Valency, oxidation state and radicals
The valency of an element is the number of electrons an atom gains, loses or shares when it forms a compound. When an atom loses or gains electrons it becomes an ion. The charge on the ion is its oxidation state. A group of atoms that stays together and carries a charge is a radical (polyatomic ion), such as sulphate SO₄²⁻.
| Positive ions | Charge | Negative ions | Charge |
|---|---|---|---|
| Sodium Na⁺, potassium K⁺, hydrogen H⁺, ammonium NH₄⁺, silver Ag⁺ | 1+ | Chloride Cl⁻, bromide Br⁻, nitrate NO₃⁻, hydroxide OH⁻, hydrogen carbonate HCO₃⁻ | 1− |
| Magnesium Mg²⁺, calcium Ca²⁺, zinc Zn²⁺, copper(II) Cu²⁺, iron(II) Fe²⁺ | 2+ | Oxide O²⁻, sulphide S²⁻, sulphate SO₄²⁻, carbonate CO₃²⁻ | 2− |
| Aluminium Al³⁺, iron(III) Fe³⁺ | 3+ | Phosphate PO₄³⁻ | 3− |
Writing formulae
A compound has no overall charge, so the positive and negative charges must cancel. Use the criss-cross method: write the symbols with their charges, then swap the numbers and write them as small subscripts (drop the signs and any 1). Put brackets round a radical when you need more than one of it.
| Compound | Ions | Formula |
|---|---|---|
| Sodium chloride | Na⁺, Cl⁻ | NaCl |
| Magnesium oxide | Mg²⁺, O²⁻ | MgO |
| Calcium hydroxide | Ca²⁺, OH⁻ | Ca(OH)₂ |
| Aluminium oxide | Al³⁺, O²⁻ | Al₂O₃ |
| Ammonium sulphate | NH₄⁺, SO₄²⁻ | (NH₄)₂SO₄ |
Naming binary inorganic compounds
Name the metal (or the less non-metallic element) first and the non-metal second with the ending -ide: NaCl sodium chloride, K₂O potassium oxide, ZnS zinc sulphide. For two non-metals, prefixes show the numbers of atoms: CO carbon monoxide, CO₂ carbon dioxide, SO₃ sulphur trioxide. Compounds with a radical end in -ate (more oxygen, SO₄²⁻ sulphate) or -ite (less oxygen, SO₃²⁻ sulphite).
Empirical and molecular formulae
The empirical formula shows the simplest whole-number ratio of atoms in a compound. The molecular formula shows the actual number of atoms in one molecule. Glucose has the molecular formula C₆H₁₂O₆ and the empirical formula CH₂O.
A compound contains 2.4 g of carbon and 0.8 g of hydrogen. (C = 12, H = 1.) Find its empirical formula.
- Divide each mass by its relative atomic mass to find the moles.
- Carbon: 2.4 ÷ 12 = 0.2 mol. Hydrogen: 0.8 ÷ 1 = 0.8 mol.
Answer: The empirical formula is CH₄.
Enter the masses of two elements in a compound and their relative atomic masses. The ratio of moles gives the formula.
Key terms
- Valency
- The number of electrons an atom gains, loses or shares when it bonds.
- Radical
- A group of atoms with an overall charge that stays together, such as SO₄²⁻.
- Empirical formula
- The simplest whole-number ratio of the atoms in a compound.
- Molecular formula
- The actual number of atoms of each element in one molecule.
Summary
- Positive and negative charges must cancel in a compound; use the criss-cross method.
- Metals that form more than one ion are named with Roman numerals, e.g. iron(III).
- Binary compounds end in -ide; radicals with oxygen end in -ate or -ite.
- Empirical formula: divide masses by Ar and take the simplest ratio; molecular formula = empirical × n.