ConstantID

Alpha particle relative atomic mass

Atomic & nuclearMeasured

Ar(α) = 4.001 506 179 129(62)

Value
4.001 506 179 129
Standard uncertainty
6.2 × 10−11
Relative uncertainty
1.5 × 10−11 (15 parts per trillion)
Status
Measured
Symbol
Ar(α)
Source
CODATA 2022 via NIST · NIST entry

Use in a calculation

# alpha particle relative atomic mass, CODATA 2022 (NIST)
alpha_particle_relative_atomic_mass = 4.001506179129  # standard uncertainty 6.2e-11

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

What it is

The mass of the alpha particle, the nucleus of helium-4 (two protons and two neutrons), 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).

alpha particle relative atomic mass: CODATA values 1998–2022 relative to the 2022 value, in parts per trillion, with one-standard-uncertainty bars−40−200+20+40parts per trillion1998200220062010201420182022

Relative uncertainty

alpha particle relative atomic mass: relative standard uncertainty in each CODATA adjustment, logarithmic scale10⁻¹²10⁻¹¹10⁻¹⁰1998200220062010201420182022
CODATAValue (uncertainty)Relative uncertaintyChange from previous
20224.001 506 179 129(62)1.5 × 10−11 +0.0 σ of 2018
20184.001 506 179 127(63)1.6 × 10−11

“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 mα/me Alpha particle-electron mass ratio 7294.299 541 71(17) Atomic & nuclear mα Alpha particle mass 6.644 657 3450(21) × 10−27 kg Atomic & nuclear mαc2 Alpha particle mass energy equivalent 5.971 920 1997(19) × 10−10 J Atomic & nuclear mαc2 Alpha particle mass energy equivalent in MeV 3727.379 4118(12) MeV Atomic & nuclear mα Alpha particle mass in u 4.001 506 179 129(62) u Atomic & nuclear M(α) Alpha particle molar mass 4.001 506 1833(12) × 10−3 kg mol−1 Atomic & nuclear mα/mp Alpha particle-proton mass ratio 3.972 599 690 252(70) Atomic & nuclear rα Alpha particle rms charge radius 1.6785(21) × 10−15 m