Question

Why config of 'cr' is 3d5 4s1 and not 3d4 4s2 and 'cu' is 3d10 4s1 and not 3d9 4s2?

Answer 1

Because the electron jumps to that orbit where the whole atom is more stable .

In Cr , the electron from s subshell jumps to d subshell to make d subshell partially filled and hence more stable

Same is the logic in the case of Cu

Hope , it helps

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