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H and He are exceptions to the Octet Rule because for them, a full shell is only 2 electrons (the 1s shell). Li and Be are metals, and so they are likely to lose electrons and become cations (Li+, Be2+). When that happens, they also revert to the "noble gas" configuration of He, which is 2 electrons. So for these four elements, a full shell doesn't consist of 8 electrons, but 2 electrons.
Additionally, the H atom does not necessarily need to fill its valence shell when forming ionic bonds. For example in an HF molecule, the H atom is the cation, meaning it has given up its single valence electron to bond with the F atom. By doing this, the H atom does not have a full valence shell, but rather has no valence shell to even fill while the F atom gains an electron to fill its valence shell.
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