Question

Why in most cases is copper written like copper (iii) or (ii) ?

Answer 1

Copper (ii) and copper (iii) are isotops of copper and these are more reactive than copper (i). So,it is used to perform reactions. This is the answer of your question.

Answer 2

Copper forms a rich variety of compounds, usually with oxidation states +1 and +2, which are often called cuprous and cupric, respectively.