Neutron mass

Chadwick identified the neutron in 1932 from the recoil of protons knocked out of paraffin by an unknown neutral radiation, and nuclear physics acquired its second nucleon. Its mass is measured most precisely not by weighing it but by watching a neutron capture on hydrogen and measuring the 2.224 MeV deuteron binding gamma — mass by way of E = mc².

The neutron outweighs the proton by 1.293 MeV/c², which is more than the 0.511 MeV rest energy of an electron. That inequality is why free neutrons decay (n → p + e⁻ + ν̄, mean life about 879 s) while bound neutrons inside a stable nucleus do not, and why the early universe froze out roughly one neutron for every seven protons — the ratio that set the primordial helium abundance at 25 % by mass. Reverse the inequality by a few hundred keV and no stars, and no chemistry, would exist.