ConstantID

Proton-neutron mass ratio

Atomic & nuclearMeasured

mp/mn = 0.998 623 477 97(40)

Value
0.998 623 477 97
Standard uncertainty
4 × 10−10
Relative uncertainty
4.0 × 10−10 (401 parts per trillion)
Status
Measured
Symbol
mp/mn
Source
CODATA 2022 via NIST · NIST entry

Use in a calculation

# proton-neutron mass ratio, CODATA 2022 (NIST)
proton_neutron_mass_ratio = 9.9862347797e-1  # standard uncertainty 4e-10

# SciPy: scipy.constants.physical_constants['proton-neutron mass ratio']

What it is

The mass of the proton divided by the mass of the neutron. Ratios like this are often measured more precisely than either quantity on its own, which is why CODATA lists them.

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).

proton-neutron mass ratio: CODATA values 1998–2022 relative to the 2022 value, in parts per trillion, with one-standard-uncertainty bars−1500−1000−5000+500+1000+1500parts per trillion1998200220062010201420182022

Relative uncertainty

proton-neutron mass ratio: relative standard uncertainty in each CODATA adjustment, logarithmic scale10⁻¹¹10⁻¹⁰10⁻⁹1998200220062010201420182022
CODATAValue (uncertainty)Relative uncertaintyChange from previous
20220.998 623 477 97(40)4.0 × 10−10 −0.3 σ of 2018
20180.998 623 478 12(49)4.9 × 10−10 −0.6 σ of 2014
20140.998 623 478 44(51)5.1 × 10−10 +0.4 σ of 2010
20100.998 623 478 26(45)4.5 × 10−10 +0.0 σ of 2006
20060.998 623 478 24(46)4.6 × 10−10 −0.8 σ of 2002
20020.998 623 478 72(58)5.8 × 10−10 +0.3 σ of 1998
19980.998 623 478 55(58)5.8 × 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 e/mp Proton charge to mass quotient 9.578 833 1430(30) × 107 C kg−1 Atomic & nuclear λC,p Proton Compton wavelength 1.321 409 853 60(41) × 10−15 m Atomic & nuclear mp/me Proton–electron mass ratio 1836.152 673 426(32) Atomic & nuclear gp Proton g factor 5.585 694 6893(16) Atomic & nuclear γp Proton gyromagnetic ratio 2.675 221 8708(11) × 108 s−1 T−1 Atomic & nuclear γp/2π Proton gyromagnetic ratio in MHz/T 42.577 478 461(18) MHz T−1 Atomic & nuclear μp Proton magnetic moment 1.410 606 795 45(60) × 10−26 J T−1 Atomic & nuclear μp/μB Proton magnetic moment to Bohr magneton ratio 1.521 032 202 30(45) × 10−3