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

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(57) should have electronic configuration as [Xe]4f16s2 but its configuration is [Xe]5d16s2. 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)

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