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.

Isotopes of beryllium (₄Be)

Main isotopes[1] Decay abun­dance half-life (t1/2) mode pro­duct 7Be trace 53.22 d ε 7Li 8Be synth 81.9 as α 4He 9Be 100% stable 10Be trace 1.387×106 y β− 10B
Standard atomic weight Ar°(Be)
	9.0121831±0.0000005[2] 9.0122±0.0001 (abridged)[3]
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Of the 10 radioisotopes of beryllium, the most stable are ¹⁰
Be with a half-life of 1.387(12) million years^{[nb 1]} 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,^{[nb 1]} 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,^[4] like ¹⁴
C.

List of isotopes

Nuclide [n 1]	Z	N	Isotopic mass (Da)[5] [n 2][n 3]	Half-life[1] [resonance width]	Decay mode[1] [n 4]	Daughter isotope [n 5]	Spin and parity[1] [n 6]	Isotopic abundance
	Excitation energy
6 Be[n 7]	4	2	6.019726(6)	5.0(3) zs [91.6(5.6) keV]	2p	4 He	0+
7 Be[n 8]	4	3	7.01692871(8)	53.22(6) d	ε	7 Li	3/2−	Trace[n 9]
8 Be[n 10]	4	4	8.00530510(4)	81.9(3.7) as [5.58(25) eV]	α[n 11]	4 He	0+
8m Be	16626(3) keV				α	4 He	2+
9 Be	4	5	9.01218306(8)	Stable			3/2−	1
9m Be	14390.3(1.7) keV			1.25(10) as [367(30) eV]			3/2−
10 Be	4	6	10.01353469(9)	1.387(12)×106 y[nb 1]	β−	10 B	0+	Trace[n 9]
11 Be[n 12]	4	7	11.02166108(26)	13.76(7) s	β− (96.7(1)%)	11 B	1/2+
					β−α (3.3(1)%)	7 Li
					β−p (0.0013(3)%)	10 Be
11m Be	21158(20) keV			0.93(13) zs [500(75) keV]	IT ?[n 13]	11 Be ?	3/2−
12 Be	4	8	12.0269221(20)	21.46(5) ms	β− (99.50(3)%)	12 B	0+
					β−n (0.50(3)%)	11 B
12m Be	2251(1) keV			233(7) ns	IT	12 Be	0+
13 Be	4	9	13.036135(11)	1.0(7) zs	n ?[n 13]	12 Be ?	(1/2−)
13m Be	1500(50) keV						(5/2+)
14 Be[n 14]	4	10	14.04289(14)	4.53(27) ms	β−n (86(6)%)	13 B	0+
					β− (> 9.0(6.3)%)	14 B
					β−2n (5(2)%)	12 B
					β−t (0.02(1)%)	11 Be
					β−α (< 0.004%)	10 Li
14m Be	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+
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1. ^mBe – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. Modes of decay:

   EC:	Electron capture
   IT:	Isomeric transition
   n:	Neutron emission
   p:	Proton emission

5. Bold symbol as daughter – Daughter product is stable.
6. ( ) spin value – Indicates spin with weak assignment arguments.
7. Intermediate in the proton–proton chain
8. Produced in Big Bang nucleosynthesis, but not primordial, as it all quickly decayed to ⁷Li
9. cosmogenic nuclide
10. Intermediate product of triple alpha process in stellar nucleosynthesis as part of the path producing ¹²C
11. Also often considered spontaneous fission, as ⁸
    Be splits into two equal ⁴
    He nuclei
12. Has 1 halo neutron
13. Decay mode shown is energetically allowed, but has not been experimentally observed to occur in this nuclide.
14. 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.^[4] 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.^[6]

The rate of delivery of ⁷
Be from the air to the ground in Japan^[6]

Beryllium-8

Main article: Beryllium-8

Beryllium-8 decays into two alpha particles with an extremely short half-life of 8.19×10⁻¹⁷ seconds, a consequence of its total ground-state energy being ~92 keV greater than that of two alpha particles. This is unusual among light N = Z nuclides and creates a bottleneck in stellar nucleosynthesis, which must be bypassed by the fusion of three alpha particles to form stable carbon-12.

Beryllium-10

Main article: Beryllium-10

Plot showing variations in solar activity, including variation in ¹⁰Be concentration which varies inversely with solar activity. (Note that the beryllium scale is inverted, so increases on this scale indicate lower beryllium-10 levels).

Beryllium-10 has a half-life of 1.39×10⁶ y, and decays by beta decay to stable boron-10 with a maximum energy of 556.2 keV.^[7][8] It is formed in the Earth's atmosphere mainly by cosmic ray spallation of nitrogen and oxygen.^{[9][10][4] 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.^{[11] 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.^[12]

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 (⁷Be@C₆₀).^[13] 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.

${\displaystyle {\begin{array}{l}{}\\{\ce {^{6}_{4}Be->[5\ {\ce {zs}}]{^{4}_{2}He}+{2_{1}^{1}H}}}\\{\ce {{^{7}_{4}Be}+e^{-}->[53.22\ {\ce {d}}]{^{7}_{3}Li}}}\\{\ce {^{8}_{4}Be->[81.9\ {\ce {as}}]{2_{2}^{4}He}}}\\{\ce {^{10}_{4}Be->[1.387\ {\ce {Ma}}]{^{10}_{5}B}+e^{-}}}\\{\ce {^{11}_{4}Be->[13.76\ {\ce {s}}]{^{11}_{5}B}+e^{-}}}\\{\ce {^{11}_{4}Be->[13.76\ {\ce {s}}]{^{7}_{3}Li}+{^{4}_{2}He}+e^{-}}}\\{\ce {^{12}_{4}Be->[21.46\ {\ce {ms}}]{^{12}_{5}B}+e^{-}}}\\{\ce {^{12}_{4}Be->[21.46\ {\ce {ms}}]{^{11}_{5}B}+{^{1}_{0}n}+e^{-}}}\\{\ce {^{13}_{4}Be->[1\ {\ce {zs}}]{^{12}_{4}Be}+{^{1}_{0}n}}}\\{\ce {^{14}_{4}Be->[4.53\ {\ce {ms}}]{^{13}_{5}B}+{^{1}_{0}n}+e^{-}}}\\{\ce {^{14}_{4}Be->[4.53\ {\ce {ms}}]{^{14}_{5}B}+e^{-}}}\\{\ce {^{14}_{4}Be->[4.53\ {\ce {ms}}]{^{12}_{5}B}+{2_{0}^{1}n}+e^{-}}}\\{\ce {^{15}_{4}Be->[790\ {\ce {ys}}]{^{14}_{4}Be}+{^{1}_{0}n}}}\\{}{\ce {^{16}_{4}Be->[650\ {\ce {ys}}]{^{14}_{4}Be}+{2_{0}^{1}n}}}\\{}\end{array}}}$

See also

Daughter products other than beryllium

• Isotopes of boron
• Isotopes of lithium
• Isotopes of helium

Notes

1. Note that NUBASE2020 uses the tropical year to convert between years and other units of time, not the Gregorian year. The relationship between years and other time units in NUBASE2020 is as follows: 1 y = 365.2422 d = 31 556 926 s