Chemical Formulae

A chemical formula is like a recipe written with symbols. It shows which atoms are present and how many of each are joined to make a substance. For example, H₂O means two hydrogen atoms and one oxygen atom in each water molecule.

Reading Formulae

• Element symbols use one capital letter, sometimes followed by a lowercase letter (Na, Cl, Fe).
• Subscripts (small numbers) show “how many”: CO₂ has 1 carbon and 2 oxygens.
• Coefficients (big numbers in front) count molecules: 2H₂O means two water molecules. Coefficients change the number of molecules; subscripts change the number of atoms in one molecule.
• State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.

Ionic Formulae (from charges)

Ionic compounds form when positive and negative ions combine so the total charge is zero:

[ \text{Total positive charge} + \text{Total negative charge} = 0 ]

• Use ion charges (e.g. Na⁺, Mg²⁺, Cl⁻, O²⁻).
• Balance charges by choosing the smallest whole-number ratio.

Empirical vs Molecular Formula

• Empirical formula: the simplest whole-number ratio of atoms (e.g. CH₂).
• Molecular formula: the actual number of atoms in a molecule (e.g. C₂H₄). It can be a multiple of the empirical formula.

To find an empirical formula from masses or percentages:

[ \text{moles} = \frac{\text{mass}}{A_r} ]

Then divide by the smallest value to get a whole-number ratio.

General Formulae in Organic Chemistry

• Alkanes: $\mathrm{C}_n\mathrm{H}_{2n+2}$
• Alkenes: $\mathrm{C}_n\mathrm{H}_{2n}$
• Alcohols: $\mathrm{C}_n\mathrm{H}_{2n+1}\mathrm{OH}$
• Carboxylic acids: $\mathrm{C}_n\mathrm{H}_{2n+1}\mathrm{COOH}$

Common Mistakes

• Changing subscripts to balance equations. Use coefficients instead.
• Wrong capital/lowercase letters (co is not Co).
• Forgetting to balance ion charges to zero in ionic formulae.