Transition metals nickel, copper, tin and iron exhibit variable valency. Nonmetals such as nitrogen and oxygen also show variable valency. The products formed as a result of reaction with the different valency atoms differ in their properties.

Why do non metals have variable valency?

The simple explanation has to do with them having electrons in the d orbital. So, there are 5 orbitals in the d subshell, and as the number of unpaired valence electrons increases, so does the oxidation state. And, btw, oxidation state is the preferred term as the term valence and valency is outdated.

What is the valency of non metals?

The number of electrons accepted by a non metal is called valency of the non metal. For example, one atom of chlorine accepts 1 electron to form a negative chloride ion (Cl^–), so chlorine is a non-metal. Here because an atom of chlorine gains 1 electron so the valency of chlorine is 1.