Some elements can have different valences. It can happen due to the change in

Valency is defined as the number of electrons that participate in a reaction. It is the number of electrons, an atom of an element can gain or loss in a reaction.

If valence electrons are 4 or less then the valency of an atom is equal to the valence electrons. And if valence electrons are more than 4, the valency is equal to 8 minus the number of electrons in the outer most shell.

For example valency of Magnesium is 2 because it has 2 valence electrons and it can donate two electrons to form a bond. The valency of sulphur is 2 because it has 6 electrons and it can accept or share 2 electrons to form a bond. The valency of carbon is 4 because it has 4 electrons and it can share 4 electrons with other atoms.

Note- Valence electrons are the electrons present in the outermost shell of an atom.