Octet Exceptions

[705170809]

I know that H, He, and B are exceptions to the octet rule, but I don't understand why they are exceptions.

[Kathryn 1F]

H and He are exceptions because they only have the 1s orbitals, so a max of 2 electrons. Therefore, it is impossible for them to form an octet.

B is an exception because it only has 3 valence electrons, so generally only forms 3 bonds (for 6 electrons), such as in BF3. However, it can form 4 bonds, but will be unstable, such as with [BF4]-.

[caseygilles 1E]

They are exceptions because they do not have enough orbitals or electrons to fill an octet and therefore can only fill a duplet. Other exceptions to the octet rule are elements in the third period and below, which can have expanded octets because of their higher energy orbitals (example: SF6 and PCl5). Other types of exceptions occur in chromium and Cooper, in which filling the 3d orbital to half shell and full shell respectively is more stable than filling the 4s orbital all the way. Hope this helps!

[KHuang1L]

H and He are exceptions because they would rather get two valence electrons to fill the 1st shell or 1s shell. B is also an exception because it is between Br, which prefers to lose two electrons to have two valence electrons, and C, which prefers to gain electrons to have a full octet.