Beryllium (4Be) has 11 known isotopes and 3 known isomers, but only one of these isotopes (9Beryllium) 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 9.012). 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 10Beryllium with a half-life of 1.387 million years and 7Beryllium with a half-life of 53.22 days. All other radioisotopes have half-lives under 15 seconds, most under 30 milliseconds. The least stable isotope is 16Beryllium, with a half-life of 650 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 8Beryllium toward alpha decay, which is favored due to the extremely tight binding of [[Helium|4Helium]] nuclei. The half-life for the decay of 8Beryllium is only 81.9 attoseconds. Beryllium is prevented from having a stable isotope with 4 protons and 6 neutrons by the very lopsided neutron–proton ratio for such a light element. Nevertheless, this isotope, [[Beryllium|10Beryllium]], has a half-life of 1.387 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 9Beryllium 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 7Beryllium, with a half-life of 53.22 d, and 10Beryllium are both cosmogenic nuclides because they are made on a recent timescale in the Solar System by spallation, like [[Carbon|14Carbon]].

List of isotopes

Beryllium-7

Beryllium-7 is an isotope with a half-life of 53.3 days that is generated naturally as a cosmogenic nuclide. The rate at which the short-lived 7Beryllium is transferred from the air to the ground is controlled in part by the weather. 7Beryllium decay in the Sun is one of the sources of solar neutrinos, and the first type ever detected using the Homestake experiment. Presence of 7Beryllium in sediments is often used to establish that they are fresh, i.e. less than about 3–4 months in age, or about two half-lives of 7Beryllium.

Beryllium-10

Beryllium-10 has a half-life of 1,390,000 y, and decays by beta decay to stable boron-10 with a maximum energy of 556.2 keV. It is formed in the Earth's atmosphere mainly by cosmic ray spallation of nitrogen and oxygen. 10Be and its daughter product have been used to examine soil erosion, soil formation from regolith, the development of lateritic soils and the age of ice cores. 10Be is a significant isotope used as a proxy data measure for cosmogenic nuclides to characterize solar and extra-solar attributes of the past from terrestrial samples.

Decay chains

Most isotopes of beryllium within the proton/neutron drip lines decay via beta decay and/or a combination of beta decay and alpha decay or neutron emission. However, 7Beryllium decays only via electron capture, a phenomenon to which its unusually long half-life may be attributed. Notably, its half-life can be artificially lowered by 0.83% via endohedral enclosure (7Be@C60). Also anomalous is 8Beryllium, which decays via alpha decay to 4Helium. This alpha decay is often considered fission, which would be able to account for its extremely short half-life.