Question

The number of lone pairs present in phosphonic acid (phosphorus acid) is .........

Hint:

When counting lone pairs, consider the bonding environment of each atom and the valency of the central atom.

Answer 1

Correct Answer: 6

Phosphonic acid, also known as phosphorus acid, has the chemical formula H₃PO₃. To determine the number of lone pairs in this molecule, we need to examine its structure:

1. Lewis Structure Analysis:
The central phosphorus (P) atom in H₃PO₃ is bonded to two hydroxyl (OH) groups and one doubly-bonded oxygen (O), with an additional lone pair. The bonds include two P-OH single bonds and one P=O double bond.

Element	Valence Electrons	Bonds	Lone Pairs
P	5	3	1
O (in OH)	3	1	2 (each)
O (double bond)	4	2	2
H	1	1	0

2. Lone Pair Calculation:
The phosphorus atom has 1 lone pair. Each of the two hydroxyl oxygen atoms has 2 lone pairs, and the double-bonded oxygen atom also holds 2 lone pairs. Therefore, the total number of lone pairs is:
1 (on P) + 2(2 on OH) + 1(2 on doubly-bonded O) = 1 + 4 + 2 = 6.

The number of lone pairs in phosphonic acid, therefore, is 6.