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According to hund's rule, electrons must occupy singularly first in the orbitals before pairing up. And if not, the atom would not be stable. Imagine if there are electrons that do not obey Hund's rule, how unstable would the electron-electron repulsion be, what would be the consequences and how are they even unstable other than electron-electron repulsion between them?

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    $\begingroup$ There are exceptions to Hund's Rule. In general Hund's Rule would apply to single atoms in free space. Once you form compounds or solid crystals, then violations of Hund's Rule are more common. $\endgroup$ – MaxW Aug 18 '16 at 20:41