Reasoning behind octet exceptions

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Why do octet exceptions exist? And what is the cause of it?

[Valerie Ayzenberg 1I]

The octet exception where the central atom can have an expanded octet exists because elements in Period 3 and after have the d orbitals in their valence shell, which they can fill with extra valence electrons. For example, phosphorus has the 3d orbital in its valence shell. The octet exception where the central atom doesn't have to have a full octet exists because the amount of valence electrons the atom has initially is not enough to reach eight valence electrons, even if all of the atom's valence electrons are in a covalent bond. For example, boron only has 3 valence electrons, so the maximum amount of covalent bonds, which if you remember uses one electron from the central atom and another electron from the bonding atom, it can form is 3 covalent bonds which results in only 6 valence electrons total. Therefore, boron, and similar atoms like it, can never achieve an octet and is part of the octet exceptions.

[Angelina Ta 1l]

Octet exceptions exist as certain elements can accommodate more or less than eight electrons in their valence shells. A cause includes smaller atoms forming stable molecules with fewer than eight electrons (e.g., hydrogen, boron, and elements in period 3 or higher).