To analyze the given equation:
\( 21H + 21H \rightarrow 42H + \text{energy} \)
- \( 21H \) represents deuterium (an isotope of hydrogen) which has a mass number of 2.
- When two deuterium nuclei combine, they can form a helium nucleus denoted as \( 42H \) (where the notation represents the combined nucleons).
In the context of the options provided:
- Each \( 21H \) has a mass number of 2.
- After fusion, \( 42H \) (which can be interpreted as a helium nuclei) results, having a mass number of 4.
Given this breakdown, here is how each statement relates to the reaction:
- There is one atom with a mass number of 2. - Incorrect. There are two atoms of \( 21H \).
- There is one atom with a mass number of 1. - Incorrect. There are no atoms in the reaction with a mass number of 1.
- There are two atoms with mass numbers of 2. - Correct. The two \( 21H \) reactants each have a mass number of 2.
- There are two atoms with mass numbers of 1. - Incorrect. Again, there are no atoms with a mass number of 1.
Therefore, the best statement that describes the mass numbers of the atoms in the reaction is:
There are two atoms with mass numbers of 2.