Isotopes of beryllium

Beryllium (₄Be) has 11 known isotopes and 3 known isomers, 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 9.0121831(5)). 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 10 radioisotopes of beryllium, the most stable are ¹⁰
Be
with a half-life of 1.387(12) million years and ⁷
Be
with a half-life of 53.22(6) d. All other radioisotopes have half-lives under 15 s, most under 30 milliseconds. The least stable isotope is ¹⁶
Be
, with a half-life of 650(130) yoctoseconds.

The 1:1 neutron–proton ratio seen in stable isotopes of many light elements (up to oxygen, and in elements with even atomic number up to calcium) 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 81.9(3.7) attoseconds.

Beryllium is prevented from having a stable isotope with 4 protons and 6 neutrons by the very large mismatch in neutron–proton ratio for such a light element. Nevertheless, this isotope, ¹⁰
Be
, has a half-life of 1.387(12) million years, which indicates unusual stability for a light isotope with such a large neutron/proton imbalance. Other possible beryllium isotopes have even more severe mismatches in neutron and proton number, and thus are even less stable.

Most ⁹
Be
in the universe is thought to be formed by cosmic ray nucleosynthesis from cosmic ray spallation in the period between the Big Bang and the formation of the Solar System. The isotopes ⁷
Be
, with a half-life of 53.22(6) d, and ¹⁰
Be
are both cosmogenic nuclides because they are made on a recent timescale in the Solar System by spallation, like ¹⁴
C
.