# Isotopes of beryllium

Beryllium ($_4$Be) 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 lopsided 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 .

## List of isotopes

| Nuclide | Z | N | Isotopic mass (Da) | Half-life [resonance width] | Decay mode | Daughter isotope | Spin and parity | Isotopic abundance |
| Isotopic abundance |||
| $^5$ Be | 4 | 1 | 5.03987(215)# | | p ? | $^4$ Li ? | (1/2+)# | |
| $^6$ Be | 4 | 2 | 6.019726(6) | 5.0(3) zs [91.6(5.6) keV] | 2p | $^4$ He | 0+ | |
| $^7$ Be | 4 | 3 | 7.01692871(8) | 53.22(6) d | ε | $^7$ Li | 3/2− | Trace |
| $^8$ Be | 4 | 4 | 8.00530510(4) | 81.9(3.7) as [5.58(25) eV] | α | $^4$ He | 0+ | |
| $^{8m}$ Be | 16626(3) keV | 16626(3) keV | 16626(3) keV | | α | $^4$ He | 2+ | |
| $^9$ Be | 4 | 5 | 9.01218306(8) | Stable | Stable | Stable | 3/2− | 1 |
| $^{9m}$ Be | 14390.3(1.7) keV | 14390.3(1.7) keV | 14390.3(1.7) keV | 1.25(10) as [367(30) eV] | | | 3/2− | |
| $^{10}$ Be | 4 | 6 | 10.01353469(9) | 1.387(12)×10$^6$ y | β$^−$ | $^{10}$ B | 0+ | Trace |
| $^{11}$ Be | 4 | 7 | 11.02166108(26) | 13.76(7) s | β$^−$ (96.7(1)%) | $^{11}$ B | 1/2+ | |
| $^{11}$ Be | 4 | 7 | 11.02166108(26) | 13.76(7) s | β$^−$α (3.3(1)%) | $^7$ Li | 1/2+ | |
| $^{11}$ Be | 4 | 7 | 11.02166108(26) | 13.76(7) s | β$^−$p (0.0013(3)%) | $^{10}$ Be | 1/2+ | |
| $^{11m}$ Be | 21158(20) keV | 21158(20) keV | 21158(20) keV | 0.93(13) zs [500(75) keV] | IT ? | $^{11}$ Be ? | 3/2− | |
| $^{12}$ Be | 4 | 8 | 12.0269221(20) | 21.46(5) ms | β$^−$ (99.50(3)%) | $^{12}$ B | 0+ | |
| $^{12}$ Be | 4 | 8 | 12.0269221(20) | 21.46(5) ms | β$^−$n (0.50(3)%) | $^{11}$ B | 0+ | |
| $^{12m}$ Be | 2251(1) keV | 2251(1) keV | 2251(1) keV | 233(7) ns | IT | $^{12}$ Be | 0+ | |
| $^{13}$ Be | 4 | 9 | 13.036135(11) | 1.0(7) zs | n ? | $^{12}$ Be ? | (1/2−) | |
| $^{13m}$ Be | 1500(50) keV | 1500(50) keV | 1500(50) keV | | | | (5/2+) | |
| $^{14}$ Be | 4 | 10 | 14.04289(14) | 4.53(27) ms | β$^−$n (86(6)%) | $^{13}$ B | 0+ | |
| $^{14}$ Be | 4 | 10 | 14.04289(14) | 4.53(27) ms | β$^−$ (> 9.0(6.3)%) | $^{14}$ B | 0+ | |
| $^{14}$ Be | 4 | 10 | 14.04289(14) | 4.53(27) ms | β$^−$2n (5(2)%) | $^{12}$ B | 0+ | |
| $^{14}$ Be | 4 | 10 | 14.04289(14) | 4.53(27) ms | β$^−$t (0.02(1)%) | $^{11}$ Be | 0+ | |
| $^{14}$ Be | 4 | 10 | 14.04289(14) | 4.53(27) ms | β$^−$α (< 0.004%) | $^{10}$ Li | 0+ | |
| $^{14m}$ Be | 1520(150) keV | 1520(150) keV | 1520(150) keV | | | | (2+) | |
| $^{15}$ Be | 4 | 11 | 15.05349(18) | 790(270) ys | n | $^{14}$ Be | (5/2+) | |
| $^{16}$ Be | 4 | 12 | 16.06167(18) | 650(130) ys [0.73(18) MeV] | 2n | $^{14}$ Be | 0+ | |
| This table header & footer: view |||||||||
* ↑ $^m$Be – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* ↑ Modes of decay: EC:Electron capture IT:Isomeric transition n:Neutron emission p:Proton emission
* ↑ Bold symbol as daughter – Daughter product is stable.
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* ↑ This isotope has not yet been observed; given data is inferred or estimated from periodic trends.
* 1 2 3 Decay mode shown is energetically allowed, but has not been experimentally observed to occur in this nuclide.
* ↑ Produced in Big Bang nucleosynthesis, but not primordial, as it all quickly decayed to $^7$Li
* 1 2 cosmogenic nuclide
* ↑ Intermediate product of triple alpha process in stellar nucleosynthesis as part of the path producing $^{12}$C
* ↑ Also often considered spontaneous fission, as $^8$ Be splits into two equal $^4$ He nuclei
* ↑ Has 1 halo neutron
* ↑ Has 4 halo neutrons

## 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 $^7$ Be is transferred from the air to the ground is controlled in part by the weather. $^7$ Be decay in the Sun is one of the sources of solar neutrinos, and the first type ever detected using the Homestake experiment. Presence of $^7$ Be 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 $^7$ Be .

## Beryllium-10

Beryllium-10 has a half-life of 1.39×10$^6$ 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. $^{10}$Be 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. $^{10}$Be 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, $^7$ Be 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 ($^7$Be@C$_{60}$). Also anomalous is $^8$ Be , which decays via alpha decay to $^4$ He . This alpha decay is often considered fission, which would be able to account for its extremely short half-life.
    ${\begin{array}{l}\\{\ce {^{5}_{4}Be->[{\ce {Unknown}}]{^{4}_{3}Li}+{^{1}_{1}H}}}\\{\ce {^{6}_{4}Be->[5\ {\ce {zs}}]{^{4}_{2}He}+{2_{1}^{1}H}}}\\{\ce {{^{7}_{4}Be}+e^{-}->[53.22\ {\ce {d}}]{^{7}_{3}Li}}}\\{\ce {^{8}_{4}Be->[81.9\ {\ce {as}}]{2_{2}^{4}He}}}\\{\ce {^{10}_{4}Be->[1.387\ {\ce {Ma}}]{^{10}_{5}B}+e^{-}}}\\{\ce {^{11}_{4}Be->[13.76\ {\ce {s}}]{^{11}_{5}B}+e^{-}}}\\{\ce {^{11}_{4}Be->[13.76\ {\ce {s}}]{^{7}_{3}Li}+{^{4}_{2}He}+e^{-}}}\\{\ce {^{12}_{4}Be->[21.46\ {\ce {ms}}]{^{12}_{5}B}+e^{-}}}\\{\ce {^{12}_{4}Be->[21.46\ {\ce {ms}}]{^{11}_{5}B}+{^{1}_{0}n}+e^{-}}}\\{\ce {^{13}_{4}Be->[1\ {\ce {zs}}]{^{12}_{4}Be}+{^{1}_{0}n}}}\\{\ce {^{14}_{4}Be->[4.53\ {\ce {ms}}]{^{13}_{5}B}+{^{1}_{0}n}+e^{-}}}\\{\ce {^{14}_{4}Be->[4.53\ {\ce {ms}}]{^{14}_{5}B}+e^{-}}}\\{\ce {^{14}_{4}Be->[4.53\ {\ce {ms}}]{^{12}_{5}B}+{2_{0}^{1}n}+e^{-}}}\\{\ce {^{15}_{4}Be->[790\ {\ce {ys}}]{^{14}_{4}Be}+{^{1}_{0}n}}}\\{\ce {^{16}_{4}Be->[650\ {\ce {ys}}]{^{14}_{4}Be}+{2_{0}^{1}n}}}\\\end{array}}$