Class 9 Science · Chapter 9

9. Atomic Foundations of Matter

Complete solutions to Revise, Reflect, Refine. CBSE Class 9 Science (NCERT Exploration).

Revise, Reflect, Refine

1Element A has one electron in its third shell. Element B has six electrons in its second shell.

A has configuration 2, 8, 1 (sodium). B has configuration 2, 6 (oxygen).

(i) How many electrons does A tend to give or take to become stable? — It tends to give 1 electron.

(ii) What kind of ion would it form? — Cation (A⁺)

(iii) How many electrons does B tend to give or take to become stable? — It tends to take 2 electrons.

(iv) What kind of ion would it form? — Anion (B²⁻)

(v) If A and B combine, what kind of bond would be formed? — Ionic bond

(vi) What would be the formula of the compound? — A₂B

Two A atoms give one electron each to one B atom (like Na₂O).

2An element X has six electrons in its outer shell and forms a diatomic molecule.

(i) Why would that be so?

X needs 2 more electrons to complete its octet. Two X atoms share two pairs of electrons with each other. In this way both get an octet and become stable. So X forms a diatomic molecule X₂ (like O₂).

(ii) What kind of bond would it form?

Covalent bond (a double bond)

(iii) and (iv) Draw the structure of the molecule X₂. Element Y has two electrons in its second shell; draw the structure that X would form with Y.

Taking X as oxygen: Y has configuration 2, 2 (beryllium). Y gives its 2 outer electrons to X. Then both get a stable configuration. They form YX (BeO).

(iii) X₂ (O₂): two shared pairs form a double covalent bond.

(iv) YX (BeO). Red dots show the electrons received from Be.

3You want to design an ionic compound where the total positive charge is 6+ and the total negative charge is 6−. Which combination gives the correct number of ions?

Answer: (iii) 2 Fe³⁺ and 3 O²⁻

2 × (+3) = +6 and 3 × (−2) = −6

In the other options, the positive and negative charges are not equal.

4Choose the correct statement(s) and correct the false statement(s).

(i) False. Correct: Elements are made up of only one kind of atoms. Compounds are made up of two or more kinds of atoms combined chemically.
(ii) False. Correct: The molecule of a compound is always made up of two or more atoms of different kinds.
(iii) False. Correct: One molecule of nitrogen gas (N₂) contains two nitrogen atoms.
(iv) True. Water (H₂O) has two hydrogen atoms covalently bonded to one oxygen atom.

5Write the chemical formulae for the following compounds.

(i) Aluminium nitrate: Al³⁺ and NO₃⁻ → Al(NO₃)₃

(ii) Calcium oxide: Ca²⁺ and O²⁻ → CaO

(iii) Ferric oxide: Fe³⁺ and O²⁻ → Fe₂O₃

6Write the formulae of the compounds formed from the following pairs of ions.

(i) Ca²⁺ and Br⁻ → CaBr₂

(ii) Al³⁺ and CO₃²⁻ → Al₂(CO₃)₃

(iii) K⁺ and SO₄²⁻ → K₂SO₄

(iv) NH₄⁺ and Cl⁻ → NH₄Cl

7Which of the following, in Fig. 9.18, correctly represents Cl⁻ ion (atomic number of chlorine = 17)?

Answer: (ii)

A chlorine atom (2, 8, 7) gains 1 electron to form Cl⁻. So Cl⁻ has 18 electrons arranged as 2, 8, 8. Figure (ii) shows this arrangement.

8Determine the formula unit mass of the following substances. (H = 1, C = 12, N = 14, O = 16, Na = 23, P = 31 u)

(i) NH₄NO₃ = 14 + (4 × 1) + 14 + (3 × 16) = 80 u
(ii) H₃PO₄ = (3 × 1) + 31 + (4 × 16) = 98 u
(iii) NaHCO₃ = 23 + 1 + 12 + (3 × 16) = 84 u

9Write the formulae for the compounds formed by the reaction of:

(i) Magnesium and nitrogen (Mg²⁺, N³⁻) → Mg₃N₂

(ii) Lithium and nitrogen (Li⁺, N³⁻) → Li₃N

(iii) Sodium and sulfur (Na⁺, S²⁻) → Na₂S

(iv) Aluminium and oxygen (Al³⁺, O²⁻) → Al₂O₃

10Complete Table 9.3 by writing the formulae of the compounds formed by the cations on the left and the anions at the top.

Cation	NO₃⁻	SO₄²⁻	PO₄³⁻
NH₄⁺	NH₄NO₃	(NH₄)₂SO₄	(NH₄)₃PO₄
Li⁺	LiNO₃	Li₂SO₄	Li₃PO₄
Al³⁺	Al(NO₃)₃	Al₂(SO₄)₃	AlPO₄
Cu²⁺	Cu(NO₃)₂	CuSO₄	Cu₃(PO₄)₂

115.3 g of sodium carbonate and 6.0 g of acetic acid react to produce 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium acetate. Verify whether the law of conservation of mass is valid.

Mass of reactants = 5.3 + 6.0 = 11.3 g
Mass of products = 2.2 + 0.9 + 8.2 = 11.3 g

The total mass of reactants equals the total mass of products. So the law of conservation of mass is valid.

12A species has 11 protons, 12 neutrons and 10 electrons.

(i) Atomic number = 11; Mass number = 11 + 12 = 23

(ii) It is a cation. It has 11 positive protons but only 10 electrons. So it has one extra positive charge.

(iii) Electronic configuration: 2, 8

(iv) It is the sodium ion (Na⁺)

13Two elements A (2, 8, 5) and B (2, 8, 7). (i) Which is more reactive? (ii) Will they form ionic or covalent bonds? (iii) Predict the formula of the compound.

(i) Element B is more reactive. It needs only 1 electron to complete its octet. A needs 3 electrons.

(ii) Covalent bonds. Both are non-metals and both need electrons. So they share electrons instead of transferring them. A shares 3 electrons, one with each of three B atoms.

(iii) Formula: AB₃ (like PCl₃)

14Assertion (A): Copper sulfate conducts electricity in the molten state but not in the solid state. Reason (R): Copper and sulfate ions are fixed in the lattice in molten state, while in solid state they can move freely.

Answer: (iii) A is true, but R is false.

The reason is reversed. In the solid state, ions are fixed in the lattice. In the molten state, they move freely and carry current.

15The species ²⁷Al, ⁸⁰Br⁻ and ²⁰¹Hg²⁺ have 13, 35 and 80 protons respectively. How many electrons and neutrons do they have?

Species	Electrons	Neutrons
²⁷Al	13 (neutral atom)	27 − 13 = 14
⁸⁰Br⁻	35 + 1 = 36	80 − 35 = 45
²⁰¹Hg²⁺	80 − 2 = 78	201 − 80 = 121

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