Chemical symbols and formulae

Chemical symbols

Each element is represented by its own chemical symbol. The periodic table shows the names and symbols of the elements. A chemical symbol:

  • consists of one or two letters
  • always starts with a capital letter, with any other letter in lower case

For example, the element mercury is shown as Hg. You must not show it as HG, hg or hG.

Formulae of elements

A chemical formula represents an element or compound in balanced equations. The formulae for most elements is just their symbol.

Some non-metal elements exist as simple molecules with two atoms joined together. We say that these elements are diatomic. To show this, their formulae contain a subscript number 2. For example:

  • hydrogen, H2
  • nitrogen, N2
  • oxygen, O2
  • fluorine, F2
  • bromine, Br2
  • chlorine, Cl2
  • iodine, I2
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The elements with a 2 in their formula are hydrogen, nitrogen and oxygen, plus the elements in group 7 (IUPAC group 17).

Formulae of simple covalent compounds

A compound contains two or more elements joined by chemical bonds. Simple covalent compounds contain non-metal atoms joined by covalent bonds. Their molecular formulae show:

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

Here are some examples and their meanings:

Name of compoundFormulaAtoms in each molecule
AmmoniaNH31 nitrogen, 3 hydrogen
Carbon dioxideCO21 carbon, 2 oxygen
MethaneCH41 carbon, 4 hydrogen
Sulfur dioxideSO21 sulfur, 2 oxygen
WaterH2O2 hydrogen, 1 oxygen