Assertion : The outer electronic configurations of Cr and Cu are ` 3d^(5)4s^(1)` and `3d^(10)4s^(1)` respectively.
Reason : Electrons are filled in orbitals in order of their increasing energies given by `(n+l)` rule.

Options

A. If both (A) and (R) are correct and (R) is the correct explanation of(A).
B. If both (A) and (R) are correçt, but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If both (A) and (R) are incorrect.

Answer

B

Explanation

According to `(n+l)` rule, 4s orbital is filled prior to 3d orbital, so expected outer configuration of Cr and to Cu is `3d^(4)4s^(2)` and `3d^(9)4s^(2)` but their actual configurations are `3d^(5)4s^(1)` and `3d^(10)4s^(1)` respectively. This is because completely filled or half-filled configurations impart extra stability due to symmetrical distribution of electrons and more exchange energy.