Why boron has 3 electrons of valency instead of 5????

Question

the electronic configuration of boron is 1s^2 2s^2p^1 so in the last layer is missing 5 electrons to fullfil to ...2p^6 and there would be 5 valency electrons. where am i wrong???

Answer 1

Atoms are basically trying to achieve a full outer electron layer (keep it simple by thinking 1st layer = 2 electrons, 2nd layer = 8 electrons, etc.)
If B is going to form bonds with ionic character, it will lose the 3 electrons - this takes less energy than attracting an extra 5 electrons.
If it is going to form bonds with covalent character, you need to remember that a covalent bond is a shared pair of electrons. B only has 3 outer electrons that can be paired off. This means it can only have a valency of 3.

Answer 2

The energy required to lose the 3 electrons in level 2 is less than the energy required to attract 5 more electrons to complete 2p6