Alpha particle mass energy equivalent

$m_αc²$ = 5.971 920 1997(19) × 10⁻¹⁰ J

Value: 5.971 920 1997 × 10⁻¹⁰ J
Standard uncertainty: 1.9 × 10⁻¹⁹ J
Relative uncertainty: 3.2 × 10⁻¹⁰ (318 parts per trillion)
Status: Measured
Symbol: $m_αc²$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# alpha particle mass energy equivalent, CODATA 2022 (NIST)
alpha_particle_mass_energy_equivalent = 5.9719201997e-10  # J, standard uncertainty 1.9e-19

# SciPy: scipy.constants.physical_constants['alpha particle mass energy equivalent']

What it is

The rest energy (mass × c²) of the alpha particle, the nucleus of helium-4 (two protons and two neutrons), in joules.

In other units

Unit	Value
CGS (cm² g s⁻²)	0.005 971 920 1997
keV kiloelectronvolt	3 727 379.4118
MeV megaelectronvolt	3727.379 4118
GeV gigaelectronvolt	3.727 379 4118
erg erg	0.005 971 920 1997

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	5.971 920 1997(19) × 10⁻¹⁰	3.2 × 10⁻¹⁰	+4.6 σ of 2018
2018	5.971 920 1914(18) × 10⁻¹⁰	3.0 × 10⁻¹⁰	+1.3 σ of 2014
2014	5.971 920 097(73) × 10⁻¹⁰	1.2 × 10⁻⁸	+1.6 σ of 2010
2010	5.971 919 67(26) × 10⁻¹⁰	4.4 × 10⁻⁸	+1.7 σ of 2006
2006	5.971 919 17(30) × 10⁻¹⁰	5.0 × 10⁻⁸	−0.2 σ of 2002
2002	5.971 9194(10) × 10⁻¹⁰	1.7 × 10⁻⁷	+0.9 σ of 1998
1998	5.971 918 97(47) × 10⁻¹⁰	7.9 × 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).