JEE Main 2024 · 4 April · morning · Single correct · +4 / −1
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
- (a) 7
- (b) 3
- (c) 5
- (d) 6
Answer and solution
Answer (b) 3
Solution
Chemistry ma’am · 3 steps
A period 2 atom has only four orbitals, so it can never form more than four bonds. Check who needs more.
- Group valency. For groups 13 to 17 the group valency is the number of valence electrons: 3, 4, 5, 6 and 7. To show it, the atom must form that many bonds.
- Period 2 limit. B, C, N, O and F have only the 2s and three 2p orbitals and no d orbitals. That caps their covalency at 4. Period 3 atoms (Al, Si, P, S) can use 3d orbitals and expand beyond 4.
Group Group valency Period 2 element Period 3 element 13 3 B: yes, BCl₃ Al: yes, AlCl₃ 14 4 C: yes, CH₄ Si: yes, SiCl₄ 15 5 N: no, maximum 4 P: yes, PCl₅ 16 6 O: no, maximum 4 S: yes, SF₆ 17 7 F: no, maximum 4 not in the list - So. Only N, O and F cannot reach their group valency, so the count is 3, option (b).
The idea to keep. Second-period elements top out at a covalency of 4 because they have no d orbitals. B and C fit under that cap. N, O and F do not, while their heavier group members can.
Answer from the NTA final answer key. Solution by PrepWiser.