The equation given is:
\[ \text{21H} + \text{21H} \rightarrow \text{42H} + \text{energy} \]
Each "H" represents a hydrogen atom. The notation "21H" indicates that each hydrogen atom has a mass number of 2 (which refers to the isotope deuterium, which consists of one proton and one neutron).
In the reaction, two atoms of "21H" combine to form one atom with a mass number of 2 (42H), which is actually a typo intended to indicate a single product of mass number 2 but is typically not how isotopes are represented.
Analyzing the statements:
- "There are two atoms with mass numbers of 2." - This is incorrect since each "21H" atom has a mass number of 2, but they are not separate atoms in the product.
- "There are two atoms with mass numbers of 1." – Incorrect; the mass number of each reactant is 2, not 1.
- "There is one atom with a mass number of 2." – This is accurate, as the product represents one atom with mass number 2.
- "There is one atom with a mass number of 1." – Incorrect; the mass number of all relevant atoms is 2.
Therefore, the best statement that describes the mass numbers of the atoms in the reaction is:
There is one atom with a mass number of 2.