How do the products of a nuclear fusion reaction differ from reactants?

The products of nuclear fusion are by far heavier than the reactants. Nuclear fusion is the combination of small atomic nuclei into larger ones.This reaction is accompanied with the release of a large amount of energy:

        ²₁H     +           ²₁H       →            ⁴₂He            +          energy

In the above reaction,

  two hydrogen atoms fuse and produce a heavier helium atom which differs from the combining atoms.

Explanation:

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