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  4. Aufbau principle does not give the correct arrangement of filling Up of atomic orbitals in

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Q. Aufbau principle does not give the correct arrangement of filling Up of atomic orbitals in

J & K CETJ & K CET 2003

Solution:

Aufbau principle does not give the correct arrangement of filling up of atomic orbitals in copper and chromium because half-filled and completely filled electronic configuration of $Cr$ and $Cu$ have lower energy and therefore, more stable.
$C r(Z=24): 1 s^{2}, 2 s^{2} 2 p^{6}, 3 s^{2} 3 p^{6} 3 d^{5}, 4 s^{1}$
$C u(Z=29): 1 s^{2}, 2 s^{2} 2 p^{6}, 3 s^{2} 3 p^{6} 3 d^{10}, 4 s^{1}$