An ion is an atom with more or less electrons than protons and therefore has a charge – either positive or negative.
If it has more electrons than protons it has a net negative charge and is known as an anion.
If it has fewer electrons than protons it has a net positive charge and is known as a cation.
The ionic state may be denoted as a following superscript (e.g. O2-, Fe2+). The number in front of the charge symbol shows how many electrons the atom has gained or lost. O2- is an oxygen atom that has gained 2 electrons.
The common ionic states of an atom are known as its valences.