The mass of an atom is less than the sum of its parts. The difference in mass is called the mass defect, and E = mc^2 shows how much energy that difference in mass has. Since mass and energy interchange in nuclear reactions, mass and energy would go on different sides of the equation. Here is what nuclear fusion looks like [tex]^3_1H + ^2_1H + (mass) \rightarrow ^4_2He + ^1_0n + (energy)[/tex] The left side is heavier and the right side has more energy. If the components are heavier, how does nuclear fission give off energy? Breaking uranium into components should absorb energy shouldn't it?