Neutron mass
Atomic & nuclearMeasured
= 1.674 927 500 56(85) × 10−27 kg
- Value
- 1.674 927 500 56 × 10−27 kg
- Standard uncertainty
- 8.5 × 10−37 kg
- Relative uncertainty
- 5.1 × 10−10 (507 parts per trillion)
- Status
- Measured
- Symbol
- Source
- CODATA 2022 via NIST · NIST entry
Use in a calculation
# neutron mass, CODATA 2022 (NIST)
m_n = 1.67492750056e-27 # kg, standard uncertainty 8.5e-37
# SciPy: scipy.constants.physical_constants['neutron mass']What it is
The rest mass of the neutron. A free neutron is slightly heavier than a proton plus an electron, so it decays (half-life about ten minutes) into a proton, an electron and an antineutrino.
Equations
- rest energy
In other units
| Unit | Value | |
|---|---|---|
| CGS (g) | 1.674 927 500 56 × 10−24 | |
| u dalton / atomic mass unit | 1.008 664 916 | unit is itself measured |
| MeV/c² MeV / c² | 939.565 421 934 | |
| GeV/c² GeV / c² | 0.939 565 421 934 |
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 | 1.674 927 500 56(85) × 10−27 | 5.1 × 10−10 | +2.7 σ of 2018 |
| 2018 | 1.674 927 498 04(95) × 10−27 | 5.7 × 10−10 | +1.3 σ of 2014 |
| 2014 | 1.674 927 471(21) × 10−27 | 1.3 × 10−8 | +1.6 σ of 2010 |
| 2010 | 1.674 927 351(74) × 10−27 | 4.4 × 10−8 | +1.7 σ of 2006 |
| 2006 | 1.674 927 211(84) × 10−27 | 5.0 × 10−8 | −0.2 σ of 2002 |
| 2002 | 1.674 927 28(29) × 10−27 | 1.7 × 10−7 | +0.9 σ of 1998 |
| 1998 | 1.674 927 16(13) × 10−27 | 7.8 × 10−8 |
“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).