Which of the following molecules contains a coordinate covalent bond?

Solution

1. The ammonia molecule ${\text{NH}}_3$ has alone pair on the nitrogen atom.
2. When ${\text{NH}}_3$ reacts with ${\text{H}}^{+}$, the nitrogen atom donates its lone pair to the hydrogen ion.
3. This sharing of alone pair from one atom to another is a coordinate covalent bond.
4. Hence the answer is (A).