What is the mass difference of an atomic nucleus?
That's where the POWER of nuclear reactions comes from!
In fission, splitting atoms are releasing the energy that held the two parts together. In fusion they are releasing the net energy gained by forming a more stable element.
From Einstein's famous equation we know that energy is related to mass by the speed of light, and the energy of photons.
Thus, fusion is not simply "adding" masses, but a nuclear-chemical change of two lighter elements into a heavier one - slightly less massive than the combination of the two lighter ones, That difference in mass is converted into energy, releasing even more energy per combination than a corresponding fission 'split' of an element.
The mass difference between a nucleus and its component parts is called binding energy.
Atomic nuclei are made up of protons and neutrons. The mass of the nucleus is less than the combined masses of the equivalent number of protons and neutrons. This is the binding energy which stops the nucleus from breaking up due to the electrostatic repulsion between protons.
In the case of Hydrogen fusion in the Sun, 4 protons fuse into Helium-4. The difference in mass between a Helium-4 nucleus and 4 protons is about 28.3MeV. Helium-4 is very stable because the binding energy would need to be replaced to split the nucleus.
The fusion of 4 protons into Helium-4 is very efficient. Most of the missing mass is converted into energy.
As part of the process, 2 of the protons get converted into neutrons. The weak force converts a proton into a neutron, a positron and an electron neutrino. The positron quickly annihilates with an electron releasing energy. The only binding energy mass which doesn't get converted into energy is the small amount in the 2 electron neutrinos.