Electronic configuration of the electrons in the outer shells of Cu & Cr are `3d^(10)4s^(1) & 3d^(5)4s^(1)` respectively and not `3d^(9)4s^(2) and 3d^(4)4s^(2)`- exaplain why.

Explanation

Half-filled and fully filled subshells have greater stability. In the case of Cu, `3d^(10)4s^(1)` configuration is more stable than `3d^(9)4s^(2)` because in `3d^(10)4s^(1)`, 3d-subshell is completely filled and 4s-subshell is half-filled.
Similarly in the case of Cr, `3d^(5)4s^(1)` configuration contains half-filled 3d-and 4s-subshells and hence this will be more stable than `3d^(4)4s^(2)` configuration.