Formulae and equations - OCR 21st CenturyFormulae of simple molecular compounds

A substance formed by the chemical combination of two or more elements is called a compound. Some compounds made of non-metal elements exist as simple molecules.

The chemical formula for a simple molecule shows:

• the symbols for each element in the compound
• the number of atoms of each element in each molecule

Example

A water molecule contains 2 hydrogen atoms and 1 oxygen atom. What is its formula?

• the symbol for hydrogen is H
• the symbol for oxygen in O
• there are 2 hydrogen atoms, so a subscript 2 is added after the H (1 is not written)
• H₂O