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Class 9 Atomic Foundations of Matter: cheat sheet
CBSE Science. The laws of chemical combination, atoms and molecules, atomic and molecular mass, ions, and writing chemical formulas from valencies.
Key ideas
- Law of conservation of mass: mass is neither created nor destroyed in a chemical reaction.
- Law of constant proportions: a compound always has its elements in the same proportion by mass (water: H to O is 1 : 8).
- Dalton: matter is made of tiny atoms that combine in small whole-number ratios.
- A molecule is the smallest particle of a substance that can exist on its own; atomicity is the number of atoms in it.
- Ions are charged: cations are positive (Na⁺), anions negative (Cl⁻). Polyatomic ions such as SO₄²⁻ act as one unit.
- To write a formula, cross over the valencies and simplify; bracket a polyatomic ion that is used more than once.
Remember
| Atomic mass unit | 1 u = 1/12 of the mass of a carbon-12 atom |
| Molecular mass | H₂O = 2 × 1 + 16 = 18 u; CO₂ = 12 + 2 × 16 = 44 u |
| Formulas | Al³⁺ and O²⁻ → Al₂O₃; Ca²⁺ and OH⁻ → Ca(OH)₂ |
| Common ions | NH₄⁺, OH⁻, NO₃⁻, CO₃²⁻, SO₄²⁻, PO₄³⁻ |
Watch out
- Forgetting the brackets in Ca(OH)₂ or (NH₄)₂SO₄.
- Adding atomic masses without multiplying by the number of each atom.
- Writing Cl for a chlorine molecule (it is Cl₂).
Worked example
Write the names and formulas of all four compounds formed by the cations NH₄ (1+) and Pb (2+) with the anions Cl (1−) and SO₄ (2−).
Working: cross the valencies; bracket a polyatomic ion used more than once
Answer: ammonium chloride: NH₄Cl; ammonium sulphate: (NH₄)₂SO₄; lead chloride: PbCl₂; lead sulphate: PbSO₄
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