Diproton

Most stable isotopes of helium

iso NA half-life DM DE (MeV) DP 3He 0.0002% is stable with 1 neutron 4He 99.9998% is stable with 2 neutrons
Standard atomic weight (Ar)	7000400260200000000♠4.002602(2)

Although there are nine known isotopes of helium (₂He) (standard atomic weight: 7000400260200000000♠4.002602(2)), only helium-3 (3
He
) and helium-4 (4
He
) are stable. All radioisotopes are short-lived, the longest-lived being 6
He
with a half-life of 7002806700000000000♠806.7 milliseconds. The least stable is 5
He
, with a half-life of 6978760000000000000♠7.6×10⁻²² s, although it is possible that 2
He
has an even shorter half-life.

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