Beryllium-8

Main isotopes of beryllium

iso NA half-life DM DE (MeV) DP 7Be trace 53.12 d ε 0.862 7Li γ 0.477 – 9Be 100% is stable with 5 neutrons 10Be trace 1.36×106 y β− 0.556 10B
Standard atomic weight (Ar)	7000901218310000000♠9.0121831(5)

Beryllium (₄Be) has 12 known isotopes, but only one of these isotopes (9
Be
) is stable and a primordial nuclide. As such, beryllium is considered a monoisotopic element. It is also a mononuclidic element, because its other isotopes have such short half-lives that none are primordial and their abundance is very low (standard atomic weight is 9.0122). Beryllium is unique as being the only monoisotopic element with both an even number of protons and an odd number of neutrons. There are 25 other monoisotopic elements but all have odd atomic numbers, and even numbers of neutrons.

Of the 11 radioisotopes of beryllium, the most stable are 10
Be
with a half-life of 1.39 million years and 7
Be
with a half-life of 53.22 days. All other radioisotopes have half-lives under 13.85 seconds, most under 0.03 seconds. The least stable isotope is 6
Be
, with a half-life measured as 5.03 × 10⁻²¹ seconds.

The natural light-element ratio of equal protons and neutron numbers is prevented in beryllium by the extreme instability of 8
Be
toward alpha decay, which is favored due to the extremely tight binding of 4
He
nuclei. The half-life for the decay of 8
Be
is only 6.7(17)×10⁻¹⁷ seconds.

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Wikipedia