Class 9 - Chapter 9: Atomic Foundations of Matter

Exercise Solution

1. Element A and Element B

A has 1 electron in the third shell → Electronic configuration = 2, 8, 1.

B has 6 electrons in the second shell → Electronic configuration = 2, 6.

(i) How many electrons does A tend to give or take?

A has 1 valence electron and tends to lose 1 electron to attain a stable octet.

Answer: A gives 1 electron.

(ii) What kind of ion would it form?

After losing 1 electron, A forms a positive ion.

A → A⁺ + e⁻

Answer: Cation (A⁺)

(iii) How many electrons does B tend to give or take?

B requires 2 electrons to complete its octet.

Answer: B takes 2 electrons.

(iv) What kind of ion would it form?

B + 2e⁻ → B²⁻

Answer: Anion (B²⁻)

(v) What kind of bond would be formed?

Electrons are transferred from A to B.

Answer: Ionic (electrovalent) bond.

(vi) What would be the formula of the compound formed?

Two A⁺ ions combine with one B²⁻ ion.

Formula: A₂B

2. Element X with six valence electrons

(i) Why does it form a diatomic molecule?

X requires two electrons to complete its octet. Two atoms share two pairs of electrons and attain stability.

(ii) What kind of bond would it form?

Answer: Covalent bond.

(iii) Structure of the molecule

X = Oxygen

Molecule formed = O₂

O = O

(iv) Molecule formed with element Y having two electrons in first shell

Y = Helium (2)

Helium has a complete duplet and generally does not combine with oxygen.

Answer: No stable compound is formed.

3. Correct combination of ions

Total positive charge = +6

Total negative charge = −6

(i) 2Al³⁺ and 3Cl⁻

Positive charge = +6

Negative charge = −3

Not correct.

(ii) 3Mg²⁺ and 1PO₄³⁻

Positive charge = +6

Negative charge = −3

Not correct.

(iii) 2Fe³⁺ and 3O²⁻

Positive charge = +6

Negative charge = −6

Correct

(iv) 3Ca²⁺ and 2SO₄²⁻

Positive charge = +6

Negative charge = −4

Not correct.

Answer: Option (iii)

4. Correct and false statements

(i)

"Elements are made up of molecules and compounds are made up of atoms."

False

Correct statement: Elements are made up of atoms or molecules, whereas compounds are made up of molecules formed by different kinds of atoms.

(ii)

"A molecule of a compound is always made up of two or more atoms of the same kind."

False

Correct statement: A molecule of a compound consists of two or more atoms of different elements chemically combined.

(iii)

"One molecule of nitrogen gas contains three nitrogen atoms."

False

Correct statement: One molecule of nitrogen gas contains two nitrogen atoms (N₂).

(iv)

"Water is made of two hydrogen atoms covalently bonded with one oxygen atom."

True

5. Chemical Formulae

(i) Aluminium nitrate

Al³⁺ + NO₃⁻

Formula: Al(NO₃)₃

(ii) Calcium oxide

Ca²⁺ + O²⁻

Formula: CaO

(iii) Ferric oxide

Fe³⁺ + O²⁻

Formula: Fe₂O₃

6. Formulae from ions

(i) Ca²⁺ and Br⁻

Formula: CaBr₂

(ii) Al³⁺ and CO₃²⁻

Formula: Al₂(CO₃)₃

(iii) K⁺ and SO₄²⁻

Formula: K₂SO₄

(iv) NH₄⁺ and Cl⁻

Formula: NH₄Cl

7. Correct representation of Cl⁻ ion

Atomic number of chlorine = 17

Electronic configuration of Cl = 2, 8, 7

Electronic configuration of Cl⁻ = 2, 8, 8

Answer: Figure (ii)

8. Formula Unit Mass

(i) NH₄NO₃

= 2 × 14 + 4 × 1 + 3 × 16

= 28 + 4 + 48

= 80 u

(ii) H₃PO₄

= 3 × 1 + 31 + 4 × 16

= 3 + 31 + 64

= 98 u

(iii) NaHCO₃

= 23 + 1 + 12 + 3 × 16

= 23 + 1 + 12 + 48

= 84 u

9. Formulae of compounds formed

(i) Magnesium and nitrogen

Mg²⁺ and N³⁻

Formula: Mg₃N₂

(ii) Lithium and nitrogen

Li⁺ and N³⁻

Formula: Li₃N

(iii) Sodium and sulfur

Na⁺ and S²⁻

Formula: Na₂S

(iv) Aluminium and oxygen

Al³⁺ and O²⁻

Formula: Al₂O₃

10. Table 9.3

11. Verification of Law of Conservation of Mass

Mass of reactants

= 5.3 g + 6.0 g

= 11.3 g

Mass of products

= 2.2 g + 0.9 g + 8.2 g

= 11.3 g

Since mass of reactants = mass of products,

Law of Conservation of Mass is verified.

12. Species having 11 protons, 12 neutrons and 10 electrons

(i) Atomic number

= Number of protons

= 11

(ii) Mass number

= Protons + Neutrons

= 11 + 12

= 23

(iii) Neutral, cation or anion?

Protons = 11

Electrons = 10

Net charge = +1

Therefore it is a cation.

(iv) Electronic configuration

10 electrons

Configuration = 2, 8

(v) Name of species

Atomic number 11 corresponds to Sodium.

Since it has lost one electron:

Species = Na⁺ (Sodium ion)

13. Elements A and B

A : 2, 8, 5

B : 2, 8, 7

(i) Which element is more reactive?

B needs only one electron to complete its octet.

Answer: B is more reactive.

(ii) Ionic or covalent bond?

Both are non-metals and gain electrons.

They combine by sharing electrons.

Answer: Covalent bond.

(iii) Formula of compound formed

A has valency 3.

B has valency 1.

Formula: AB₃

14. Assertion–Reason

Assertion (A): Copper sulphate conducts electricity in molten state but not in solid state.

Reason (R): Copper sulphate ions are fixed in solid state but move freely in molten state.

Both statements are true and the reason correctly explains the assertion.

Answer: (i)

15. Electrons and neutrons

(i) ²⁷Al

Atomic number = 13

Electrons = 13

Neutrons = 27 − 13 = 14

(ii) ⁸⁰Br

Atomic number = 35

Electrons = 35

Neutrons = 80 − 35 = 45

(iii) ²⁰¹Hg

Atomic number = 80

Electrons = 80

Neutrons = 201 − 80 = 121