The number of nitrogen atoms in a semicarbazone molecule of acetone is ___.

Semicarbazone is formed when a ketone (or aldehyde) reacts with semicarbazide (H₂N-NH-CO-NH₂) in a condensation reaction, releasing water.

Acetone is (CH₃)₂CO. The reaction of acetone with semicarbazide is:

$$(CH_3)_2CO + H_2N-NH-CO-NH_2 \rightarrow (CH_3)_2C=N-NH-CO-NH_2 + H_2O$$

The semicarbazone of acetone is: $$(CH_3)_2C=N-NH-CO-NH_2$$

Now count the nitrogen atoms in this molecule: the first N is in the C=N group (imine/azomethine nitrogen), the second N is in the -NH- group, and the third N is in the terminal -NH₂ group of the carbamoyl part.

Total nitrogen atoms = 3.

Therefore, the number of nitrogen atoms in the semicarbazone of acetone is $$\boxed{3}$$.