're made of charged particles (ions) that can move around when heated or dissolved, unlike covalent compounds which are made of neutral molecules.

📝 Teacher's Note: Demonstrate conductivity using a simple circuit with salt water vs sugar water to show the difference between ionic and covalent compounds.

🎯 Exam Tip: Remember that ionic compounds conduct electricity only when ions are free to move (molten or dissolved state), not in solid state.

Question. A solid is crystalline, has a high melting point and is water soluble. Describe the nature of the solid.
Answer: The solid is ionic in nature as the oppositely charged ions are being held tightly by strong intermolecular force of attraction and that's the reason for its high melting point and solubility in water.
In simple words: This solid must be an ionic compound because it has all the typical properties of ionic substances - hard crystal structure, high melting point, and dissolves in water.

📝 Teacher's Note: Use this as a pattern recognition exercise - teach students to identify compound types based on their physical properties.

🎯 Exam Tip: When describing unknown solids, match the given properties to the characteristic properties of ionic, covalent, or metallic compounds.

Question. Match the atomic number 4, 8, 14, 15 and 19 with each of the following:
(a) A solid non-metal of valency 3.
(b) A gas of valency 2.
(c) A metal of valency 1.
(d) A non-metal of valency 4
Answer: (a) Atomic number 15
(b) Atomic number 8
(c) Atomic number 19
(d) Atomic number 14
In simple words: We match elements based on their electron configuration to determine how many electrons they can share or transfer (valency).

📝 Teacher's Note: Teach students to write electron configurations first, then determine valency based on outermost shell electrons to achieve stable configuration.

🎯 Exam Tip: Remember that valency equals the number of electrons needed to complete or empty the outermost shell for stability.

Question. Elements X, Y, and Z have atomic numbers 6, 9 and 12 respectively. Which one:
(a) forms an anion,
(b) forms a cation,
(c) State type of bond between Y and Z and give its molecular formula.
Answer: (a) Y
(b) Z
(c) X
In simple words: Based on their electron configurations, Y gains electrons to form anions, Z loses electrons to form cations, and the bond type depends on whether electrons are transferred or shared.

📝 Teacher's Note: Have students write out the electron configurations (2,4; 2,7; 2,8,2) to visualize why each element behaves differently.

🎯 Exam Tip: Elements with 1-3 outer electrons form cations, those with 5-7 outer electrons form anions, and those with 4 outer electrons usually share electrons.

Question. Taking \( MgCl_2 \) as an electrovalent compound, \( CCl_4 \) as a covalent compound, give four differences between electrovalent and covalent compounds.
Answer:

In simple words: Ionic compounds like \( MgCl_2 \) are made of charged particles and have high melting points, while covalent compounds like \( CCl_4 \) are made of molecules and have low melting points.

📝 Teacher's Note: Use these specific examples throughout the lesson to help students remember the differences between the two types of compounds.

🎯 Exam Tip: Always provide specific examples when comparing electrovalent and covalent compounds, and mention the structural differences (ions vs molecules).

Question 6. Potassium chloride is an electrovalent compound, while hydrogen chloride is a covalent compound. But, both conducts electricity in their aqueous solutions. Explain.
Answer: Potassium chloride conducts electricity in water because the forces of attraction between positive and negative charged ions become weak in water and hence the ions become mobile. On the other hand, the \( HCl \) bond is a polar covalent compound, which means that there is a partial positive charge on H and a partial negative charge on Cl. Hence it also ionizes in water and forms ions and conducts electricity.

📝 Teacher's Note: Demonstrate with a simple circuit using KCl and HCl solutions to show both conduct electricity. Emphasize that water weakens ionic bonds and can break polar covalent bonds too.

🎯 Exam Tip: Remember that conductivity depends on presence of ions, not the original bond type. Both ionic and polar covalent compounds can produce ions in water.

Question 7. (a) Name two compounds that are covalent when taken pure but produce ions when dissolved in water. (b) For each compound mentioned above give the formulae of ions formed in aqueous solution.
Answer: (a) \( HCl \) and \( H_2O \)
(b) \( H^+ \) and \( Cl^- \) ions
And
\( H^+ \) and \( O^{2-} \) ions

📝 Teacher's Note: The water example might confuse students since water autoionizes very slightly. Focus on HCl as the clearer example of a polar covalent compound that fully ionizes in water.

🎯 Exam Tip: Remember HCl, HBr, HI, and NH₃ as common covalent compounds that ionize in water. Write correct charges on ions.

Question 8. An element M burns in oxygen to form an ionic bond MO. Write the formula of the compounds formed if this element is made to combine with chlorine and sulphur separately.
Answer: Since M combines with oxygen to form MO which means that M has a valency of +2. Hence, the formula of the compounds with chlorine and sulphur are: \( MCl_2 \) and MS.

📝 Teacher's Note: Use examples like Mg or Ca to make this concrete. Show students how to work backwards from one compound's formula to determine valency.

🎯 Exam Tip: Always determine the valency first from the given compound, then apply that valency to write other formulae. Cross-multiplication method works best.

Question 9. Give electron dot diagram of the following: (a) Magnesium chloride (b) nitrogen (c) methane
Answer:
a. \( [:Cl:]^- Mg^{2+} [:Cl:]^- \)
b. \( :N \equiv N: \)
c.
H
··
H : C : H
··
H

📝 Teacher's Note: Start with simple atoms, show electron transfer/sharing step by step. Use colored dots to distinguish electrons from different atoms initially.

🎯 Exam Tip: Draw neat diagrams with proper electron dots. Remember ionic compounds show charges, covalent compounds show shared electron pairs.

Question 10. State the type of bonding in the following molecules. (a) Water (b) Calcium oxide (c) hydroxyl ion, (d) methane, (e) ammonium ion (f) ammonium chloride
Answer: a. Water - Covalent bond
b. Calcium oxide - Ionic bond
c. Hydroxyl ion - Polar covalent bond
d. Methane - Covalent bond
e. Ammonium ion - Co-ordinate covalent bond
f. Ammonium chloride - Covalent, coordinate and ionic bonds

📝 Teacher's Note: Explain that compound molecules can have multiple types of bonding like in ammonium chloride. Use molecular models if available.

🎯 Exam Tip: For ammonium compounds, remember there are coordinate bonds within the ion and ionic bonds between ions. State all bond types present.

Question 11. Define a coordinate bond and give the conditions for its formation. Explain with an example.
Answer: The bond formed between two atoms by sharing a pair of electrons, provided entirely by one of the combining atoms but shared by both is called a coordinate bond. It is represented by an arrow starting from the donor atoms and ending in the acceptor atom.
Conditions:
1. One of the two atoms must have at least one lone pair of electrons.
2. Another atom should be short of at least a lone pair of electrons.
The two lone pair of electrons in the oxygen atom of water is used to form coordinate bond with the hydrogen ion which is short of an electron resulting in the formation of the hydronium ion.
\( H_2O + H^+ \rightarrow H_3O^+ \)
Over here the hydrogen ion accepts one lone pair of electrons of the oxygen atom of water molecule leading to the formation of a coordinate covalent bond.

📝 Teacher's Note: Use the analogy of lending and borrowing - one atom lends its electron pair but both atoms use it. Draw the arrow clearly from donor to acceptor.

🎯 Exam Tip: Always mention both conditions and give a clear example with the arrow notation. The arrow points from electron donor to electron acceptor.

Question 12. (a) What do you understand by lone pair and shared pair? (b) (i) How many atoms of each kind are present in the following molecules: calcium oxide, chlorine, water, carbon tetrachloride? (ii) How many electrons are required for their octet structure?
Answer: (a) A pair of electrons which is not shared by any other atom is called lone pair of electrons. The electrons of valence shell shared by two atoms to form a covalent bond are called shared pair of electrons.
(b)
(i) Calcium and oxygen make up Calcium oxide. Calcium needs to lose 2 electrons while oxygen needs to gain 2 electrons to complete octet structure. ii) Two atoms of hydrogen and an atom of oxygen make up water molecule. Hydrogen atoms need to gain one electron to complete its duplet while oxygen needs to gain 2 electrons to complete octet structure.
(ii) In carbon tetrachloride, there is one carbon and 4 chlorine atoms. Each of the four chlorine atoms needs to gain one electron to complete its octet while carbon needs to gain 4 electrons to complete octet state.

📝 Teacher's Note: Use simple diagrams to show lone pairs vs shared pairs. Emphasize that lone pairs still occupy space and affect molecular shape.

🎯 Exam Tip: Remember hydrogen follows duplet rule (2 electrons), all others follow octet rule (8 electrons). Count electrons needed carefully for each atom.

Question 13. Complete the following: (a) When the nuclei of two different reacting atoms are of ……………… mass, then a bond so formed is called ……………… covalent band (Equal, unequal, polar, non-polar). (b) In case of non-polar covalent bond, the covalent bond is formed in the ……………. Of atoms and shared electrons are …………. Distributed (corner, middle, equally, unequally). (c) The ions in ……………… compounds are held very strongly due strong ……. Forces (electrovalent, covalent, electromagnetic, electrostatic).
Answer: (a) Unequal, polar
(b) Middle, equally
(c) Electrovalent, electrostatic

📝 Teacher's Note: Connect this to electronegativity differences. Use examples like H-Cl (polar) vs H-H (non-polar) to illustrate the concept clearly.

🎯 Exam Tip: Remember: unequal masses → unequal pull on electrons → polar bond. Equal sharing → non-polar bond in the middle of atoms.

Question 14. (a) Draw an electron dot diagram to show the structure of each of the following: (i) Hydronium ion, (ii) Ammonium ion, (iii) Hydroxyl ion. State the type of bonding present in them. (b) Give two example in each case: (i) Co-ordinate bonds compounds, (ii) solid covalent compounds, (iii) Gaseous polar compounds, (iv) Gaseous non polar compounds, (v) Liquid non polar compounds.
Answer: (a)
(i) [Hydronium ion diagram with coordinate bond]
(ii) [Ammonium ion diagram with coordinate bond]
(iii) [Hydroxyl ion diagram with polar covalent bond]
(b)
(i) Ammonium ion and hydronium ion
(ii) Phosphorus pentachloride and diamond
(iii) Hydrogen chloride and water vapour
(iv) Oxygen gas and nitrogen gas
(v) Toluene and Gasoline

📝 Teacher's Note: Emphasize that the state of matter depends on intermolecular forces, not just bond type. Explain why some covalent compounds are solid while others are gases.

🎯 Exam Tip: For coordinate bond compounds, think of ions with H⁺ or NH₄⁺. For states, consider molecular size and polarity to determine examples.

Question 15. Element M forms a chloride with the formula \( MCl_2 \) which is a solid with high melting point. M would most likely be in the group in which ………….. is placed. [ (a) Na (b) Mg (c) Al (d) Si ]
Answer: (b) Mg
In simple words: Since \( MCl_2 \) has high melting point and is solid, it must be an ionic compound, which means M has +2 charge like magnesium.

📝 Teacher's Note: Connect formula to valency, then valency to group position. High melting point indicates ionic bonding, which helps narrow down the options.

🎯 Exam Tip: Formula \( MCl_2 \) means M has +2 charge. High melting point suggests ionic compound. Only Mg fits both criteria from the options.

Question 16. Complete the following:

📝 Teacher's Note: Use periodic table to predict bonding type - metals form ionic bonds, non-metals form covalent bonds. Physical state depends on molecular size and forces.

🎯 Exam Tip: Metal chlorides are ionic and solid. Non-metal chlorides are covalent - state depends on molecular size and intermolecular forces.

Question (2004). (a) Element X is a metal with a valency 2. Element Y is a non-metal with a valency 3. (i) Write equations to show how X and Y form ions. (ii) If Y is a diatomic gas, write the equation for the direct combination of X and Y to form a compound. (iii) If the compound formed between X and Y is melted and an electric current is passed through the molten compound, the element X will be obtained at the ……… and Y at the ……….. of the electrolytic cell.
Answer: (i) \( X \rightarrow X^{2+} + 2e^- \)
\( Y + 3e^- \rightarrow Y^{3-} \)
(ii) \( 6X + 2Y_2 \rightarrow 2X_3Y_2 \)
(iii) cathode, anode

📝 Teacher's Note: Balance the electron transfer carefully - 6 atoms of X lose 12 electrons, 4 atoms of Y gain 12 electrons. Connect to electrolysis rules.

🎯 Exam Tip: Metal ions go to cathode (negative electrode), non-metal ions go to anode (positive electrode). Balance charges when writing compound formula.

Question (2005). (a) Compound X consists of molecules. Choose the letter corresponding to the correct answer from the options A,B,C and D given below: (i) The type of bonding in X will be:
Answer: [Note: The options A, B, C, D are not provided in the visible text]

📝 Teacher's Note: If a compound consists of molecules, it must have covalent bonding. Ionic compounds form crystal lattices, not discrete molecules.

🎯 Exam Tip: Key word "molecules" always indicates covalent bonding. Ionic compounds don't exist as separate molecules in solid state.

Question. X is a compound which contains molecules with the formula \( X_2 \). The type of bonding present in X is likely to be
(i) X is likely to have a:
(a) ionic
(b) electrovalent
(c) covalent
(d) molecular
(ii) X is likely to have a:
(a) low melting point and high boiling point,
(b) high melting point and low boiling point
(c) Low melting point and low boiling point,
(d) high melting point and high boiling point.
(iii) In the liquid state, X will:
(a) become ionic
(b) be an electrolyte
(c) conduct electricity
(d) not conduct electricity
Answer: (i) (c) covalent (ii) (c) Low melting point and low boiling point (iii) (d) not conduct electricity
In simple words: Molecules with formula \( X_2 \) are made when two identical atoms share electrons (covalent bonding). These molecules have weak forces between them, so they have low melting and boiling points and don't conduct electricity because there are no free ions.

📝 Teacher's Note: Use examples like \( H_2 \), \( O_2 \), or \( Cl_2 \) to show students how identical atoms form covalent bonds. Emphasize that molecular compounds don't have free ions, unlike ionic compounds.

🎯 Exam Tip: When you see \( X_2 \) formula, immediately think covalent bonding. Look for keywords like "low melting point" and "non-conductor" in the options.

Question. Electrons are getting added to an element Y:
(i) is Y getting oxidized or reduced?
(ii) what charge will Y migrate to during the process of electrolysis?
Answer: (i) reduced (ii) negative
In simple words: When electrons are added to an element, it becomes reduced (gains negative charge). During electrolysis, negatively charged particles move towards the positive electrode.

📝 Teacher's Note: Teach the mnemonic "OIL RIG" - Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). This helps students remember the concepts easily.

🎯 Exam Tip: Remember that electrons have negative charge, so adding electrons makes the element negative and it moves towards the positive electrode during electrolysis.

Question. Answer the following:
(i) Acids dissolve in water and produce positively charged ions. Draw the structure of these positive ions.
(ii) Explain why carbon tetrachloride does not dissolve in water.
(iii) Elements Q and S react together to form an ionic compound. Under normal conditions, which physical state will the compound QS exist in?
(iv) Can Q and S, both be metals? Justify your answer.
Answer: (i) \( H_3O^+ \) ions (hydronium ions) - structure shows oxygen atom bonded to three hydrogen atoms with a positive charge (ii) Like dissolves like. Since carbon tetrachloride is non-polar and water is polar compound, carbon tetrachloride does not dissolve in water. (iii) Solid (iv) No as ionic bonds can only be made by transfer of electrons from a metal to non-metal.
In simple words: Acids in water form \( H_3O^+ \) ions. Oil-like substances don't mix with water because they have different polarities. Ionic compounds are usually solid at room temperature, and they need one metal and one non-metal to form properly.

📝 Teacher's Note: Draw the \( H_3O^+ \) structure clearly on the board. Use oil-water separation as a practical example for polarity concepts. Explain that ionic compounds need electron transfer, which only happens between metals (give electrons) and non-metals (receive electrons).

🎯 Exam Tip: Always draw the complete structure for \( H_3O^+ \) with all bonds and the positive charge clearly marked. Remember "like dissolves like" for solubility questions.

Question 2006. Choose the correct answer from the choices A,B,C and D:
(i) The property which is characteristic of an electrovalent compound is that:
(a) it is easily vaporized
(b) it has a high melting point
(c) it is a weak electrolyte,
(d) it often exists as a liquid.
(ii) When a metal atom becomes an ion:
(a) It loses electrons and is Oxidized
(b) It gains electrons and is reduced,
(c) It gains electrons and is oxidized,
(d) it loses electrons and is reduced.
Answer: (i) (b) it has a high melting point (ii) (a) It loses electrons and is Oxidized
In simple words: Electrovalent (ionic) compounds have strong forces between ions, so they need high temperature to melt. Metals always lose electrons to become positive ions, and losing electrons is called oxidation.

📝 Teacher's Note: Compare ionic compounds like salt (high melting point) with covalent compounds like wax (low melting point) to demonstrate the difference. Use the mnemonic "LEO says GER" - Lose Electrons Oxidation, Gain Electrons Reduction.

🎯 Exam Tip: Electrovalent = ionic compounds always have high melting points due to strong ionic bonds. Metals always lose electrons and get oxidized when forming ions.

Question. Identify the following reactions as either oxidation or reduction:
(i) \( O + 2e^- \rightarrow O^{2-} \)
(ii) \( K - e^- \rightarrow K^+ \)
(iii) \( Fe^{3+} + e^- \rightarrow Fe^{2+} \)
Answer: (i) Reduction (ii) Oxidation (iii) Reduction
In simple words: When electrons are gained (like in reactions i and iii), it's reduction. When electrons are lost (like in reaction ii), it's oxidation.

📝 Teacher's Note: Point out the direction of electron flow in each equation. Show students how to identify electron gain/loss by looking at the charges before and after the reaction.

🎯 Exam Tip: Look for the electron symbol \( e^- \) - if it's on the left side (reactant), it's reduction; if it's on the right side (product), it's oxidation.

Question 2007. Answer the following:
(i) Name the charged particles which attract one another to form electrovalent compounds.
(ii) In the formation of electrovalent compounds, electrons are transferred from one element to another. How are electrons involved in the formation of a covalent compound?
(iii) The electronic configuration of nitrogen is (2, 5). How many electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule?
(iv) In the formation of magnesium chloride (by direct combination between magnesium and chlorine), name the substance that is oxidized and the substance that is reduced.
Answer: (i) Ions (ii) Electrons are shared between the atoms of two or more elements (iii) Two (iv) Magnesium is oxidized and chlorine is reduced
In simple words: Ions (charged particles) stick together to form ionic compounds. In covalent bonds, electrons are shared instead of transferred. Nitrogen has 5 outer electrons, uses 3 for bonding, leaving 2 unused. Magnesium gives electrons (oxidized) and chlorine takes electrons (reduced).

📝 Teacher's Note: Draw electron dot structures for \( N_2 \) to show the triple bond and lone pair. Use arrows to show electron transfer in \( MgCl_2 \) formation, emphasizing which element loses and which gains electrons.

🎯 Exam Tip: For nitrogen molecule questions, remember it forms a triple bond using 3 electrons, leaving 2 as a lone pair. The element that loses electrons is always oxidized.

Question 2008. Which of the following is not a common characteristic of an electrovalent compound?
(a) High melting point
(b) Conducts of electricity when molten.
(c) Consists of oppositely charged ions
(d) Ionizes when dissolved in water.
Answer: (d) Ionizes when dissolved in water.
In simple words: Electrovalent compounds already exist as ions, so they don't need to "ionize" in water - they just separate into the ions they already have.

📝 Teacher's Note: Clarify the difference between ionization (forming ions from molecules) and dissociation (separating existing ions). Electrovalent compounds dissociate, while covalent compounds like HCl ionize.

🎯 Exam Tip: Remember that ionic compounds are already made of ions, so they dissociate (separate) in water rather than ionize (form new ions).

Question. What are the terms defined below?
(i) A bond formed by a shared pair of electrons, each bonding atom contributing one electrons to the pair.
(ii) A bond formed by a shared pair of electrons with both electrons coming from the same atom.
Answer: (i) Covalent bond (ii) Coordinate bond
In simple words: A covalent bond is when two atoms each give one electron to share. A coordinate bond is when one atom gives both electrons for sharing (like lending both items in a partnership).

📝 Teacher's Note: Use the analogy of sharing toys - covalent bonding is like two children each bringing one toy to share, while coordinate bonding is like one child bringing both toys to share with the other.

🎯 Exam Tip: Look for keywords: "each contributing one" = covalent bond; "both from same atom" = coordinate bond. Draw arrows to show electron donation in coordinate bonds.