4 ms·

I assume that is the mass of an unbound proton.

• Maursault 5y ago
  A proton in a hydrogen atom does not have the same mass as a proton in a Helium atom, nor do any protons in any particular element have the same mass as the protons in any of the other elements. Again, the mass of a proton varies.
  • shirleyquirk 5y ago
    Why does it vary? Changes in gluon/quark energy due to nucleon-nucleon interactions?
    • Maursault 5y ago
      Yes, in nucleons of different sizes, the amount of binding energy changes according to quantum mechanics. The result is the average mass per proton is different in different isotopes. The mass differences are explained by the energy differences. Anyway, idk about you, but this really bothers me.