I was told the mass defect of an atom is the difference in mass of the atom versus it's particles, for example:

carbon 12 is less massive than 6 protons + 6 neutrons + 6 electrons.

this negative difference means carbon has a negative mass defect, thus energy is released when carbon is formed, or energy is required to split carbon into it's particles.

more mass -> less mass + energy



for deuterium + deuterium -> either (neutron + He-3) or (proton + tritium), I would assume that a less massive product would also yield more energy. In these reactions, the reactants are the same, but I'm reading that the He-3 reaction generates 3.27 MeV and the tritium reaction generates 4.03 MeV. (image bellow from this link). is this value per nucleon or something?

enter image description here

Since the tritium reaction yields more energy, how can it be that tritium is heavier (more massive) than He-3?

Tritium: 3.0160492 u
He-3:    3.0160293 u

If tritium has more mass than He-3, shouldn't that reaction produce less energy?

Is the extra proton and neutron in each reaction weighed differently?


1 Answer 1


The quoted energy release value is per overall reaction.

Note that tritium is two neutrons plus a proton while He-3 is two protons plus a neutron. Neutrons are very slightly heavier than protons which means that tritium doesn't weigh the same as He-3.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.