Naming of Oxo-anions

Oxo-anions are two components coming together. They are Oxo from the word oxygen and anions which is a negatively charged atom. Examples are PO₄ ^3- , MnO₄ ^–.

Rules for naming Oxo-anions.

Find the number of Oxygen atoms in the oxo-anions using the following prefixes: Mono (1 oxygen), di (2 oxygen atoms), tri (3 oxygen atoms), tetra (4 oxygen atoms), pent (5 oxygen atoms), heria (6 oxygen atoms), hepta (7 oxygen atoms), etc.

The name of the oxo-anion which usually ends in ‘ate’ is written. Example, the oxo-anion MnO₄ ^– is known as Manganate.

The oxidation number of the first element of the oxo-anion is calculated and written in a bracket.

Example: MnO₄ ^–

The oxygen atom in the oxo-anion is four (4) and this is due to the 4 in front of the first oxygen (O). The four (4) oxygen atoms in the oxo-anion gives the name tetraoxo (tetra from four (4) and oxo from oxygen). The oxo-anion is manganate.

From the calculation of oxidation number in the previous lessons, the oxidation number of oxygen (O) is -2.

Let’s calculate the number to be written in front of the tetraoxomanganate or the oxo-anion. MnO4 –

The oxidation number of Mn is not known so it will be represented by x. .: x + 4 (-2) = -1

The -1 is from the charge on the oxo-anion which is the (-) on top of the oxygen.

 x +4 (-2) = -1
 x – 8     = -1
  x        = -1+8
x          = +7

.: the number in front of the tetraoxomanganate will be +7 which will be written in Roman numerals as (VII).

Hence the systematic name of MnO₄ ^– is tetraoxomanganate (VII). The word ion must be written at the end.

.: MnO₄ ^– – tetraoxomanganate ion.

NB: When ion is not written when naming an oxo-anion, the systematic name is very wrong.