Question

how is valency changed?

Answer 1

The chemical properties of elements depend upon the number of electrons in the outer most shell of an atom. These electrons are called valence electrons and thus determines the valency of the atom or element In representative elements, the valency is generally equal to either n or (8 - n), where n is number of valence electrons in the atom.
In a period valence electrons increase from 0 to 8 on moving from left to right.

In a group, the number of valence electrons remains the same and therefore all elements in a group exhibit same valency e.g., all elements of group 1 have valency one and those of group 2 havevalency two. However the transition elements exhibit variable valency

Answer 2

Valency is changed on their atomic number.The balance electrons left on the last shell which is not stable is the valency.A stable elment has electronic configuration of 2,8,8,8.... .