Explanation to Octet Rule Exceptions [ENDORSED]

[Rohita Thammineni 2D]

Hi! Can someone explain why H, He, Li, and Be are exceptions to the octet rule? Is it simply because they are too small to hold that many electrons? In that case, why do the exceptions stop at Beryllium specifically? Thanks!

[Sanjana Sivakumar 2E]

Hello!

H, He, Li, and Be are exceptions because they all hold less than 8 electrons. More specifically, when H and He have full shells, they contain only 2 electrons. Also, since Li and Be are metals, they will most likely become cations (since usually metals become cations by losing an electrons). And when that happens, I think they then have a noble gas configuration (or something along the lines of that) so it will also have 2 electrons. I believe that there are more exceptions, but these are the main ones.

Hope this helps!

[Hannah Jin 1J]

H, He, Li, and B are exceptions because they don't have enough electrons, so 8 valence electrons would not make it more stable. Rather, these all hold 2 valence electrons to be satisfied. From my understanding, boron and aluminum are also exceptions to the octet rule (though different from the other 4 elements in terms of how many electrons they hold), but we did not cover it in the lecture (yet).

[Hayden Jackson]

You are correct they are just simply to small to achieve the octet which is why they are exceptions. There are some more exceptions when you get into bonding as certain atoms are able to go above 8 valence electrons to form covalent bonds.

[gracebinder3I]

Hi!
H and He are exceptions because the first shell (1s) only contains two electrons. Thus, one (1s1) or two (1s2) electrons in the first orbital completes the shell more than if there were 8 electrons in the first shell. It would make He and H more unstable if they abided by the octet rule. Li and Be are exceptions for the same reason but with the second shell (2s). The exceptions stop with beryllium because after then, the p-orbitals begin as to allow the octet rule to begin.
I hope this helps!

[Alyssa Cua 2J]

Hi!
H and He are exceptions because the first shell (1s) only contains two electrons. Thus, one (1s1) or two (1s2) electrons in the first orbital completes the shell more than if there were 8 electrons in the first shell. It would make He and H more unstable if they abided by the octet rule. Li and Be are exceptions for the same reason but with the second shell (2s). The exceptions stop with beryllium because after then, the p-orbitals begin as to allow the octet rule to begin.
I hope this helps!

Really good explanation! Thank you!

[Naman Jain 3F]

Second-row elements are always assumed to follow the octet rule, as their valence orbitals (2s and 2p) can only hold 8 electrons. Elements that exceed the octet rule are only for Period 3 (row 3) and beyond.

[Gavin Trent 2F]

The first 4 elements (H, He, Li, Be) are exceptions to the octet rule because they have very few valence electrons, and are therefore highly unlikely to form an octet by gaining or losing electrons.

[Santiago Chang 2K]

Hi!
H and He try to fill a 1s orbital, while Li and Be try to fill their 2s orbital. s-orbitals hold 2 electrons, rather than the 8 that is formed from an s- and a p- orbital together, and that is why they are an exception to the octet rule. I hope this helps!