What is the valency of copper? How to find the chemical formula of copper sulphate?

Copper shows two common valencies, +1 and +2, and the chemical formula of copper(II) sulphate is CuSO₄.

Valency tells how many electrons an atom uses to bond. Copper can lose one electron (cuprous, +1) or two electrons (cupric, +2). Sulphate is a polyatomic ion, SO₄²⁻, with a fixed charge of −2. When copper has valency +2, it pairs neatly with sulphate’s −2 to give CuSO₄. 

When copper is +1, two Cu⁺ are needed to balance one SO₄²⁻, giving Cu₂SO₄ (which is much less common and unstable in water compared with CuSO₄).