Common ions

An ion is denoted by the symbol of the parent atom, followed by the sign as a superscript and, where necessary, the number of charges.

Some ions to be aware of

Ion	Name	Formed from	Number of electrons exchanged
Na+	sodium ion	the sodium atom Na	loss of 1 electron
Cl⁻	chloride ion	the chlorine atom Cl	gain of 1 electron
Cu²⁺	copper(II) ion	the copper atom Cu	loss of 2 electrons
Ca²⁺	calcium ion	the calcium atom Ca	loss of 2 electrons

The small number before the symbol indicates how many electrons have been exchanged: Cu²⁺ means that the copper atom has lost two electrons, not just one. If there is no number, as in Na⁺, this means that only one electron is involved.

Where are they found?

• Table salt (sodium chloride) is made up of Na+ and Cl- ions, not metallic sodium and gaseous chlorine.
• Blue copper sulphate solutions, used for example to treat vines, owe their colour to Cu²⁺ ions.
• Calcium ions (Ca²⁺), present in water, are responsible for the limescale found in pipes and kettles.

Key points

A cation retains the name of the element (‘sodium ion’, ‘copper ion’) because it has simply lost electrons. An anion, on the other hand, often changes its suffix (chlorine → chloride) to indicate that it has gained electrons. This difference in suffix is not a mere detail: it allows us to tell, simply from the name, whether we are talking about a cation or an anion.