Sulfur-31
Properties and data of the isotope 31S.
Sulfur-31 is a radioisotope of the chemical element sulfur, which has 15 neutrons in its atomic nucleus in addition to the element-specific 16 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 31. The very short-lived, only artificially produced, unstable and thus radioactive nuclide has no practical significance; the study of 31S is exclusively for academic purposes and experimental research.
According to a report published in 1940, the artificial isotope was first produced and identified by the nuclear reaction 28Si(α,n)31S [1].
See also: List of individual Sulfur isotopes (and general data sources).
General data
Name of the isotope:Sulfur-31; S-31Symbol:31S or 3116SMass number A:31 (= number of nucleons)Atomic number Z:16 (= number of protons)Neutrons N:15Nucleon pairing (Z - N):even - oddNuclear ratio (N/Z ratio):0.9375 (= neutron-proton ratio)Neutron excess (N-Z):-1Isotopic mass:30.97955700(25) u (atomic weight of Sulfur-31)Nuclide mass:30.9707798 u (calculated nuclear mass without electrons)Mass excess:-19.04253 MeVMass defect:0.275617252 u (per nucleus)Nuclear binding energy:256.73581653 MeV (per nucleus)
8.28180053 MeV (average binding energy per nucleon)Separation energy:SN = 13.0546(3) MeV (first neutron)
SP = 6.13065(24) MeV (first proton)Half-life:2.5534(18) sDecay constant λ:0.27146047644707 s-1Specific activity α:5.449244090232 × 10+21 Bq g-1
147276867303.58 Ci g-1Spin and parity:
(nuclear angular momentum)1/2+Magnetic dipole moment:μ(μN) = 0.48793(8)Nuclear g-factor:gl = 0.97586Mirror nucleus:Phosphorus-31Year of discovery:1940
Radioactive Decay
Half-life T½ = 2.5534(18) s.
Decay
mode | Daughter | Probability | Decay energy | γ energy
(intensity) |
|---|
| EC/β+ | 31P | 100 % | 5.39801(23) MeV | |
Parent Nuclides
Direct parent isotopes are: 32Ar, 31Cl.
Isotones and Isobars
The following table shows the atomic nuclei that are isotonic (same neutron number N = 15) and isobaric (same nucleon number A = 31) with Sulfur-31. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.
| Z | Isotone N = 15 | Isobar A = 31 |
|---|
| 5 | 20B | |
| 6 | 21C | |
| 7 | 22N | |
| 8 | 23O | |
| 9 | 24F | 31F |
| 10 | 25Ne | 31Ne |
| 11 | 26Na | 31Na |
| 12 | 27Mg | 31Mg |
| 13 | 28Al | 31Al |
| 14 | 29Si | 31Si |
| 15 | 30P | 31P |
| 16 | 31S | 31S |
| 17 | 32Cl | 31Cl |
| 18 | 33Ar | 31Ar |
| 19 | 34K | 31K |
| 20 | 35Ca | |
| 21 | 36Sc | |
External data and identifiers
Literature and References
[1] - L. D. P. King, D. R. Elliott:
Short-Lived Radioactivities of 14Si27, 16S31, and 18A35.
In: Physical Review, 58, 846, (1940), DOI 10.1103/PhysRev.58.846.
Last update: 2025-12-30
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