Neutron Mass

Rest mass of the neutron, slightly exceeding the proton's — the margin that lets free neutrons decay.

Specifications & Metrics

symbol

mₙ

status

Measured (CODATA 2022)

value

1.67492750056 × 10⁻²⁷ ± 8.5 × 10⁻³⁷ kg

Relative Scale in Constants

Core Principles & Analysis

• Exceeds the proton's mass by 1.293 MeV/c², enough for free neutrons to beta-decay in about 15 minutes.
• Bound inside stable nuclei the decay is blocked energetically, which is why ordinary matter persists.
• The neutron-proton mass difference set the helium fraction produced in Big Bang nucleosynthesis.

Open in the Simulator — run this concept live