An anion is a negatively charged ion, formed when an atom (usually a non-metal) gains one or more electrons, for example Cl− or O2−.
An anion is a negatively charged ion, formed when an atom gains one or more electrons. The atom is usually a non-metal. Malay: anion · Chinese: 阴离子.
When an atom gains electrons, it now has more electrons than protons, so the extra negative charge is no longer cancelled and the particle carries a net negative charge. This negative ion is an anion. Non-metals form anions easily because they already have nearly full outer shells and only need a few more electrons to reach the stable arrangement of the nearest noble gas. The size of the negative charge equals the number of electrons gained, so a Group 17 non-metal gains one electron to form a 1- anion, and a Group 16 non-metal gains two to form a 2- anion.
Example. A chlorine atom (2.8.7) gains one electron to become the anion Cl− (2.8.8). An oxygen atom (2.6) gains two electrons to become O2− (2.8). Nitrate, NO3−, and sulfate, SO4 2-, are polyatomic anions. In an ionic bond, the anion is attracted to the cation.
Confusion to avoid. Do not mix up an anion with a cation. An anion is negative and is usually formed by a non-metal gaining electrons; a cation is positive and is usually formed by a metal losing electrons. Remember that an anion is attracted to the positive anode in electrolysis, which is one way to keep the two straight. It also helps to note that the name of a simple anion usually ends in “-ide”, as in chloride or oxide, while many polyatomic anions such as nitrate and sulfate end in “-ate”.
The anion is the other half of every ionic compound in this chapter, and balancing its charge against the cation’s is how a correct ionic formula is worked out in SPM Chemistry.
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