ConstantID

Neutron relative atomic mass

Atomic & nuclearMeasured

Ar(n) = 1.008 664 916 06(40)

Value
1.008 664 916 06
Standard uncertainty
4 × 10−10
Relative uncertainty
4.0 × 10−10 (397 parts per trillion)
Status
Measured
Symbol
Ar(n)
Source
CODATA 2022 via NIST · NIST entry

Use in a calculation

# neutron relative atomic mass, CODATA 2022 (NIST)
neutron_relative_atomic_mass = 1.00866491606  # standard uncertainty 4e-10

# SciPy: scipy.constants.physical_constants['neutron relative atomic mass']

What it is

The mass of the neutron, the electrically neutral nucleon, divided by the atomic mass constant (one twelfth of the mass of a carbon-12 atom). It has no 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).

neutron relative atomic mass: CODATA values 1998–2022 relative to the 2022 value, in parts per trillion, with one-standard-uncertainty bars−1000−5000+500+1000parts per trillion1998200220062010201420182022

Relative uncertainty

neutron relative atomic mass: relative standard uncertainty in each CODATA adjustment, logarithmic scale10⁻¹¹10⁻¹⁰10⁻⁹1998200220062010201420182022
CODATAValue (uncertainty)Relative uncertaintyChange from previous
20221.008 664 916 06(40)4.0 × 10−10 +0.2 σ of 2018
20181.008 664 915 95(49)4.9 × 10−10

“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).

Related constants

Atomic & nuclear λC,n Neutron Compton wavelength 1.319 590 903 82(67) × 10−15 m Atomic & nuclear μn/μe Neutron-electron magnetic moment ratio 1.040 668 84(24) × 10−3 Atomic & nuclear mn/me Neutron-electron mass ratio 1838.683 662 00(74) Atomic & nuclear gn Neutron g factor −3.826 085 52(90) Atomic & nuclear γn Neutron gyromagnetic ratio 1.832 471 74(43) × 108 s−1 T−1 Atomic & nuclear γn/2π Neutron gyromagnetic ratio in MHz/T 29.164 6935(69) MHz T−1 Atomic & nuclear μn Neutron magnetic moment −9.662 3653(23) × 10−27 J T−1 Atomic & nuclear μn/μB Neutron magnetic moment to Bohr magneton ratio −1.041 875 65(25) × 10−3