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

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Answer

B

Explanation

According to (n + l) rule, 4s orbital is filled prior to 3d orbital, so expected outer configuration of Cr and 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.