It’s easier to determine the elecron configurations for the p-block elements in periods 1,2,3 than to determine the electrons configurations for the rest of the p-block elements in the periodic table beacause

It’s easier to determine the elecron configurations for the p-block elements in periods 1,2,3 than to determine the electrons configurations for the rest of the p-block elements in the periodic table beacause from period 4, specifically from the element 31 (Ga), the atoms start to fill the d orbitals, and the energy levels of the 3d orbitals ara quite similar to the energy levels of 4p orbitals. So, for the elements Cr and Cu the right configurations do not match the configurations predicted using Aufbau method and Hund rules. Those are not the only exceptions but the two first. All is due to the proximity of the energy of the d and p orbitals and the fact that the rearrangement of the electrons result in a lower energy level.

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