Magnesium-21

Properties and data of the isotope ²¹Mg.

Contents

General Data

Radioactive Decay

Parent Nuclides

Isotones und Isobares

Literature

Magnesium-21 is a radioisotope of the chemical element magnesium, which has 9 neutrons in its atomic nucleus in addition to the element-specific 12 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 21. The very short-lived, only artificially produced, unstable and thus radioactive nuclide has no practical significance; the study of ²¹Mg is exclusively for academic purposes and experimental research.

The first observation of the proton-rich isotope was reported in 1963 [1]: Magnesium-21 was identified by observing delayed protons in a Si particle detector that accumulated when a Mg template was irradiated with 97 MeV protons.

See also: List of individual Magnesium isotopes (and general data sources).

General data

Name of the isotope:Magnesium-21; Mg-21Symbol:²¹Mg or 2112MgMass number A:21 (= number of nucleons)Atomic number Z:12 (= number of protons)Neutrons N:9Isotopic mass:21.0117058(8) u (atomic weight of Magnesium-21)Nuclide mass:21.0051229 u (calculated nuclear mass without electrons)Mass excess:10.90388 MeVMass defect:0.160178828 u (per nucleus)Nuclear binding energy:149.20561721 MeV (per nucleus)
7.10502939 MeV (average binding energy per nucleon)Separation energy:S_N = 14.6452(20) MeV (first neutron)
S_P = 3.2356(13) MeV (first proton)Half-life:120.5(4) msDecay constant λ:5.7522587598336 s^-1Specific activity α:1.649567262596 × 10⁺²³ Bq g^-1
4458289898908.9 Ci g^-1Spin and parity:
(nuclear angular momentum)5/2+Magnetic dipole moment:μ(μ_N) = -0.983(7)Nuclear g-factor:g_l = -0.3932Charge radius:0 femtometer fmMirror nucleus:Fluorine-21Year of discovery:1963

Radioactive Decay

Half-life T_½ = 120.5(4) ms respectively 1.205 × 10^-1 seconds s.

Decay mode	Daughter	Probability	Decay energy
EC/β⁺	²¹Na	66.9 %	13.0885(8) MeV
β⁺, p	²⁰Ne	32.6 %	10.663 MeV
β⁺, α	¹⁶F	< 0.5 %	6,534 MeV

Parent Nuclides

Direct parent isotope is: ²²Si.

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 9) and isobaric (same nucleon number A = 21) with Magnesium-21. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 9	Isobar A = 21
3	¹²Li
4	¹³Be
5	¹⁴B	²¹B
6	¹⁵C	²¹C
7	¹⁶N	²¹N
8	¹⁷O	²¹O
9	¹⁸F	²¹F
10	¹⁹Ne	²¹Ne
11	²⁰Na	²¹Na
12	²¹Mg	²¹Mg
13	²²Al	²¹Al
14	²³Si
15	²⁴P

Data changes (history)

At this point, a newer and more accurate value (2024) for the half-life of magnesium-21 was adopted [3]. In the common data tables, t_1/2 is often given as 120.0(4) ms [NuBase 2020], 122(3) ms or 121.9(6) ms [2].

External data and identifiers

Adopted Levels, Gammas:NuDat 21Mg

Literature and References

[1] - R. Barton et al.:
Observation of delayed proton radioactivity.
In: Canadian Journal of Physics, 41(12), (1963), DOI 10.1139/p63-201.

[2] - Yu-Ting Wang et al.:
β-delayed particle emission from ²¹Mg.
In: The European Physical Journal A, 54,107, (2018), DOI 10.1140/epja/i2018-12543-1.

[3] - E. A. M. Jensen, S. T. Nielsen, A. Andreyev et al.:
Detailed study of the decay of ²¹Mg.
In: The European Physical Journal A, 60, 153, (2024), DOI 10.1140/epja/s10050-024-01376-6.