Concept:Maximum covalency of an element equals the number of valence orbitals available for bonding; it can exceed the group valency if empty orbitals are present.
Explanation:- Aluminium has nine valence orbitals (3s, three 3p, five 3d), not five.
Option A is false.
- Boron has three valence electrons and an empty 2p orbital.
It can accept a lone pair to form four bonds (e.g.,
BF4−​), so its maximum covalency is four.
Option B is correct.
- Beryllium has four valence orbitals (2s and three 2p), not three.
Its maximum covalency is only two.
Option C is false.
- Magnesium can expand its octet using 3d orbitals, giving a maximum covalency of six (e.g.,
[Mg(NH3​)6​]2+), not four.
Option D is false.
Answer:Option B: Boron has a maximum covalency of four.
