Selina Concise Solutions for ICSE Class 7 Chemistry Chapter 4 Atoms Molecules and Radicals

Points to Remember :

Point 1: Atoms of the most elements are composed of still smaller particles known as fundamental particles or subatomic particles. They are protons, electrons and neutrons.
Everything around us is built from these three tiny building blocks inside the atom. Scientists call them subatomic because they are smaller than the atom itself.
Teacher's Tip: Think of an atom as a tiny solar system where these particles are the parts that make it work.
Exam Tip: To get full marks, always list all three particles: protons, electrons, and neutrons.

Point 2: Protons are the positively charged particles present in an atom.
These particles live in the center of the atom and have a plus (+) charge. The number of protons is unique for every different type of element.
Teacher's Tip: Remember 'P' for Proton and 'P' for Positive charge.
Exam Tip: Always specify that protons are found inside the nucleus of the atom.

Point 3: Electrons are the negatively charged particles present in an atom and its mass is 1/1837 the mass of one hydrogen atom.
Electrons are incredibly light and zip around the outside of the atom's center. Because their mass is so small, they don't really affect the total weight of the atom much.
Teacher's Tip: Imagine electrons as tiny, fast bees buzzing around a hive.
Exam Tip: Mention the specific fraction 1/1837 to show you have studied the details of electron mass.

Point 4: Neutrons are particles with no electrical charge. Its mass is equal to one atomic mass unit.
Neutrons act like the "glue" that helps hold the center of the atom together. They are heavy particles, just like protons, and carry a neutral charge.
Teacher's Tip: Remember 'N' for Neutron and 'N' for Neutral (no charge).
Exam Tip: If asked for the mass, always state it is approximately 1 amu (atomic mass unit).

Point 5: The central part of an atom is called nucleus, which contains both protons and neutrons.
The nucleus is like the brain or the engine room located right in the middle of the atom. It holds almost all the weight of the atom because it contains the heavy particles.
Teacher's Tip: Think of the nucleus as the pit inside a cherry.
Exam Tip: Clearly state that the nucleus is positively charged because it contains protons.

Point 6: Shells or orbits around the nucleus.
These are the specific paths or layers where electrons stay while they move around the center. Electrons cannot just go anywhere; they must stay in these designated energy levels.
Teacher's Tip: Compare shells to the different lanes on a running track.
Exam Tip: Remember that shells are labeled with letters starting from K, L, M, and N.

Point 7: Atomic number refers to the number of protons present in an atom. It is denoted by the alphabet Z.
The atomic number is like the ID card or fingerprint of an element. If you change the number of protons, you change the element entirely.
Teacher's Tip: Just like every student has a unique roll number, every element has a unique atomic number.
Exam Tip: Always use the symbol Z to represent Atomic Number in your formulas.

Point 8: Mass number - Mass number is the sum of number of protons and neutrons present in the nucleus of an atom. Mass number (A) = Number of protons + Number of neutrons.
This number tells us how much the "heavy" part of the atom weighs. We ignore electrons here because they are too light to count toward the total mass.
Teacher's Tip: A = P + N is the easiest way to remember this formula.
Exam Tip: To find the number of neutrons, subtract the Atomic Number (Z) from the Mass Number (A).

Point 9: Atomic mass - The mass of an atom is known as its atomic mass.
Since atoms are too small to weigh on a regular scale, we use a special system to measure them. It represents the total matter present in a single atom.
Teacher's Tip: Atomic mass and mass number are very similar but not exactly the same thing in advanced science.
Exam Tip: Use the unit u or amu when writing the value of atomic mass.

Point 10: Relative atomic mass is the mass of an atom of an element as a multiple of the standard atomic mass unit.
This is a comparison tool used to see how heavy one atom is compared to a standard carbon atom. It helps scientists work with very large numbers of atoms more easily.
Teacher's Tip: It is like saying a bag of apples is twice as heavy as a bag of oranges.
Exam Tip: Relative atomic mass has no units because it is a comparison ratio.

Point 11: Atoms of different elements combine to form molecules so as to attain chemical stability.
Atoms don't like being alone if their outer shells aren't full, so they "team up" with others. This partnership makes them much more stable and less likely to react.
Teacher's Tip: Atoms are like social people; they are happier when they have friends to bond with.
Exam Tip: Explain that "stability" usually means having a full outermost shell of electrons.

Point 12: A positively charged ion is called a cation and a negatively charged ion is called anion.
Ions are created when an atom loses or gains electrons. If it loses an electron, it becomes positive; if it gains one, it becomes negative.
Teacher's Tip: Think of a "Cat-ion" as being "Paws-itive" (Positive).
Exam Tip: Be careful! Metals usually form cations while non-metals usually form anions.

Point 13: The valency of an element is the number of electrons donated or accepted by its ‘atom’ during chemical combination.
Valency is like the "combining power" or the number of hands an atom has to hold onto others. It determines how many other atoms it can link up with.
Teacher's Tip: Think of valency as the number of available seats on a bus.
Exam Tip: Valency is always a whole number and does not have a plus or minus sign.

Point 14: There are some elements with more than one valency e.g., iron, copper, tin, lead.
Certain elements can change how many bonds they form depending on the situation. This is called "variable valency" and happens mostly with heavy metals.
Teacher's Tip: Think of these elements as "multi-talented" workers who can do different jobs.
Exam Tip: When writing names for these, use Roman numerals like Iron (II) or Iron (III) to show which valency is used.

Point 15: Two or more different non-metals that collectively accept or donate one or more electrons and become negatively or positively charged in the process are called radicals.
Radicals are groups of atoms that act like a single team with a charge. They move together during chemical reactions as if they were just one particle.
Teacher's Tip: A radical is like a football team; many players, but they play as one unit.
Exam Tip: Common radicals include Sulphate (SO₄^2-}) and Nitrate (NO₃^-}).

EXERCISE

Question 1: Define the following terms :
1. Atom : An atom is the smallest indivisible unit of an element OR Atom is the smallest unit of matter.
2. Molecule : Molecule is the smallest unit of a compound (or an element) which always has an independent existence.
3. Radicals : A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.
4. Valency : It is the number of electrons donated or accepted by the valence shell of an atom during chemical combination.
5. Periodic table represents the tabular arrangement of elements in horizontal rows called periods and vertical columns called groups in order to classify the elements and their systematic study.
Answer: Verbatim as above.
These definitions form the foundation of chemistry. Understanding these terms helps you describe how matter is structured and how it interacts.
Teacher's Tip: Use flashcards to memorize these five key definitions perfectly.
Exam Tip: In the definition of an atom, always mention that it "exhibits the properties of that element."

Question 2: Write the names of the elements present in the following compounds.
1. Common salt : Sodium, chlorine.
2. Ammonia : Nitrogen, hydrogen.
3. Sulphuric acid : Hydrogen, sulphur, oxygen.
4. Glucose : Carbon, hydrogen, oxygen.
5. Sodium hydroxide : Sodium, oxygen, hydrogen.
6. Acetic acid : Carbon, hydrogen, oxygen.
Answer: Verbatim as above.
Compounds are made by combining different elements together. Even common things like sugar (glucose) or salt have specific chemical recipes.
Teacher's Tip: Most acids contain hydrogen, and "hydroxides" always contain oxygen and hydrogen together.
Exam Tip: Learn the chemical names along with the common names (like "sodium chloride" for common salt).

Question 3: What does each of the following represent ?
1. 2CO₂} = 2 molecules of carbon dioxide.
2. 2H₂}S = 2 molecules of hydrogen sulphide.
3. 5H₂SO₄} = 5 molecules of sulphuric acid.
4. 6NaNO₃} = 6 molecules of sodium nitrate.
Answer: Verbatim as above.
The big number in front tells you how many separate molecules there are. The small numbers below tell you how many atoms are inside each individual molecule.
Teacher's Tip: Think of the big number like the number of boxes, and the small numbers like the items inside each box.
Exam Tip: Never confuse the coefficient (big number) with the subscript (small number) during calculations.

Question 4: Write the symbols and valencies of the following radicals:
(a) Magnesium ion
(b) Ammonium
(c) Carbonate
(d) Nitrate
(e) Oxide
(f) Bisulphate
(g) Aluminium ion
Answer:
(a) Magnesium ion: Mg²⁺, Valency: 2
(b) Ammonium: NH₄⁺, Valency: 1
(c) Carbonate: CO₃^2-, Valency: 2
(d) Nitrate: NO₃^-, Valency: 1
(e) Oxide: O^2-, Valency: 2
(f) Bisulphate: HSO₄^-, Valency: 1
(g) Aluminium ion: Al³⁺, Valency: 3
Symbols use superscripts to show the charge, while the valency is just the number itself. These ions and radicals are the parts we use to build chemical formulas.
Teacher's Tip: Notice that metals like Magnesium and Aluminium have plus charges, while most non-metal groups have minus charges.
Exam Tip: When writing the symbol, the charge must be in the top-right corner, and the number of atoms in the bottom-right.

Question 5: Name the following radicals :
1. SO₄^2-
2. HCO₃^-
3. OH^-
4. Cr₂O₇^2-
Answer:
1. O₄^2- = Sulphate
2. HCO₃ = Bicarbonate
3. OH = Hydroxide
4. Cr₂O₇² = Dichromate
Recognizing these symbols is key to reading chemical names. Each specific combination of atoms has its own special "group name."
Teacher's Tip: The prefix "bi-" often means there is an extra hydrogen atom in the radical.
Exam Tip: Be very careful with the number of oxygens; SO₃ is different from SO₄.

Question 6: 1. Name one ion for each of the valencies +1, +2 and +3.
2. Name one ion for each of the valencies -1, -2 and -3.
Answer:
1. +1 = Sodium Na⁺
+2 = Calcium Ca²⁺
+3 = Aluminium Al3⁺
2. -1 = Chlorine Cl^-
-2 = Oxygen O^2-
-3 = Nitrogen N3-
Different elements have different abilities to lose or gain electrons. This determines the charge of the ion they will eventually form.
Teacher's Tip: Group 1 elements like Sodium are always +1, and Group 2 like Calcium are always +2.
Exam Tip: Always include the symbol and the charge when asked to "name an ion" for full marks.

Question 7: The valency of calcium is 2. Write the valencies of other radical in the following :
1. CaO
2. Ca(OH)₂
3. CaCO₃
4. CaCl₂
Answer:
1. O = 2
2. OH = 1
3. CO₃ = 2
4. Cl = 1
In a neutral compound, the total positive valency must equal the total negative valency. Since we know Calcium is 2, we can figure out the others by looking at the ratios.
Teacher's Tip: Think of it as a balancing scale; both sides must match to be stable.
Exam Tip: If there is a subscript on the other part (like the 2 in CaCl₂}), you must divide the total valency by that number to find the valency of a single ion.

Question 8: Write the names of the following compounds :
(a) (NH₄)₂SO₄
(b) Ca(NO₃)₂}
(c) FeS
(d) Na₃PO₄
(e) NH₄OH
(f) CuCO₃
(g) HgO
(h) ZnCl₂
(i) ZnS
(j) H₂S
Answer:
(a) Ammonium Sulphate
(b) Calcium Nitrate
(c) Iron Sulphide
(d) Sodium Phosphate
(e) Ammonium Hydroxide
(f) Copper Carbonate
(g) Mercuric Oxide
(h) Zinc Chloride
(i) Zinc Sulphide
(j) Hydrogen Sulphide
Naming compounds is like putting a first name and a last name together. The first part is usually the positive metal or radical, and the second is the negative one.
Teacher's Tip: When a compound ends in just one non-metal element, the name usually ends in "-ide."
Exam Tip: Practice writing these names neatly; spelling errors in chemical names can lead to point deductions.

Question 9: Write the molecular formulae of:
(a) Sodium sulphide
(b) Magnesium oxide
(c) Calcium hydroxide
(d) Hydrogen chloride
(e) Sulphuric acid
(f) Iron (II) sulphide
(g) Iron (III) sulphate
(h) Nitric acid
(i) Calcium phosphate
(j) Aluminium sulphate
(k) Magnesium nitride
Answer:
(a) Na₂S
(b) MgO
(c) Ca(OH)₂
(d) HCl
(e) H₂SO₄
(f) FeS
(g) Fe₂(SO₄)₃
(h) HNO₃
(i) Ca₃(PO₄)₂
(j) Al₂(SO₄)₃
(k) Mg₃N₂
To write these correctly, you use the "criss-cross" method with valencies. The valency of one part becomes the subscript for the other part.
Teacher's Tip: If the valencies are the same (like +2 and -2 in MgO), they cancel out and you don't write any subscripts.
Exam Tip: Use parentheses () whenever a radical needs a subscript larger than 1, like in Al₂(SO₄)₃.

Question 10: The valency of sodium is one, write the molecular formula for the following compounds of sodium.
1. sodium oxide : Na₂O
2. sodium sulphate : Na₂SO₄
3. sodium carbonate : Na₂CO₃
4. sodium hydroxide : NaOH
5. sodium nitrate : NaNO₃
Answer: Verbatim as above.
Since Sodium has a valency of 1, it needs two atoms to balance with a valency-2 radical like Oxygen or Sulphate. For valency-1 radicals like Hydroxide, only one Sodium is needed.
Teacher's Tip: Practice this set specifically, as Sodium is the most common metal used in Class 6 chemistry.
Exam Tip: Remember to write the symbol for Sodium as Na, not S.

Question 11: What is variable valency ? Give two examples of elements showing variable valency.
Answer: There are some elements with more than one valency. They are said to have variable valency, e.g. Iron, copper.
Some atoms are flexible and can bond in different ways depending on what they are reacting with. This leads to the formation of different compounds with different properties.
Teacher's Tip: Imagine a person who can hold either 2 or 3 bags at once depending on how heavy they are.
Exam Tip: For Iron, the common valencies are +2 (ferrous) and +3 (ferric).

Question 12: Give the group number of following elements present in periodic table
1. Magnesium : IIA
2. Carbon : IVA
3. Sulphur : VIA
4. Neon : Zero
Answer: Verbatim as above.
The group number tells you how many electrons are in the outer shell of an atom. Elements in the same group often share similar chemical personalities.
Teacher's Tip: Group "Zero" elements are the Noble Gases that don't like to react.
Exam Tip: Use Roman Numerals like IIA or VIA when writing group numbers in the periodic table.

Question 13: An element belongs to group VA. What would be its valency? Name two such elements.
Answer: Elements of group VA has valency 3. Two elements : Nitrogen and phosphorus.
Atoms in Group VA have 5 electrons in their outer shell, so they need 3 more to reach a stable 8. This need to gain 3 electrons gives them a valency of 3.
Teacher's Tip: To find the valency for groups V through VII, subtract the number of outer electrons from 8.
Exam Tip: Always link Nitrogen and Phosphorus as "cousins" in Group VA.

Question 14: An element belongs to group II. What would be its valency? Write the formula of molecules of compounds it will form with elements in VA, VIA and VIIA groups.
Answer: Valency 2.
Example using Magnesium (Mg²⁺):
With VA (N^3-): Mg₃N₂
With VIA (O^2-): MgO
With VIIA (Cl^-}): MgCl₂}
Group II elements have 2 electrons to give away, so their valency is 2. By criss-crossing this valency with others, we can predict exactly how they will combine.
Teacher's Tip: Criss-cross the charges: the '2' from Mg goes to the bottom of the other element.
Exam Tip: Show the "crossing" arrows in your rough work to ensure you don't make a mistake in the final formula.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks:
Answer:
1. Atoms are neutral.
2. An ion with positive charge is called cation.
3. An ion with negative charge is called anion.
4. 2H₂ means two atoms of hydrogen.
5. Ozone is a triatomic molecule.
6. Metals have variable valency.
7. Chemical name of caustic soda is sodium hydroxide NaOH.
Fill-in-the-blank questions test your quick recall of fundamental definitions. These facts are the basic building blocks for solving harder chemistry problems.
Teacher's Tip: Ozone is O₃}, which is why it is called "tri-atomic" (3 atoms).
Exam Tip: Always include the chemical formula (NaOH) if you know it, even in a fill-in-the-blank question.

2. Tick correct answer.

(a) The valency of iron in Fe₂O₃ is
1. 1
2. 2
3. 3
4. 6
Answer: 3. 3
In Fe₂O₃, the subscript '3' on the Oxygen comes from the valency of the Iron. This is the reverse of the criss-cross method used to find the valency of the metal.
Teacher's Tip: Just look at the number after the other element to find the metal's valency.
Exam Tip: Remember Fe₂O₃ is "Ferric Oxide" where iron has its higher valency of 3.

(b) Which of the following has valency 4 ?
1. aluminium
2. oxygen
3. carbon
4. phosphorus
Answer: 3. carbon
Carbon is in Group IV, meaning it has 4 electrons in its outer shell and can form 4 bonds. This high valency is why carbon can build so many complex structures in nature.
Teacher's Tip: Carbon is the "LEGO" piece of life because it has 4 connectors.
Exam Tip: Memorize that Silicon also has a valency of 4 because it is below carbon in the same group.

(c) The sulphate radical is written as SO₄^2-. What is the formula of calcium sulphate ?
1. Ca(SO₄)₂
2. Ca₂(SO₄)
3. Ca(SO₄)₃
4. CaSO₄
Answer: 4. CaSO₄
Calcium has a valency of +2 and Sulphate has a valency of -2. Since the numbers are the same, they perfectly balance each other in a 1-to-1 ratio.
Teacher's Tip: Think of it as a 2-for-2 trade; you only need one of each item to make it fair.
Exam Tip: Never write subscripts if the valencies are equal; just write the symbols next to each other.

(d) Which of the following exhibit variable valency ?
1. calcium
2. copper
3. carbon
4. chlorine
Answer: 2. copper
Copper is a transition metal that can have a valency of either +1 (cuprous) or +2 (cupric). Most simple elements in the first few groups have only one fixed valency.
Teacher's Tip: Transition metals in the middle of the periodic table are the ones that usually show variable valency.
Exam Tip: Iron and Copper are the most common examples of variable valency given in exams.

3. State the term for the following:
1. The number of atoms present in a molecule of an element atomicity.
2. The symbolic representation of a molecule molecular formula.
3. A group of atoms that react as a single unit molecule (Note: Radicals also fit this).
4. The combining capacity of an element valency.
5. The tabular arrangement of elements in horizontal rows and vertical columns periodic table.
Answer: Verbatim as above.
Technical terms allow scientists to communicate very complex ideas with just one word. Learning these is like learning the "language" of science.
Teacher's Tip: Think of "Atomicity" as "Atom-count."
Exam Tip: If the definition mentions "reacting as a single unit with a charge," use the term "Radical" instead of "Molecule."

ADDITIONAL QUESTIONS

1. Define the following terms:
1. Ions - An atom which becomes charged by losing or gaining electrons is called an ion.
2. Chemical bond - A chemical bond is the binding force between two or more atoms of a molecule.
3. Atomic number - Atomic number refers to the number of protons present in an atom. It is denoted by Z.
4. Mass number - Mass number is the sum of the number of protons and neutrons present in the nucleus of an atom. It is denoted by A.
5. Relative atomic mass - Relative atomic mass is the mass of an atom of an element as a multiple of the standard atomic mass unit.
Answer: Verbatim as above.
Ions and bonds are how atoms transform into the substances we see in the real world. Atomic numbers and mass numbers help us categorize these changes mathematically.
Teacher's Tip: A "Bond" is like a chemical "Glue" holding atoms together.
Exam Tip: Always mention the specific letters (Z and A) when defining atomic and mass numbers.

2. Answer the following questions.
1. What does an atom consist of ? Explain.
Answer: An atom consists protons, electrons and neutrons. Protons are positively charged, electrons have negative charge while neutrons have no charge.
The core of the atom holds the protons and neutrons, while the electrons orbit on the outside. This structure makes the atom mostly empty space with a very dense center.
Teacher's Tip: If an atom were as big as a stadium, the nucleus would be a small pea in the middle.
Exam Tip: To get full marks, describe where each particle is located (inside or outside the nucleus).

2. Give the charge and the mass of an electron, proton and neutron present in an atom.
Answer:
(i) Proton: Charge: Positive, Mass: One atomic mass unit (amu)
(ii) Electron: Charge: Negative, Mass: 1/1837 times the mass of one hydrogen atom.
(iii) Neutron: Charge: No charge, Mass: One atomic mass unit, equal to mass of proton.
Knowing these values allows us to calculate how much a specific atom weighs and how it will react electrically. Protons and neutrons contribute almost all the weight, while electrons handle the "social" reactions.
Teacher's Tip: Electrons are so light they are practically weightless in Class 6 math!
Exam Tip: Use a table to present this answer to make it very clear for the examiner.

3. Name two particles found inside the nucleus. What are they collectively called ?
Answer: Protons and Neutrons. Mass number.
These two heavy particles form the "nucleus team." Because they are the only ones with significant mass, their sum gives the atom its mass number.
Teacher's Tip: Sometimes they are also called "nucleons" because they live in the nucleus.
Exam Tip: Be sure to distinguish between the "particles" (protons/neutrons) and the "total count" (mass number).

4. (a) Name the particles which revolve around the nucleus.
(b) What are orbits or shells ?
(c) What is the maximum number of electrons present in:
1. first shell
2. second shell
3. third shell and
4. fourth shell
(d) The maximum number of electrons, an atom can have in the outermost shell.
Answer:
(a) Electron.
(b) The fixed circular paths along which electrons revolve round the nucleus.
(c) First Shell (K) = 2, Second Shell (L) = 8, Third Shell (M) = 18, Fourth (N) = 32
(d) 8
Atoms organize their electrons in layers called shells, following the 2n² rule. No matter how large the atom is, the very last shell can never hold more than 8 electrons.
Teacher's Tip: Use the formula 2n² where n is the shell number to calculate the max electrons.
Exam Tip: Remember the "Octet Rule": atoms are most stable when their outer shell has exactly 8 electrons.

5. Elements X and Y have 1 and 7 electrons in their outermost shell respectively.
1. Which element will lose electron ? X
2. Which element will gain electron ? Y
3. Which will form positive ion ? X
4. Which will form negative ion ? Y
5. What will be the charge present of the molecule XY after chemical combination ? electrically neutral
Answer: Verbatim as above.
Atoms with few outer electrons (1 to 3) prefer to give them away, while those with many (5 to 7) prefer to steal them to reach 8. When they combine, the loss and gain cancel out to make a neutral molecule.
Teacher's Tip: Giving away an electron makes you positive; it's like getting rid of a "negative" habit!
Exam Tip: Always state that a complete molecule is "electrically neutral" to show total charge is zero.

6. (a) What is the number of electrons donated by a magnesium atom and number of electrons accepted by a sulphur atom during their chemical combination?
(b) How many units of charge are developed on each atom ?
(c) What is the number of electrons after the transfer of electrons in magnesium and sulphur atoms.
(d) Diagrammatically represent the transfer of electrons.
Answer:
(a) Two, Two
(b) Two
(c) Eight
(d) Magnesium (Mg) shows two dots in outer shell moving to Sulphur (S) which has six dots.
Magnesium gives its 2 outer electrons to Sulphur, which only needs 2 more to finish its set of 8. This transfer creates an ionic bond between the two atoms.
Teacher's Tip: This is like a friend giving you the last two pieces of a puzzle so your set is complete.
Exam Tip: When drawing electron dot diagrams, use 'x' for one atom's electrons and '.' for the other's to make the transfer visible.

3. What do you understand by the following terms valency, valence electrons and valence shell ?
1. Valency - The capacity of the atom of an element to form chemical bonds.
2. Valence electrons - The electrons present in the valence shell are called valence electrons.
3. Valence shell - The outer most shell of an atom is known as its valence shell.
Answer: Verbatim as above.
These three "valence" terms all describe the outermost part of the atom where all the chemical action happens. The inner electrons stay protected, but the outer ones are what interact with the world.
Teacher's Tip: Think of the valence shell as the "surface" or "face" of the atom.
Exam Tip: Do not confuse valence electrons (the total count) with valency (the number of bonds it can form).

4. Name the following radicals and give their valencies :
1. Sulphate SO₄ - Valency: 2
2. Carbonate CO₃ - Valency: 2
3. Hydroxide OH - Valency: 1
4. Dichromate Cr₂O₇ - Valency: 2
Answer: Verbatim as above.
Radicals are special groups of atoms that stay together as a single unit during chemical reactions. They have their own set valency just like individual elements do.
Teacher's Tip: Most common radicals you'll study in Class 6 have a valency of 1 or 2.
Exam Tip: Memorize the valency and the formula together; if you forget the subscript, the valency might change.

5. (i) What is variable valency ? (ii) Give three examples of elements having variable valency.
Answer:
(i) Variable valency - There are some elements with more than one valency. They are said to have variable valency.
(ii) e.g., iron, copper, tin, lead
Because of how their electrons are arranged, these atoms can sometimes use electrons from an inner shell to make extra bonds. This results in different "versions" of the element that can react differently.
Teacher's Tip: Think of a multi-tool that can act as a screwdriver or a wrench.
Exam Tip: If asked for three, Copper, Iron, and Lead are the easiest to remember.

6. (i) Name three elements each whose ions have valency +1, +2 and +3 respectively. (ii) Name three non-metals/radicals which have valency -1, -2, and -3.
Answer:
1. +1 = Hydrogen, Sodium, Potassium.
+2 = Calcium, Magnesium, Zinc.
+3 = Aluminium, Chromium, Ferric [Fe (III)]
2. -1 = Chlorine, Bromine, Iodine.
-2 = Oxide, Sulphide, Sulphate.
-3 = Nitrogen, Nitride Phosphate.
Grouping elements by their valency is a common way to organize chemistry labs. Knowing these lists helps you quickly solve formulas during a test.
Teacher's Tip: Elements in the same column of the periodic table will usually have the same valency.
Exam Tip: Be careful with "-ide" and "-ate" endings; they refer to different types of ions and radicals.

7. Define: (a) Ionic bond (b) Covalent bond
Answer:
(a) Ionic bond - A bond formed by transfer of electrons between atoms is called ionic bond.
(b) Covalent bond - When atoms of different non-metals neither donate nor accept electrons and hence no ions are formed, such a bond is called covalent bond.
In an ionic bond, one atom "gives" and the other "takes." In a covalent bond, both atoms "share" their electrons equally to stay stable.
Teacher's Tip: Ionic is "giving/taking," Covalent is "sharing."
Exam Tip: Mention that ionic bonds happen between metals and non-metals, while covalent bonds happen between non-metals.

8. Give two examples for each of ionic and covalent bond.
Answer:
Example of ionic bonds - Sodium chloride and calcium oxide.
Example of covalent bond - Water, carbon dioxide.
Ionic bonds are usually strong and form crystals like salt. Covalent bonds form discrete molecules like the ones in the air we breathe.
Teacher's Tip: Most things that dissolve well in water and conduct electricity are ionic.
Exam Tip: Sodium Chloride (NaCl) and Water (H₂O) are the standard examples for these two bond types.

9. Write electronic configuration of oxygen, fluorine, sodium, silicon, argon.
Answer:
Element | At. Number | Electronic configuration (K L M N)
1. Oxygen | 8 | 2 6
2. Fluorine | 9 | 2 7
3. Sodium | 11 | 2 8 1
4. Silicon | 14 | 2 8 4
5. Argon | 18 | 2 8 8
Electronic configuration is the map of where electrons live in an atom. We fill the inner shell (K) with 2 electrons first before moving to the L and M shells.
Teacher's Tip: The K shell is the "VIP" area - it only has room for 2 electrons.
Exam Tip: Always make sure your total electron count (2+8+...) equals the Atomic Number of the element.

10. Who originated the word ‘atom’ ?
Answer: Democritus
Democritus was an ancient Greek philosopher who imagined that if you kept cutting matter, eventually you'd hit a piece you couldn't cut anymore. He named this piece 'atomos'.
Teacher's Tip: History and science often go hand-in-hand; he thought of this over 2000 years ago!
Exam Tip: Be sure to spell the name correctly: D-e-m-o-c-r-i-t-u-s.

11. What does the word ‘atom’ mean ?
Answer: “atom” means “indivisible”
It comes from the Greek word 'atomos', where 'a-' means 'not' and '-tomos' means 'cut'. So, it literally means something that cannot be cut further.
Teacher's Tip: Even though we now know we can split atoms, we still use this name to honor history.
Exam Tip: Use the word "indivisible" exactly as it is written in the textbook for full marks.

12. State Dalton’s atomic theory.
Answer:
1. Matter is made up of tiny particles called atoms
2. Atoms are indivisible.
3. Atom can be neither be created nor destroyed.
4. Atoms of same element are identical in mass and properties
5. Atoms of different elements differ in mass and properties
John Dalton was the first to give us a modern framework for understanding how chemistry works. His theory explained why substances react in fixed proportions.
Teacher's Tip: Think of Dalton's atoms like solid billiard balls.
Exam Tip: Try to memorize at least 4 of these 5 points to answer long-form questions effectively.

13. How did Niels Bohr visualise the atom ?
Answer: Niels Bohr explained that atom is like a solar system with an empty space with a minute central nucleus, like Sun and electrons like planets revolve around it.
This model was revolutionary because it explained why electrons don't just crash into the center. They stay in their orbits just like Earth stays in its path around the Sun.
Teacher's Tip: Imagine the nucleus as the "Sun" and the electron shells as the "Planetary Orbits."
Exam Tip: Use the term "Planetary Model" if you need to describe Bohr's visualization in one phrase.

14. Name the particle or particles you would expect to find in :
1. The orbit of an atom : electrons
2. The nucleus of an atom : protons and neutrons
Answer: Verbatim as above.
Every atom is like a house with two main sections: the yard (orbits) and the inner rooms (nucleus). Different particles have their specific permanent "rooms."
Teacher's Tip: If you are outside the nucleus, you will only see electrons.
Exam Tip: Never forget that the nucleus has TWO types of particles, not just one.

15. An atom has 13 protons and 14 neutrons.
1. What is the atomic number of the atom ? 13
2. How many electrons would be present in this atom ? 13
3. What is the atomic mass of the atom ? 27
Answer: Verbatim as above.
The atomic number is just the proton count (13). In a neutral atom, protons and electrons must match (13), and the total mass is the sum of center particles (13+14=27).
Teacher's Tip: This specific atom is Aluminium! You can check that by looking up number 13 on a periodic table.
Exam Tip: Always show your addition (13 + 14 = 27) so the teacher can see your logic.

16. Define :
1. An Atom: Smallest particle of an element that can exist and have properties of an element.
2. Atomic number : Number of protons present in the nucleus of an atom.
3. Mass number : Number of protons and neutrons present in the nucleus of an atom.
4. Atomic weight : It is the ratio that tells how many times an atom of an element is heavier than atom of Hydrogen.
Answer: Verbatim as above.
These technical definitions are slightly different versions of the ones we saw earlier. They help provide a more formal scientific perspective on the structure of atoms.
Teacher's Tip: "Atomic Weight" is a comparison, so think of it like a seesaw with Hydrogen on the other side.
Exam Tip: When defining Mass Number, be sure to mention that it's specifically for particles inside the "nucleus."

17. A certain particle ‘X’ has 17 protons, 18 electrons and 20 neutrons.
1. What is such a particle called? anion
2. What is the mass number of X? 35
3. What is the atomic number of X? 17
4. Does the particle have an excess of electrons or a deficiency of electrons? excess of electrons
5. What is the charge on the particle? -1 (negative)
Answer: Verbatim as above.
Since there is one more electron than protons (18 vs 17), the particle is not neutral anymore. This extra negative charge transforms the atom into an ion, specifically an anion.
Teacher's Tip: Proton count (17) = Atomic Number (17). Protons + Neutrons (17+20) = Mass Number (35).
Exam Tip: If electrons are MORE than protons, it's negative. If they are LESS, it's positive.

18. Fill in the blanks :
Answer:
1. If an atom gains an electron, it is called an anion.
2. If an atom loses an electron, it acquires positive a charge.
3. A charged particle is called an ion.
4. An atom is electrically neutral.
5. An excess of electrons produces a negative charge.
6. A deficiency of electrons produces a positive charge.
Atoms want to be neutral, but they will become charged ions to find stability. Losing "negative" electrons makes them positive, while gaining them makes them negative.
Teacher's Tip: Think of it as a bank account; losing debt (negative) makes you richer (positive)!
Exam Tip: Memorize that "excess" = "more" and "deficiency" = "less."