Class 9 Science

Chapter 9 Solutions — Atomic Foundations of Matter

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Solutions

Overview

Step-by-step NCERT solutions for Atomic Foundations of Matter (Chapter 9, NCERT Class 9 Science) — every question and answer worked out in full, not just the final result. You can also read the Atomic Foundations of Matter textbook chapter.

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What these solutions cover

All 15 questions in Atomic Foundations of Matter are solved in the PDF. Here's what's inside, exercise by exercise:

Revise, Reflect, Refine

1.
A particular element (A) has one electron in its third shell. There is another element (B) with six electrons in its second shell.
- (i) How many electrons does A tend to give or take to become stable?
- (ii) What kind of ion would it form?
- (iii) How many electrons does B tend to give or take to become stable?
- (iv) What kind of ion would it form?
- (v) If A and B were to combine, what kind of bond…
2.
An element X has six electrons in its outer shell and forms a diatomic molecule.
- (i) Why would that be so?
- (ii) What kind of bond would it form?
- (iii) Draw the structure of the molecule it would form.
- (iv) A certain other element Y has two electrons in its second shell. Draw the structure of the molecule that X would form with Y.
3.
You want to design a new ionic compound where the total positive charge is 6+ and the total negative charge is 6−. Which of the following combinations gives the correct number of ions?
- (i) 2 Al³⁺ and 3 Cl⁻
- (ii) 3 Mg²⁺ and 1 PO₄³⁻
- (iii) 2 Fe³⁺ and 3 O²⁻
- (iv) 3 Ca²⁺ and 2 SO₄²⁻
4.
Choose the correct statement(s) and correct the false statement(s).
- (i) Elements are made up of molecules and compounds are made up of atoms.
- (ii) The molecule of a compound is always made up of two or more atoms of the same kind.
- (iii) One molecule of nitrogen gas contains three nitrogen atoms.
- (iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.
5.
Write the chemical formulae for the following compounds.
- (i) Aluminium nitrate
- (ii) Calcium oxide
- (iii) Ferric oxide
6.
Write the formulae of the compounds formed from the following pairs of ions.
- (i) Ca²⁺ and Br⁻
- (ii) Al³⁺ and CO₃²⁻
- (iii) K⁺ and SO₄²⁻
- (iv) NH₄⁺ and Cl⁻
Which of the following, in Fig. 9.18, correctly represents Cl⁻ ion (Atomic number of chlorine = 17)?
8.
Determine the formula unit mass of the following substances.
- (i) Ammonium nitrate (NH₄NO₃), used as a nitrogen fertiliser, which is essential for plant growth.
- (ii) Phosphoric acid (H₃PO₄), used to make phosphate fertiliser and detergents.
- (iii) Sodium hydrogencarbonate (NaHCO₃), used to relieve acidity and helps in digestion.
9.
Write the formulae for the compounds formed by the reaction of:
- (i) Magnesium and nitrogen
- (ii) Lithium and nitrogen
- (iii) Sodium and sulfur
- (iv) Aluminium and oxygen
Complete Table 9.3 by writing the formulae of the compounds formed by the cations on the left and the anions at the top. LiNO₃ is given as an example. Cations: NH₄⁺, Li⁺ (example: LiNO₃), Al³⁺, Cu²⁺ Anions: NO₃⁻, SO₄²⁻, PO₄³⁻
5.3 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.
12.
If a species has 11 protons, 12 neutrons and 10 electrons then:
- (i) what is its atomic number and mass number?
- (ii) is it neutral, a cation or an anion? Explain.
- (iii) write its electronic configuration.
- (iv) name the species.
13.
Two elements, A and B, have the following configurations — A: 2, 8, 5 and B: 2, 8, 7.
- (i) Which element is more reactive?
- (ii) Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing.
- (iii) Predict the formula of the compound they would form.
14.
Assertion (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. Choose the correct option:
- (i) Both A and R are true, and R is the correct explanation of A.
- (ii) Both A and R are true, but R is not the correct explanation of A.
- (iii) A is…
The species ²⁷Al, ⁸⁰Br⁻ and ²⁰¹Hg²⁺ have 13, 35 and 80 protons, respectively. How many electrons and neutrons do they have?

Sample

Worked examples

3 questions from Atomic Foundations of Matter, solved step by step.

Revise, Reflect, Refine

You want to design a new ionic compound where the total positive charge is 6+ and the total negative charge is 6−. Which of the following combinations gives the correct number of ions? (i) 2 Al³⁺ and 3 Cl⁻ (ii) 3 Mg²⁺ and 1 PO₄³⁻ (iii) 2 Fe³⁺ and 3 O²⁻ (iv) 3 Ca²⁺ and 2 SO₄²⁻

A balanced ionic compound requires total positive charge = total negative charge in magnitude.

(i) 2 Al³⁺ and 3 Cl⁻: Positive charge = 2 × 3 = 6+. Negative charge = 3 × 1 = 3−. 6+ ≠ 3−. Not balanced.

(ii) 3 Mg²⁺ and 1 PO₄³⁻: Positive charge = 3 × 2 = 6+. Negative charge = 1 × 3 = 3−. 6+ ≠ 3−. Not balanced.

(iii) 2 Fe³⁺ and 3 O²⁻: Positive charge = 2 × 3 = 6+. Negative charge = 3 × 2 = 6−. 6+ = 6−. Balanced. ✓

(iv) 3 Ca²⁺ and 2 SO₄²⁻: Positive charge = 3 × 2 = 6+. Negative charge = 2 × 2 = 4−. 6+ ≠ 4−. Not balanced.

Only option (iii) gives total positive charge = 6+ and total negative charge = 6−.

Answer: (iii) 2 Fe³⁺ and 3 O²⁻. Total positive charge = 2 × 3 = 6+; total negative charge = 3 × 2 = 6−. Charges balance.

Revise, Reflect, Refine

Write the chemical formulae for the following compounds. (i) Aluminium nitrate (ii) Calcium oxide (iii) Ferric oxide

(i) Aluminium nitrate: Al³⁺ and NO₃⁻ (nitrate ion, charge −1).

Cross-multiply valencies: Al³⁺ × 1 and (NO₃)⁻ × 3.

Formula: Al(NO₃)₃

(ii) Calcium oxide: Ca²⁺ and O²⁻.

Both have charge magnitude 2; they combine in a 1:1 ratio.

Formula: CaO

(iii) Ferric oxide: Fe³⁺ (ferric = Fe³⁺) and O²⁻.

Cross-multiply valencies: Fe × 2 and O × 3 (to balance 2 × 3 = 6+ and 3 × 2 = 6−).

Formula: Fe₂O₃

Answer: (i) Al(NO₃)₃ (ii) CaO (iii) Fe₂O₃

Revise, Reflect, Refine

Write the formulae of the compounds formed from the following pairs of ions. (i) Ca²⁺ and Br⁻ (ii) Al³⁺ and CO₃²⁻ (iii) K⁺ and SO₄²⁻ (iv) NH₄⁺ and Cl⁻

(i) Ca²⁺ and Br⁻: Cross-multiply — Ca × 1, Br × 2. Verify: 1 × 2 = 2+ and 2 × 1 = 2−. Balanced.

Formula: CaBr₂

(ii) Al³⁺ and CO₃²⁻: Cross-multiply — Al × 2, CO₃ × 3. Verify: 2 × 3 = 6+ and 3 × 2 = 6−. Balanced.

Formula: Al₂(CO₃)₃

(iii) K⁺ and SO₄²⁻: Cross-multiply — K × 2, SO₄ × 1. Verify: 2 × 1 = 2+ and 1 × 2 = 2−. Balanced.

Formula: K₂SO₄

(iv) NH₄⁺ and Cl⁻: Both have charge magnitude 1; combine in 1:1 ratio.

Formula: NH₄Cl

Answer: (i) CaBr₂ (ii) Al₂(CO₃)₃ (iii) K₂SO₄ (iv) NH₄Cl

All 15 questions in Atomic Foundations of Matter are solved step by step in the Solutions PDF above.

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Chapter 9 Solutions — Atomic Foundations of Matter

Overview

What these solutions cover

Revise, Reflect, Refine

Worked examples

Write the chemical formulae for the following compounds. (i) Aluminium nitrate (ii) Calcium oxide (iii) Ferric oxide

Write the formulae of the compounds formed from the following pairs of ions. (i) Ca²⁺ and Br⁻ (ii) Al³⁺ and CO₃²⁻ (iii) K⁺ and SO₄²⁻ (iv) NH₄⁺ and Cl⁻

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