How is the charge on an ion calculated ?

Atoms are electrically neutral. A positive ion (or cation) is formed when an atom loses one or more electrons. An electron carries one unit of negative charge. So, the number of units of positive charge only a cation is equal to the number of electrons lost by the concerned atom.

Thus, the number of units of positive charge only an ion is equal to the positive electrovalency of that element.

This can also be explained as follows:

In a neutral atom, the number of electrons is equal to number of protons. So, when electrons are lost by an atom, it is left with more protons than electrons, i.e., the number of protons in a cation is more than the number of electrons. This excess positive charge only the remaining part of the atom (called cation) is the net charge only the ion.

This is illustrated below.

Sodium ion is formed when sodium atom loses one electron, i.e.

Sodium atom (Na) → Sodium ion (Na⁺) + e^–

11 protons in nucleus 11 protons in nucleus

11 electrons in shells 10 electrons in shells

Excess positive charge on sodium ion = No. of protons in Na⁺ – No. of electrons in Na⁺=11-10=1

So, sodium ion carries one unit of positive charge i.e., sodium ion carries a charge of +1. Sodium ion, therefore, is written as Na⁺

Sulphur atom(S) gains two electrons to form sulphide only (S^2-) i.e.

Sulphur atom(S) + 2e^–→ Sulphide ion (S^2-)

16 protons in nucleus 16 protons in nucleus

16 electrons in shells 18 electrons in shells

Then,

Excess negative charge on sulphide ion = No of electrons in S^2- ion – No. of protons in S^2- ion=18-16=2

So, the sulphide ion carries two units of negative charge, i.e., the sulphide ion carries a charge of -2 units. Sulphide ion, therefore, is written as S^2-.