1. Identify semicarbazone formation

A semicarbazone is formed when a carbonyl compound reacts with semicarbazide.

Semicarbazide has the structure: $\mathrm{H_2N-CO-NH-NH_2}$

It contains 3 nitrogen atoms.

2. Reaction with acetone

Acetone is: $\mathrm{(CH_3)_2CO}$

On reaction with semicarbazide, it forms acetone semicarbazone: $\mathrm{(CH_3)_2C=NNHCONH_2}$

3. Count nitrogen atoms in the product

In $\mathrm{(CH_3)_2C=NNHCONH_2}$ there are:

• one nitrogen in $=N-$
• one nitrogen in $-NH-$
• one nitrogen in $-NH_2$

So total nitrogen atoms $= 3$.

4. Final answer

The number of nitrogen atoms in semicarbazone of acetone is: $\boxed{3}$

5. Comparison with stored answer

Stored correct answer = 3, which matches the derived answer.