An ion is an atom or group of bonded atoms which have lost or gained one or more electrons, making them negatively or positively charged. An ion consisting of a single atom is called a monatomic ion. A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion (pronounced ; an-eye-on) due to its attraction to anodes. A positively-charged ion, which has fewer electrons than protons, is known as a cation (pronounced ; cat-eye-on) due to its attraction to cathodes. A polyatomic anion that contains oxygen is sometimes known as an oxyanion.
Ions are denoted in the same way as electrically neutral atoms and molecules except for the presence of a superscript indicating the sign of the net electric charge and the number of electrons lost or gained, if more than one. For example: H+, SO42−. An alternate way of denoting charge is like this: SO4-2.