Exceptions to octet rule

[Nancy Hu - 4E]

Can someone help explain why H, He, Li, and Be are exceptions to the octet rule?

[Sofia Ban]

H and He are exceptions to the Octet Rule because for them, a full shell contains only 2 electrons (1s shell). Li and Be are metals, so they're likely to lose electrons, therefore becoming cations (Li+, Be2+). When this happens, they revert to the "noble gas" configuration of He, which is 2 electrons. So for these four elements, a full shell consists of 2 electrons, not eight.

In addition, the H-atom doesn't need to fill its valence shell when forming ionic bonds. In an HF molecule, the H-atom is the cation, meaning it gave up its single valence electron to bond with the F-atom. The H-atom doesn't have a full valence shell, but rather has no valence shell to fill while the F-atom gains an electron to fill its valence shell.