Assertion (A): The electronic configuration of Cr in the ground state is [Ar] $4 s^{1} 3 d^{5}$ instead of [Ar] $4 s^{2} 3 d^{4}$
Reason (R): The Average exchange energy associated with $4 s^{1} 3 d^{5}$ configuration is more when compared to that of [Ar] $4 s^{2} 3 d^{4}$

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Both (A) and (R) are true and (R) is the correct explanation of (A)

(A) is true but (R) is false

Both (A) and (R) are false

Both (A) and (R) are true and (R) is not the correct explanation of (A)

answer is A.

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Detailed Solution

However, we also know, half filled is more stable than partially filled orbitals. Also, 3d⁵ is more stable and less energy than 4s² configuration. In case of Chromium, after the 4s² 3d⁴ configuration is attained, an electron from the 4s orbital jumps to 3d subshell because 3d⁵ is a much more stable configuration than 3d⁴. That's why final configuration for Chromium is 4s¹ 3d⁵ and not 3d⁴ 4s².

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