free neutron

A free neutron is a neutron that exists outside of an atomic nucleus. While neutrons can be stable when bound inside nuclei, free neutrons are unstable and decay with a lifetime of just under 15 minutes (885.7 ± 0.8 s). The only possible decay mode, via the weak nuclear force, is into a proton, an electron, and an electron antineutrino (antineutrino), the proton and electron forming a hydrogen atom:
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