Question

Why cu+ salts are colorless but cu2+ salts are colorful

Answer 1

the colour of transition elements is due to the presence of unpaired electrons.
cu+ is colourless as its outermost configuration is 3d10 ...so there are no unpaired electrons which causes the colour .
whereas cu+2 has 3d9 outer configuration . so the presence of unpaired electrons makes it coloured

Answer 2

Answer : Because $Cu^+$ do not contain any unpaired electron whereas $Cu^{2+}$ contain one unpaired electron.

Explanation:

Atomic number of copper is 29.

The electronic configuration of copper will be,

$1s^22s^23s^23p^64s^13d^{10}$

The electronic configuration of $Cu^+$ will be,

$1s^22s^23s^23p^63d^{10}$

The electronic configuration of $Cu^{2+}$ will be,

$1s^22s^23s^23p^63d^{9}$

As we know that the unpaired electrons present in the ion show color due to the transition of the electrons from ground state to the excited state.

In case of $Cu^+$ there is no unpaired electrons that means the shells are fully filled. So, it will not show any color. That's why $Cu^+$ salts are colorless.

Whereas In case of $Cu^{2+}$,  there is one unpaired electron. That's why $Cu^{2+}$ salts are colorful.