Why Cu(l) is colourless and Cu(ll) is blue in colour ?

Explanation

Transition Element : These are the substances which have partially filled d-subshell is called transition element.
Now, Electronic configuration of Cu (Atomic No. 29) is.
`1 s^2 2 s^2 2p^6 3s^2 3p^6 3d^10 4s^1`

Cu (I) is colourless because they have fully filled d-subshell so d-d transition is not possible. But Cu (II) is Blue in colour because there·is possible d-d Transition and have partilly filled d-subshell so, they show colour.