Question

Boron is an exception to the octet rule and can take up to 10 electrons in its valence shell. True or false?

Answer 1

False. Boron only has three valance electrons which mean that at most it can only share with three other elements giving it an illusion of 6 valance.

Answer 2

Only atom in which the d orbitals are close to the Fermi energy (the energy of the valence electrons) can accomodate more than 8 electrons in the valence shell, because the participation of the d orbitals is necessary. That means no elements in Periods 1 and 2 (including boron) qualify.

Boron is an exception to the octet rule in that it is sometimes electron DEFICIENT, meaning it has fewer than 8 electrons in its valence shell. For example in boron trifluoride, BF3, boron shares only three pairs (six) valence electrons with the fluorines.

As for why it does this: this is a subtle question, but we adduce that boron is only barely nonmetallic at all -- forming covalent bonds -- and it is very small, with only 5 electrons held very closely to the nucleus. We can assume that any electrons it shares with other atoms will be held very close to the B nucleus, which means the electron-electron repulsion will be unusually high. Adding in all 8 electrons may result in so much electron-electron repulsion, without the stronger nuclear-electronic attraction you find to the right of boron in the table (e.g. in carbon, nitrogen, etc), that the molecules is unstable.