I am trying to wrap my head around the relationship between binding energy and mass defect. I have read that the difference between the binding energies of the products and reactants of a nuclear equation is equal to the energy equivalent of the mass defect. Using this, along with exact masses of a proton, electron and deuteron I have calculated the following:

mass difference = 7.49089 x 10^-31 kg

Converting this mass to energy via E = mc^2

E = 6.732476 x 10^-14 J = 0.420208 MeV

This value is far from the value I have managed to find on the internet, which states that the binding energy of deuteron is approximately 2.2 MeV.

Am I wrong in my knowledge that the binding energy is equivalent to the mass defect, or is there some other explanation for the disparity in the binding energy I calculated and the value found on the internet?

  • $\begingroup$ Deuteron consists of a proton and a neutron, while you say it is proton and electron - perhaps a typo. $\endgroup$ Oct 15, 2021 at 8:28

1 Answer 1


It'll be clearer if you first list the particles (with their mass) that a deuteron contains. Deuteron is a deuterium's nucleus which contains a proton and a neutron (not electron, although outside a nucleus, a neutron further decays to form a proton, electron and antineutrino. However, for now it's enough to consider just a neutron).

From wikipedia you can find easily the mass of these three particles:

M$_\text{proton} = 938.272$ MeV/$c^2$

M$_\text{neutron} = 939.565$ MeV/$c^2$

M$_\text{deuteron} = 1875.612$ MeV/$c^2$

With a simple arithmetic:

M$_\text{proton}$ $+$ M$_\text{neutron}$ $-$ M$_\text{deuteron}$ $= 938.272 + 939.565 - 1875.612 = 2.237 $ MeV/$c^2$, which is the number you're looking for.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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