Board exam focus — Atoms and Molecules (CBSE & HBSE)

CBSE emphasises the laws of chemical combination, mole concept numericals and writing chemical formulae using valency, while HBSE focuses on definitions, symbols, atomic and molecular mass calculations and direct valency-based formula questions.

Laws of Chemical Combination

Background

The laws of chemical combination, established by Antoine Lavoisier and Joseph Proust, govern how elements combine to form compounds.

Law of Conservation of Mass

Mass can neither be created nor destroyed in a chemical reaction.

The total mass of the reactants equals the total mass of the products.

Example: when 5.3 g of sodium carbonate reacts with 6.0 g of ethanoic acid, the products (sodium ethanoate, carbon dioxide and water) also weigh 11.3 g total.

Law of Constant (Definite) Proportions

In a chemical substance, the elements are always present in definite proportions by mass.

• In water (H₂O), hydrogen and oxygen are always present in the ratio 1 : 8 by mass, no matter the source.
• In carbon dioxide (CO₂), carbon and oxygen are always in the ratio 3 : 8 by mass.

Dalton's Atomic Theory

John Dalton proposed that the smallest particle of matter is the atom. Its postulates:

1. All matter is made of very tiny particles called atoms, which are indivisible.
2. Atoms are never created or destroyed in a chemical reaction (explains conservation of mass).
3. Atoms of a given element are identical in mass and chemical properties.
4. Atoms of different elements have different masses and properties.
5. Atoms combine in the ratio of small whole numbers to form compounds (explains constant proportions).
6. The relative number and kinds of atoms are constant in a given compound.

CBSE trap: The law of conservation of mass is verified using a closed conical flask so that gaseous products do not escape; otherwise mass appears to decrease.

Atoms, Symbols, Molecules, Ions and Valency

Atoms

An atom is the smallest particle of an element that may or may not exist independently but takes part in a chemical reaction. Atoms are extremely small; their radius is measured in nanometres (1 nm = 10⁻⁹ m).

Symbols of Elements

A symbol is a short representation of an element. IUPAC approves the names. Symbols are derived from the English or Latin name.

The first letter of a symbol is always capital; the second (if any) is always small (e.g., Co = cobalt, but CO = carbon monoxide).

Atomic Mass

The atomic mass of an element is the relative mass of its atom compared with 1/12th the mass of one carbon-12 atom (1 u = atomic mass unit). For example, the atomic mass of hydrogen = 1 u and oxygen = 16 u.

Molecules

A molecule 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 (He = monoatomic, O₂ = diatomic, O₃ = triatomic, P₄, S₈).
• Molecule of an element: contains the same type of atoms (e.g., O₂, N₂).
• Molecule of a compound: contains different types of atoms (e.g., H₂O, CO₂).

Ions

An ion is a charged particle formed when an atom or group of atoms loses or gains electrons.

• Cation = positively charged (Na⁺, Ca²⁺) — formed by losing electrons.
• Anion = negatively charged (Cl⁻, O²⁻) — formed by gaining electrons.
• Polyatomic ions: a group of atoms carrying charge, e.g., sulphate (SO₄²⁻), nitrate (NO₃⁻), ammonium (NH₄⁺), carbonate (CO₃²⁻).

Valency

Valency is the combining capacity of an element — the number of electrons it loses, gains or shares to complete its octet.

Writing Chemical Formulae (Criss-Cross Method)

1. Write the symbols of the ions side by side with valencies on top.
2. Cross over the valencies as subscripts.
3. Simplify the ratio; enclose polyatomic ions in brackets if subscript > 1.

Examples: Al₂O₃ (aluminium oxide), CaCl₂ (calcium chloride), Ca(OH)₂ (calcium hydroxide), Na₂SO₄ (sodium sulphate).

HBSE/CBSE trap: For a polyatomic ion taken more than once, always use brackets — Ca(OH)₂, NOT CaOH₂.

Molecular Mass, Formula Unit Mass and the Mole Concept

Molecular Mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms in one molecule. It is expressed in atomic mass units (u).

• Water (H₂O) = (2 × 1) + 16 = 18 u
• Carbon dioxide (CO₂) = 12 + (2 × 16) = 44 u

Formula Unit Mass

For ionic compounds, we use the term formula unit mass — the sum of atomic masses of all atoms in a formula unit.

• Sodium chloride (NaCl) = 23 + 35.5 = 58.5 u

The Mole Concept

A mole is the amount of a substance that contains exactly 6.022 × 10²³ particles (atoms, molecules or ions). This number is called the Avogadro number (Nₐ or N₀).

• 1 mole of any substance = 6.022 × 10²³ particles.
• The mass of 1 mole of a substance = its molar mass (in grams), numerically equal to atomic/molecular mass in u.
• 1 mole of atoms of an element = gram atomic mass.
• 1 mole of molecules = gram molecular mass.

Key Mole Relationships

Example: Molar mass of water = 18 g/mol, so 18 g of water = 1 mole = 6.022 × 10²³ molecules.

CBSE trap: Atomic mass and molar mass are numerically equal but have different units — atomic mass is in u, molar mass is in g/mol. Always carry units in mole numericals.

Frequently asked questions

Are these Atoms and Molecules notes free?

Yes — the Atoms and Molecules notes for Science (Class 9) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Atoms and Molecules notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Atoms and Molecules chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Atoms and Molecules.