Neutron-proton mass difference
Atomic & nuclearMeasured
= 2.305 574 61(67) × 10−30 kg
- Value
- 2.305 574 61 × 10−30 kg
- Standard uncertainty
- 6.7 × 10−37 kg
- Relative uncertainty
- 2.9 × 10−7 (291 parts per billion)
- Status
- Measured
- Symbol
- Source
- CODATA 2022 via NIST · NIST entry
Use in a calculation
# neutron-proton mass difference, CODATA 2022 (NIST)
neutron_proton_mass_difference = 2.30557461e-30 # kg, standard uncertainty 6.7e-37
# SciPy: scipy.constants.physical_constants['neutron-proton mass difference']What it is
How much heavier the neutron is than the proton. The difference is larger than the electron mass, which is why a free neutron can decay into a proton, an electron and an antineutrino.
In other units
| Unit | Value | |
|---|---|---|
| CGS (g) | 2.305 574 61 × 10−27 | |
| u dalton / atomic mass unit | 0.001 388 449 48 | unit is itself measured |
| eV/c² electronvolt / c² | 1 293 332.51 | |
| MeV/c² MeV / c² | 1.293 332 51 | |
| GeV/c² GeV / c² | 0.001 293 332 51 |
Converted with ConstantID's unit table; digits are rounded to the precision of the value, and a long exact value is rounded only for display. The relative uncertainty is unchanged by conversion to an exactly defined unit.
How the value changed, 1998–2022
Each point is one CODATA adjustment, plotted as its difference from the 2022 value; the bars show one standard uncertainty (an arrowhead means the bar runs off the chart).
Relative uncertainty
| CODATA | Value (uncertainty) | Relative uncertainty | Change from previous |
|---|---|---|---|
| 2022 | 2.305 574 61(67) × 10−30 | 2.9 × 10−7 | +0.3 σ of 2018 |
| 2018 | 2.305 574 35(82) × 10−30 | 3.6 × 10−7 |
“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).