Class 9 Science Chapter 3: Atoms and Molecules

Atoms and Molecules is Chapter 3 of NCERT Class 9 Science. It covers the laws of chemical combination, Dalton's atomic theory, symbols and atomic masses of elements, molecules and ions, and how to write chemical formulae and calculate molecular mass. Watch the eight video lessons below, then practise with the notes and solved examples on this page.

What you will learn

The law of conservation of mass and the law of constant proportions
The postulates of Dalton's atomic theory and how they explain these laws
Symbols of elements, the atomic mass unit and common atomic masses
Molecules, atomicity and ions, including polyatomic ions
Writing chemical formulae using valency and finding molecular mass and formula unit mass

Video Lessons

Atoms and Molecules - Part 1

Atoms and Molecules - Part 2

Atoms and Molecules - Part 3

Atoms and Molecules - Part 4

Atoms and Molecules - Part 5

Atoms and Molecules - Part 6

Atoms and Molecules - Revision

Atoms and Molecules - Part 7

Key Concepts and Formulas

Laws of Chemical Combination

Two experimental laws laid the foundation for the idea of atoms. Both deal with masses of substances taking part in chemical reactions.

Total mass of reactants = Total mass of products
Water: H : O = 1 : 8 (by mass)
Carbon dioxide: C : O = 3 : 8 (by mass)

Law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction
Law of constant proportions: in a chemical substance the elements are always present in definite proportions by mass, whatever the source or method of preparation

Dalton's Atomic Theory

John Dalton explained the laws of chemical combination by proposing that matter is made of atoms.

All matter is made of very tiny particles called atoms
Atoms are indivisible and can neither be created nor destroyed in a chemical reaction (this explains conservation of mass)
Atoms of a given element are identical in mass and chemical properties; atoms of different elements differ
Atoms combine in the ratio of small whole numbers to form compounds, and the relative number and kind of atoms in a compound is fixed (this explains constant proportions)
Modern science shows atoms are made of smaller particles, so the "indivisible" idea is no longer fully true

Atoms and Symbols

An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are incredibly small; their radius is measured in nanometres.

1 nm = 10⁻⁹ m

IUPAC approves symbols: the first letter is always capital and the second letter (if any) is small, as in H, C, Ca, Co, Cl
Some symbols come from Latin names: Na (sodium, natrium), K (potassium, kalium), Fe (iron, ferrum), Cu (copper), Ag (silver), Au (gold)

Atomic Mass

Since a single atom is too light to weigh directly, scientists use a relative scale based on carbon-12.

1 u = 1/12 × mass of one carbon-12 atom
H = 1 u    C = 12 u    N = 14 u    O = 16 u
Na = 23 u  Mg = 24 u   Al = 27 u   S = 32 u
Cl = 35.5 u   K = 39 u   Ca = 40 u

Molecules and Atomicity

A molecule is a group of two or more atoms chemically bonded together. It is the smallest particle of an element or compound that can exist independently and shows all the properties of that substance.

Atomicity: number of atoms in one molecule of an element
Monoatomic: He, Ne, Ar; diatomic: H₂, O₂, N₂, Cl₂; triatomic: O₃; tetra-atomic: P₄; polyatomic: S₈
Molecules of compounds contain atoms of different elements, for example H₂O, NH₃ and CO₂

Ions and Chemical Formulae

Compounds formed by metals and non-metals are usually made of ions. The combining capacity of an element is called its valency.

Cation: positively charged ion (Na⁺, Mg²⁺, Al³⁺, NH₄⁺); anion: negatively charged ion (Cl⁻, O²⁻, S²⁻)
Polyatomic ions: OH⁻, NO₃⁻, HCO₃⁻, CO₃²⁻, SO₄²⁻, PO₄³⁻
To write a formula, write the symbols with valencies, criss-cross the valencies, and reduce to the simplest ratio; the metal or positive ion is written first
Put a polyatomic ion in brackets when more than one of it is present, for example Ca(OH)₂

Molecular Mass and Formula Unit Mass

Formula unit mass is used for ionic compounds such as NaCl and is calculated in the same way as molecular mass.

Molecular mass = sum of atomic masses of all atoms
NaCl: 23 + 35.5 = 58.5 u (formula unit mass)

Solved Examples

Example 1: On heating, 4.0 g of calcium carbonate gives 1.76 g of carbon dioxide and calcium oxide. Find the mass of calcium oxide.

CaCO₃ → CaO + CO₂
4.0 = mass of CaO + 1.76
∴ Mass of CaO = 4.0 − 1.76 = 2.24 g

Example 2: Magnesium and oxygen combine in the ratio 3 : 2 by mass to form magnesium oxide. How much oxygen reacts with 12 g of magnesium, and how much oxide forms?

Oxygen = 12 × 2/3 = 8 g
MgO = 12 + 8 = 20 g
∴ 8 g oxygen reacts; 20 g MgO forms

Example 3: Find the molecular mass of glucose, C₆H₁₂O₆.

= (6 × 12) + (12 × 1) + (6 × 16)
= 72 + 12 + 96
∴ Molecular mass = 180 u

Example 4: Find the molecular mass of nitric acid, HNO₃.

= 1 + 14 + (3 × 16)
= 1 + 14 + 48
∴ Molecular mass = 63 u

Example 5: Find the formula unit mass of aluminium sulphate, Al₂(SO₄)₃.

= (2 × 27) + 3 × (32 + 4 × 16)
= 54 + 3 × 96
= 54 + 288
∴ Formula unit mass = 342 u

Example 6: Write the formulae of magnesium nitrate and calcium phosphate.

Mg²⁺ and NO₃⁻ → criss-cross → Mg(NO₃)₂
Ca²⁺ and PO₄³⁻ → criss-cross → Ca₃(PO₄)₂
∴ Mg(NO₃)₂ and Ca₃(PO₄)₂

Example 7: Find the ratio by mass of elements in calcium carbonate, CaCO₃.

Ca : C : O = 40 : 12 : (3 × 16)
= 40 : 12 : 48
∴ Ratio = 10 : 3 : 12

Common Mistakes to Avoid

Writing symbols with the wrong case, such as "CO" for cobalt; cobalt is Co, while CO is carbon monoxide.
Forgetting brackets around polyatomic ions; Mg(NO₃)₂ is correct, MgNO₃₂ is wrong.
Multiplying only the first atom inside a bracket; in Al₂(SO₄)₃ the 3 multiplies both S and O.
Using "molecular mass" for ionic compounds in board answers; say formula unit mass for compounds like NaCl and K₂CO₃.

Quick Revision

Mass of reactants = Mass of products
1 u = 1/12 of mass of a carbon-12 atom
Molecular mass = sum of atomic masses
Atomicity: He 1, O₂ 2, O₃ 3, P₄ 4, S₈ 8