An ion is an atom or group of atoms in which the number of electrons is not equal to the number of protons, giving it a net positive or negative electrical charge. An anion is an ion that is negatively charged, and is attracted to the anode (positive electrode) in electrolysis. A cation has a net positive charge, and is attracted to the cathode (negative electrode) during electrolysis.
Anion originated from the Greek word ano, meaning ‘up’. The word cation originated from its Greek counterpart kata, which means ‘down’.
A good way to remember which type of ion is positively charged is to use the following mnemonic devices:
- Anion: A Negative ION
- CATion: PAWSitive (cats have paws)
When a cation like sodium is represented as (Na+), the ‘+’ charge that accompanies it, indicates that it has one less electron than the total number of protons. This uneven equation of electrons and protons enables sodium to have a positive charge. Similarly, when the chloride anion is represented with (Cl-), the ‘–‘charge indicates that it has one less proton than the total number of electrons. If the ‘+’ or the ‘-’ sign is accompanied by a number like +4 or -2, then it implies that: The cation with +4 charge has 4 less electrons than the total number of protons and The anion with -2 charge has two less protons than the total number of electrons.
As a result of being negatively and positively charged, anions and cations usually form bonds. These are known as ionic bonds and exist due to the mutual attraction of oppositely charged ions. They form a crystal bond, in which oppositely charged ions are bound to each other.
Examples of Anions include oxide (O2-), sulfide (S2-), fluoride (F-), chloride (Cl-), bromide (Br-), iodide (I-), nitride (N3-) and hydride (H-).
Cation examples include sodium (Na+), Iron (Fe2+), and Lead (Pb2+)