How atoms combine in fixed ratios and how chemists count them
Two laws laid the foundation of chemistry. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. The law of constant proportions states that a pure compound always contains the same elements in the same ratio by mass; water always has hydrogen and oxygen in the mass ratio 1 : 8. Dalton explained these laws by proposing that matter is made of tiny indivisible atoms.
Atomic mass is measured relative to one-twelfth of a carbon-12 atom, in atomic mass units. Molecular mass is the sum of the atomic masses in a molecule, so H₂O has a mass of 2 × 1 + 16 = 18 u. Chemists count particles in moles: one mole contains 6.022 × 10²³ particles, called Avogadro’s number, and the mass of one mole in grams equals the molecular mass. Formulae of ionic compounds are written by crossing over valencies, such as Al₂O₃ for aluminium oxide.
Why it matters
The mole concept lets pharmacists measure correct doses, factories calculate how much raw material they need, and chemists predict how much product a reaction will give. Writing correct formulae is essential for every chemical equation. This topic prepares you for stoichiometry, solutions and concentration calculations in Class 11.
Key terms
- Law of constant proportions
- A chemical compound always contains the same elements combined in the same fixed ratio by mass.
- Atomic mass unit
- One-twelfth of the mass of a carbon-12 atom, used to express atomic masses.
- Mole
- The amount of substance containing 6.022 × 10²³ particles.
- Molecular mass
- The sum of the atomic masses of all atoms in a molecule.