Chemistry: Atoms, Molecules, and Chemical Equations
Formula and equation items are easy marks, but only if your atom counting is disciplined.
From atoms to formulae
- 原子 is the smallest particle of an element; 分子 is a group of atoms bonded together; イオン is an atom or group that has gained or lost electrons.
- A 化学式 gives the kind and number of atoms. An イオン式 adds the charge as a superscript: Na⁺, Ca²⁺, Cl⁻, SO₄²⁻.
- Metals and ionic crystals such as NaCl and CuO are written as empirical formulae; molecular substances such as H₂O, CO₂ and CH₄ are written as molecules.
Balancing an equation
- Write correct formulae - never change a subscript to balance.
- Count each element on both sides.
- Adjust coefficients only, starting with an element that appears in fewest formulae.
- Recount every element, then check charge if ions are involved.
Ions in solution
Acids release H⁺, アルカリ release OH⁻, and 中和 gives water plus a salt. A precipitate forms when ions pair into an insoluble compound: AgCl, BaSO₄ and CaCO₃ are all white.
Worked example. Aluminium burns: Al + O₂ → Al₂O₃. Left side has 2 O, right side has 3 O; the lowest common multiple is 6, so use 3O₂ and 2Al₂O₃. That forces 4 Al on the left. Balanced: 4Al + 3O₂ → 2Al₂O₃. Check: 4 Al each side, 6 O each side.
Traps. Balancing 2H₂ + O₂ → 2H₂O and then 'simplifying' it to H₂ + O → H₂O; dropping the ² in SO₄²⁻; giving a 化学式 where an イオン式 is required; writing a coefficient of 1.