Question

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.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

According to Aufbau principle, orbitals are filled in the order of their energy which is decided by the (n+l) value for each orbital (wheren=principal quantum number and l=azimuthal quantum number). Thus 4s orbital is filled prior to 3d orbital and hence expected outer shell electronic configuration of $Cr(Z=24)$ and $Cu(Z=29)$ is $3d^{4}4s^2$ and $3d^{9}4s^2$, respectively. But their actual configurations are $3d^{5}4s^1$ and $3d^{10}4s^1$, respectively. This is because half-filled and completely filled configurations impart extra stability due to two reasons:

1. Symmetrical distribution of electrons in all degenerated orbitals. This makes a configuration stable.

2. High Exchange energy:In half-filled orbitals, the electrons can exchange among all five orbitals and will have high stabilisation due to high exchange energy.

Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.