Question

Assertion (A) $Cr$ and $Cu$ are extra stable elements in the $3 d$-series.
Reason (R) Half and completely filled set of orbitals are relatively more stable.

• A. Both $A$ and $R$ are correct; $R$ is the correct explanation of $A$.
• B. Both $A$ and $R$ are correct; $R$ is not the correct explanation of $A$.
• C. $A$ is correct; $R$ is incorrect.
• D. $R$ is correct; $A$ is incorrect.

Answer 1

Answer: (A)

Half and completely filled set of orbitals are relatively more stable. Thus, electronic configuration of $Cr$ is $3 d^{5} 4 s^{1}$ instead of $3 d^{4} 4 s^{2}$, the energy gap between the two sets ( $3 d$ and $4 s$ ) of orbitals is small enough to prevent electron entering the $3 d$-orbitals. Similarly, in the case of $Cu$, the configuration is $3 d^{10} 4 s^{1}$ and not $3 d^{9} 4 s^{2}$.