The mass of an H-atom is less than the sum of the masses of a proton and electron. Why is this?
There is something called the strong nuclear force (strong interaction) that binds protons and neutrons (and other particles) together. This force overall reduces the energy of the system - energy released as heat. The binding energy (not of electrons to protons but of protons to neutrons) becomes greater as elements become heavier as you increase atomic mass until you reach iron, at which point it begins to decrease. This is why fusing hydrogen atoms together releases energy and splitting uranium does the same. so from the einstein's equation the binding force is responsible,
