anion

anion - n 
a negatively charged ion 

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If an atom, or atoms, has a balanced number of electrons (negative charge) and protons (positive charge) they are neutral overall. However, if they are not balanced, they will be charged. These charged species are called ions.

Cations are ions that are positively charged. Anions are ions that are negatively charged. Ions are charged atoms or molecules. If a balanced atom loses one or more electrons, it will become a positively charged cation. If a balanced atom gains one or more electrons, it will become a negatively charged anion. 

An anion has more electrons than protons, consequently giving it a net negative charge. For an anion to form, one or more electrons must be gained, typically pulled away from other atoms with a weaker affinity for them. The number of electrons gained, and so the charge of the ion, is indicated after the chemical symbol, e.g. chlorine (Cl) gains one electron to become Cl-, whilst oxygen (O) gains two electrons to become O2-.

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