3.1 Formulae

2026 Syllabus Objectives

  1. Core: State the formulae of the elements and compounds named in the subject content
  2. Core: Define the molecular formula of a compound as the number and type of different atoms in one molecule
  3. Core: Deduce the formula of a simple compound from the relative numbers of atoms present in a model or a diagrammatic representation
  4. Core: Construct word equations and symbol equations to show how reactants form products, including state symbols
  5. Supplement: Define the empirical formula of a compound as the simplest whole number ratio of the different atoms or ions in a compound
  6. Supplement: Deduce the formula of an ionic compound from the relative numbers of the ions present in a model or a diagrammatic representation or from the charges on the ions
  7. Supplement: Construct symbol equations with state symbols, including ionic equations
  8. Supplement: Deduce the symbol equation with state symbols for a chemical reaction, given relevant information

Chemical Symbols and the Periodic Table

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The Language of Chemistry 🔬

The system of chemical symbols and formulae is designed to be understood internationally, transcending cultural, linguistic, and national boundaries. This universal system was invented by Swedish chemist Jöns Jacob Berzelius (1779–1848).

Chemical Symbols

🔑 Key Definition: A chemical symbol is a letter or group of letters representing an element in a chemical formula.

Rules for writing chemical symbols:

  • Each element has a unique chemical symbol based on its name
  • Symbols consist of either:
    • A single capital letter (e.g., hydrogen = H\mathrm{H}, carbon = C\mathrm{C})
    • A capital letter followed by a lowercase letter (e.g., aluminium = Al\mathrm{Al}, calcium = Ca\mathrm{Ca})

Symbols from Latin Names

Some elements have symbols derived from their Latin names rather than their English names:

ElementLatin NameChemical Symbol
SilverargentumAg\mathrm{Ag}
GoldaurumAu\mathrm{Au}
CoppercuprumCu\mathrm{Cu}
SodiumnatriumNa\mathrm{Na}
PotassiumkaliumK\mathrm{K}
IronferrumFe\mathrm{Fe}
LeadplumbumPb\mathrm{Pb}

Diatomic Elements ⚡

Seven elements exist naturally as diatomic molecules (molecules made of two atoms):

  1. Hydrogen: H2\mathrm{H}_2
  2. Nitrogen: N2\mathrm{N}_2
  3. Oxygen: O2\mathrm{O}_2
  4. Fluorine: F2\mathrm{F}_2
  5. Chlorine: Cl2\mathrm{Cl}_2
  6. Bromine: Br2\mathrm{Br}_2
  7. Iodine: I2\mathrm{I}_2

Important: These seven elements form an "L" shape on the right side of the Periodic Table (with hydrogen separately on the top left).

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