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Atomic Foundations of Matter – Class 9 Science notes, important questions and MCQs

NCERT Class 9 Science, chapter 9. Notes, key terms, important questions with answers and MCQs.

Chapter overview

All matter is made of tiny particles — atoms and molecules. Chemical formulas, molecular mass and the mole help us count particles and compare amounts.

Laws and atoms

Law of conservation of mass (Lavoisier) and law of constant proportions (Proust).
Dalton's atomic theory: matter is made of indivisible atoms; atoms of an element are identical.
Atomic mass is measured in u (1/12 of a carbon-12 atom).

Molecules, ions and formulas

A molecule is the smallest particle of a substance that can exist independently (O₂, H₂O).
Ions are charged particles: cations (Na⁺) and anions (Cl⁻); polyatomic ions: SO₄²⁻, NO₃⁻, NH₄⁺.
Write formulas by crossing valencies: Al³⁺ and O²⁻ → Al₂O₃.

Mole concept

1 mole = 6.022 × 10²³ particles (Avogadro number).
Molar mass = atomic or molecular mass in grams.
Number of moles = given mass / molar mass.

Key terms

Atomicity: number of atoms in a molecule (O₂ = 2, O₃ = 3)
Molecular mass: sum of atomic masses of all atoms in a molecule
Mole: amount containing 6.022 × 10²³ particles

Formulas

n = m / M
Particles = n × 6.022 × 10²³
Molecular mass of H₂O = 2(1) + 16 = 18 u

Important questions with answers

Find the molecular mass of H₂SO₄ (H = 1, S = 32, O = 16).2 marks

2(1) + 32 + 4(16) = 2 + 32 + 64 = 98 u.

Write the chemical formulae of aluminium oxide, calcium chloride and sodium carbonate using valencies.3 marks

Al³⁺ and O²⁻ give Al₂O₃; Ca²⁺ and Cl⁻ give CaCl₂; Na⁺ and CO₃²⁻ give Na₂CO₃.

Define the mole. Find (i) the number of moles in 52 g of helium, (ii) the mass of 0.5 mole of water, and (iii) the number of molecules in 8 g of oxygen gas (He = 4, H = 1, O = 16).5 marks

A mole is the amount of substance containing 6.022 × 10²³ particles (Avogadro number). (i) 52/4 = 13 mol. (ii) Molar mass of H₂O = 18 g, so 0.5 × 18 = 9 g. (iii) O₂ has molar mass 32 g; 8 g = 0.25 mol, so molecules = 0.25 × 6.022 × 10²³ = 1.5055 × 10²³.

Quick revision

1 mole = 6.022 × 10²³.
Molar mass of water = 18 g.
Constant proportions: water always has H : O = 1 : 8 by mass.

Exam tips

Write the valency above each symbol and cross it to get the formula.
Remember atomic masses: H 1, C 12, N 14, O 16, Na 23, Cl 35.5.

Practice MCQs

Avogadro number is
1. 9.8
2. 3 × 10⁸
3. 6.022 × 10²³
4. 1.6 × 10⁻¹⁹
Show answer
Answer: (c) 6.022 × 10²³. Particles in one mole.
Molecular mass of CO₂ is
1. 28 u
2. 32 u
3. 44 u
4. 12 u
Show answer
Answer: (c) 44 u. 12 + 2 × 16.
The atomicity of ozone (O₃) is
1. 1
2. 2
3. 3
4. 4
Show answer
Answer: (c) 3. Three atoms.
Number of moles in 36 g of water is
1. 1
2. 2
3. 3
4. 18
Show answer
Answer: (b) 2. 36/18.
The formula of aluminium oxide is
1. Al₂O₃
2. AlO
3. Al₃O₂
4. AlO₃
Show answer
Answer: (a) Al₂O₃. Valencies 3 and 2 crossed.
Who stated the law of constant proportions?
1. Lavoisier
2. Avogadro
3. Dalton
4. Proust
Show answer
Answer: (d) Proust. Proust.
Number of molecules in 8 g of oxygen gas (O₂) is
1. 1.5055 × 10²³
2. 6.022 × 10²³
3. 3.011 × 10²³
4. 12.044 × 10²³
Show answer
Answer: (a) 1.5055 × 10²³. 8/32 = 0.25 mol × 6.022 × 10²³.
Formula unit mass of CaCl₂ (Ca = 40, Cl = 35.5) is
1. 75.5
2. 115
3. 110
4. 111
Show answer
Answer: (d) 111. 40 + 71 = 111 u.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.