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Electronic Configuration of Atoms

Following the above three principles, we can build up the electronic configuration of an atom, perhaps with one exception that half-filled or fully-filled degenerate orbitals are more stable than the other possible configurations. Two common examples are of Cr and Cu. 
Following the three rules, namely, aufbau, Pauli and Hund's rules, their configurations would have been
24Cr        1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d4
29Cu        1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d9
But the actual configurations are
24Cr        1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5
29Cu        1s2, 2s22, 2p6, 3s2, 3p6, 4s1, 3d10
This is because the 3d5 4s1 (half filled) configuration is more stable than 3d4 4s2 in Cr whereas 3d10 4s1 (completely filled) is more stable than 3d9 4s2 in Cu. In the above representations, superscripts represent the number of electrons assigned to the corresponding orbitals. Table 4.7 describes the electronic configurations of some atoms.

Electronic configurations of atoms up to atomic number 36



Stability of Half Filled and Completely Filled Orbitals

Chromium and copper have five and ten electrons in 3d orbitals rather than four and nine as their position would have indicated. 
The reason is that fully filled orbitals and half-filled orbitals have extra stability. Thus the p3,d5,d10,f7,f14 etc., configurations, which are either fully filled or half-filled are more stable. Chromium and copper therefore, adopt the d5 and d10 configurations in preference to the d4 and d9 configurations.

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