Number of elements from the following that CANNOT form compounds with valencies which match with their respective group valencies is ______. B, C, N, S, O, F, P, Al, Si

We need elements that CANNOT form compounds with valencies matching their group valencies.

Group valencies: B(3), C(4), N(5), S(6), O(6), F(7), P(5), Al(3), Si(4)

Elements that cannot exhibit their group valency:

- N (Group 15, valency 5): Cannot form NCl₅ as it lacks d-orbitals. Cannot show valency 5 in covalent compounds. ✓

- O (Group 16, valency 6): Cannot show valency 6 due to lack of d-orbitals. ✓

- F (Group 17, valency 7): Cannot show valency 7 as it's the most electronegative element and has no d-orbitals. ✓

All other elements can exhibit their group valency. So 3 elements cannot.

The correct answer is Option 2: 3.