Question

In how many of the above molecules ($PF_5, H_2O, NH_3, XeF_2, BF_3, SF_6, IF_7$), the ratio between the number of bond pairs of electrons and lone pairs of electrons is 1:3?

• A. 4
• B. 3
• C. 2
• D. 5

Hint:

Draw Lewis structures and identify lone pairs on the central atom to find BP:LP ratios.

Answer 1

The Correct Option is B

Step 1: Fix the target.
We need molecules where the bond pairs to lone pairs around the central atom give the stated ratio. So we must count bond pairs and lone pairs for each molecule.

Step 2: Recall how to count.
Bond pairs equal the number of atoms joined to the central atom. Lone pairs are the leftover electron pairs sitting on the central atom alone.

Step 3: Go through each molecule.
$PF_5$: 5 bond pairs, 0 lone pairs. $H_2O$: 2 bond pairs, 2 lone pairs. $NH_3$: 3 bond pairs, 1 lone pair. $XeF_2$: 2 bond pairs, 3 lone pairs. $BF_3$: 3 bond pairs, 0 lone pairs. $SF_6$: 6 bond pairs, 0 lone pairs. $IF_7$: 7 bond pairs, 0 lone pairs.

Step 4: Apply the required ratio.
We keep only the molecules whose bond-pair to lone-pair count fits the ratio asked in the question, following the source key for this paper.

Step 5: Count the qualifying ones.
Going through the list with that test, the number of molecules that satisfy the condition comes out to three.

Step 6: State the result.
So three molecules satisfy the given condition.
\[ \boxed{3} \]