Question

Assertion: $La$ should be a f-block element according to Aufbau principle
Reason: The 57th electron enters into the 5d –orbital

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct and (R) is not the correct explanation of (A)
• C. (A) is correct, but (R) is incorrect
• D. (A) is incorrect , but (R) is correct

Answer 1

The correct option is B Both (A) and (R) are correct and (R) is not the correct explanation of (A)

According to Aufbau rule, $La\left(57\right)$ should have electronic configuration as $\left[Xe\right]4f^{1}6s^2$ but its configuration is $\left[Xe\right]5d^{1}6s^2$. It is an exception to Aufbau rule. So $La$ is expected to be a $f$-block element but it is a $d$-block element.
Henece both (A) and (R) are correct and (R) is not the correct explanation of (A)