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When listening to the lecture, professor mentioned that the exceptions to the octet rule include H, He, Be, and Li. I was wondering if anyone could explain why these are exceptions to the octet rule. Is it because of the low number of valence electrons? Thanks in advance!
Valence electrons are a big part of the reason they do not need to have an octet in bonding. Looking at the periodic table we can see that B and Al are in the np^1 configuration. This makes it harder for them to gain enough 8 electrons and instead they often bond with 6. Also, the octet rule is not a strict guideline that will always be followed. It is just a pattern that is based on trends seen in bonding. We see elements with expanded octets as well. Hope this helps!
H, He, Li, and Be are exceptions to the octet rule because their electron configurations do not reach the 2p orbitals. This means that they only have the 1s and 2s orbitals and do not have the capacity to have 8 electrons in their valence shells.
As mentioned above, H, He, Li, and Be are exceptions to the octet rule because they only consist of the s-orbital. Since H and He only contain the 1s orbital, and Li and Be only contain the 1s and 2s orbitals, none of these elements can hold 8 valence electrons. Without containing the p orbitals, these elements have no way of reaching the full 8 valence electrons, so that excludes them from the octet rule.
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