Atomic Foundations of Matter

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.

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