The mass number of reactants in fission and fusion are different because the processes themselves are different:

Fission:

* Reactant: A heavy atom (e.g., Uranium-235)

* Mass Number: Typically in the range of 230-250.

* Process: A heavy atom is split into two or more lighter atoms, releasing a tremendous amount of energy.

Fusion:

* Reactants: Two light atoms (e.g., Deuterium and Tritium)

* Mass Number: Typically in the range of 2-6.

* Process: Two light nuclei combine to form a heavier nucleus, releasing a tremendous amount of energy.

Key Point: In both fission and fusion, the total mass number of the products is slightly less than the total mass number of the reactants. This difference in mass is converted into energy, as described by Einstein's famous equation E=mc².