Amongst SF₄, XeF₄, CF₄ and H₂O, the number of species with two lone pairs of electrons is _____________.

Solution

Chemistry ma’am · 2 steps

Count lone pairs on the central atom only: the electrons it keeps after making its bonds.

1. Counting rule. Each F or H forms one single bond, and the central atom puts one of its own valence electrons into each bond. The electrons it has left over pair up as lone pairs.

   lone pairs = (valence electrons of centre - number of bonds)/2

   Species	Valence electrons of centre	Bonds	Lone pairs	Shape
   SF₄	6 (S)	4	(6-4)/2 = 1	see-saw
   XeF₄	8 (Xe)	4	(8-4)/2 = 2	square planar
   CF₄	4 (C)	4	(4-4)/2 = 0	tetrahedral
   H₂O	6 (O)	2	(6-2)/2 = 2	bent

2. So. Only XeF₄ and H₂O have two lone pairs on the central atom, so the answer is 2.

The common slip: 3. Giving S in SF₄ two lone pairs, as in H₂O, gives 3, not 2. S uses 4 of its 6 electrons in bonds, so only one pair is left.

The idea to keep. In VSEPR questions, lone pairs always means lone pairs on the central atom. Subtract the bonding electrons from its valence electrons and halve what is left.

Answer from the NTA final answer key. Solution by PrepWiser.