The Actual Numbers You Need
I spent way too many hours watching people mix up atomic mass units with kilograms when they didn't need to, so let me just lay out the three masses cleanly before anything else. The proton sits at 1.67262192 × 10^-27 kg, which is also 1.00727647 u or roughly 938.272 MeV/c². The neutron is slightly heavier at 1.67492749 × 10^-27 kg, or 1.00866492 u, or about 939.565 MeV/c². The electron is where most people get tripped up because it's so small compared to the nucleons — 9.10938370 × 10^-31 kg, which is 5.48579909 × 10^-4 u, or roughly 0.510999 MeV/c². If you need the ratio, the neutron is about 1.001378 times heavier than the proton, and the proton is about 1836 times heavier than the electron. That last number comes up constantly in plasma physics and quantum chemistry calculations.
Mass Of Proton Electron Neutron
The three values listed above are your baseline. CODATA 2018 is the standard reference most people cite, though 2022 adjustments have made minor tweaks to the last few significant figures. Unless you're doing something at the level of few-body QED calculations, the 2018 values are more than sufficient and you won't run into any practical issues with them. Here's where people actually hit trouble in practice. I was running a mass defect calculation for a project a few years ago involving light nuclei, and I kept getting binding energy values that were consistently off by about 0.3%. Turns out I'd been using the bare nucleon masses and forgetting that the neutron-proton mass difference matters when you're computing beta decay thresholds. The neutron is heavier than the proton by about 1.293 MeV/c², and that difference determines whether a nucleus can undergo beta-minus or beta-plus decay at all. I recalculated everything using the mass excess values from the AME2020 table instead, and the results came into alignment immediately. Using mass excess or atomic mass tables directly rather than plugging in bare nucleon masses is the real workaround here. It saves you from propagating rounding errors across multiple calculations. The deeper issue nobody talks about is that the "mass" of a proton or neutron isn't really a fixed number in the way you might think. Most of the proton's mass comes from the gluon field energy and quark kinetic energy inside it, not from the Higgs mechanism giving the constituent quarks their bare mass. The three valence quarks (two up, one down) only contribute about 9 MeV/c² of the proton's total 938 MeV/c². The rest is QCD binding energy. This matters when you're working at high precision in nuclear physics or when you're interpreting results from lattice QCD simulations. For regular chemistry or introductory physics, it's a fun fact. For anything involving precise mass spectrometry or nuclear reaction cross-sections, it's the thing that will bite you if you ignore it.
Another common pitfall: the electron mass in atomic mass unit calculations. When you look up the mass of an atom on a periodic table, that's the atomic mass, which includes the electrons. But if you're doing nuclear physics calculations and you use the atomic mass of hydrogen (which includes the electron) as a proxy for the proton mass, you introduce a small but real error. The hydrogen-1 atomic mass is 1.007825 u, and subtracting the electron mass of 0.000549 u gives you roughly 1.007276 u for the proton. The difference is tiny in absolute terms, but in high-precision work involving large numbers of atoms, it compounds. I once saw a student's thesis reject a hypothesis because they'd neglected the electron mass correction across a dataset of several thousand samples. It was an 0.05% error that accumulated enough to shift their regression line outside the confidence interval. The free neutron is also unstable, decaying with a half-life of about 10.2 minutes into a proton, electron, and electron antineutrino. That decay energy comes from the mass difference between the neutron and the sum of its decay products. The Q-value is about 0.782 MeV. Inside a nucleus, the neutron can be stable if the resulting nucleus has lower total mass, which is why you see stable neutrons in everything from carbon-12 to uranium-238. The same neutron, outside a nucleus, falls apart in minutes. This is another place where beginners get confused and think the neutron mass value is inconsistent between sources. It's not. The mass is the same. The stability depends entirely on the nuclear environment. For most people reading this, the takeaway is straightforward. Use the CODATA values. If you're doing chemistry calculations with molar masses, use the atomic mass unit scale and look up values from a standard table like the one published by IUPAC. If you're doing nuclear physics, work in MeV/c² and use mass excess values from the AME tables. Don't convert between units mid-calculation unless you have to — each conversion introduces another opportunity for rounding error. And remember that the mass of a bound nucleus is always less than the sum of its parts, which is where nuclear binding energy comes from, and that mass defect is what powers everything from stars to nuclear reactors.
Get the Full Details
If you need a quick reference sheet, the NIST website publishes the CODATA recommended values with full uncertainty budgets. It's not the most exciting read, but it's the source most other sources trace back to. I keep it bookmarked and go there whenever I'm unsure about which edition a paper is citing or whether a value has been updated.
