Formulae 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


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

What is the formula for sulfur trioxide? Each molecule is made of 1 sulfur atom and 3 oxygen atoms.


Carbon dioxideCO2
Carbon monoxideCO
Nitrogen dioxideNO2
Nitrogen monoxideNO
Sulfur dioxideSO2
Methane CH4