To determine the number of nitrogen atoms in one molecule of serotonin, we need to carefully analyze its chemical formula, which is [tex]\( C_{10}H_{12}N_2O \)[/tex].
The subscript numbers next to each chemical symbol in a molecular formula indicate the number of atoms of that element in a single molecule. Let’s break down the formula:
- [tex]\( C_{10} \)[/tex]: This indicates there are 10 carbon atoms.
- [tex]\( H_{12} \)[/tex]: This indicates there are 12 hydrogen atoms.
- [tex]\( N_2 \)[/tex]: This indicates there are 2 nitrogen atoms.
- [tex]\( O \)[/tex]: This indicates there is 1 oxygen atom.
Since we are specifically interested in the number of nitrogen atoms, we focus on the part of the formula denoted by [tex]\( N_2 \)[/tex]. The subscript ‘2’ following the ‘N’ indicates that there are 2 nitrogen atoms.
Thus, the number of nitrogen atoms in one molecule of serotonin is:
[tex]\[
\boxed{2}
\][/tex]
The correct answer is [tex]\( \text{A. 2 atoms} \)[/tex].
