Question

why octet rule is not applicable on BF3 .

Answer 1

With an odd number of electrons, at least one atom in the molecule will have to violate the octet rule.

Answer 2

Octet rule simply means that the central atom (in a molecule) must be surrounded with 8 valence electrons.

Let me explain why BF₃ doesn't follow the octet rule in comparison with NH₃ which follows it.

So, Usually, atoms without octet configuration (all except noble gases) for example Nitrogen try to share electrons with other atoms so as to attain octet configuration.

They do so, to attain stability.

Noble gases already have octet configuration which is reason why they are so inert or stable.

Ammonia (NH₃) is formed by Nitrogen by sharing its electrons mutually with Hydrogen. After formation of Ammonia, there are 8 electrons in total, around the central Nitrogen atom in NH₃ as shown in the picture below.

Thus NH₃ follows the octet rule.

While in BF₃, after it formation, there are only 6 electrons around the central atom 'Boron'; which means it does not follow the octet rule.

Hope it helps!!