Question

Copper(l) is diamagnetic where as copper (ll) is paramagnetic. Why?

Answer 1

A substance is paramagnetic when it has at least one unpaired electron and a substance is diamagnetic when it has no unpaired electron.
In case of Cu, the configuration of Cu is $\left[Ar\right]3d^{10}4s^1$

In case of Cu(I) configuration is $\left[Ar\right]3d^{10}$

As there is no unpaired electron in Cu(I), therefore Cu(I) is diamagnetic in nature.
On the other hand in Cu(I) the configuration of Cu(II) is $\left[Ar\right]3d^9$

It has one unpaired electron in one of its orbital. So, Cu(II) is paramagnetic.
Hence, Cu(I) is diamagnetic and Cu(II) is paramagnetic.