Phosphorus-31
Properties and data of the isotope 31P.
Phosphorus-31 is a stable, naturally occurring isotope of the chemical element phosphorus, which has 16 neutrons in its atomic nucleus in addition to the element-specific 15 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 31.
Phosphorus-31 was first identified as a distinct isotope in 1920 by measuring its mass spectrum [1].
See also: List of individual Phosphorus isotopes (and general data sources).
General data
Name of the isotope:Phosphorus-31; P-31Symbol:31P or 3115PMass number A:31 (= number of nucleons)Atomic number Z:15 (= number of protons)Neutrons N:16Nucleon pairing (Z - N):odd - evenNuclear ratio (N/Z ratio):1.0666666666667 (= neutron-proton ratio)Neutron excess (N-Z):1Isotopic mass:30.9737619977(8) u (atomic weight of Phosphorus-31)Nuclide mass:30.9655334 u (calculated nuclear mass without electrons)Mass excess:-24.44054 MeVMass defect:0.2822521383 u (per nucleus)Nuclear binding energy:262.91617331 MeV (per nucleus)
8.48116688 MeV (average binding energy per nucleon)Separation energy:SN = 12.31101(7) MeV (first neutron)
SP = 7.296553(22) MeV (first proton)Half-life:stableSpin and parity:
(nuclear angular momentum)1/2+Magnetic dipole moment:μ(μN) = + 1.130925(5)Charge radius:3.1889(19) femtometer fmMirror nucleus:Sulfur-31Year of discovery:1920
Parent Nuclides
Direct parent isotopes are: 31S, 31Si, 32Cl.
Occurrence
The element phosphorus has only one stable isotope, phosphorus-31; it is therefore classified as a monoisotopic element.
Comparison of the natural Phosphorus isotopes including isotopic abundance (mole fraction of the isotope mixture in percent):
| | Atomic Mass ma | Quantity | Half-life | Spin |
|---|
Phosphorus
Isotopic mixture | 30.973761998 u | 100 % | | |
| Isotope 31P | 30.9737619977(8) u | 100 % | stable | 1/2+ |
NMR data
Nuclear magnetic properties and parameters of the NMR active Nuclide 31P
Isotope:31P-NMRQuantity:100 %Spin:1/2+Nuclearmagnetic moment
μ/μN:+ 1.130925(5)Gyromagnetic ratio γ:10.841 · 107 rad T-1 s-1
17.25399 MHz T-1Nuclear g-factor:gl = 2.26185Resonance frequency:v0 = 17.2515 MHz at 1 TRelative Sensitivity:[related to 1H = 1.000]:
0.06652 (H0 = const.)
0.4052 (v0 = const.)
0.0665 (at natural abundance)
relative receptivity as compared to 13C:
391 (at natural abundance)Reference compound:
(conditions)85% phosphoric acid (H3PO4)Frequency ratio:Ξ(31P) = 40.480742 % [based on the reference substance] The nuclide phosphorus-31 has an atomic nucleus of medium sensitivity and a large range of chemical shifts; 31P-NMR spectra show sharp lines.
Isotones and Isobars
The following table shows the atomic nuclei that are isotonic (same neutron number N = 16) and isobaric (same nucleon number A = 31) with Phosphorus-31. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.
| Z | Isotone N = 16 | Isobar A = 31 |
|---|
| 5 | 21B | |
| 6 | 22C | |
| 7 | 23N | |
| 8 | 24O | |
| 9 | 25F | 31F |
| 10 | 26Ne | 31Ne |
| 11 | 27Na | 31Na |
| 12 | 28Mg | 31Mg |
| 13 | 29Al | 31Al |
| 14 | 30Si | 31Si |
| 15 | 31P | 31P |
| 16 | 32S | 31S |
| 17 | 33Cl | 31Cl |
| 18 | 34Ar | 31Ar |
| 19 | 35K | 31K |
| 20 | 36Ca | |
| 21 | 37Sc | |
| 22 | 38Ti | |
External data and identifiers
Literature and References
[1] - F. W. Aston:
The Constitution of the Elements.
In: Nature, 105, 547, (1920), DOI 10.1038/105547a0.
[2] - Marvin Alberts, Nina Hartrampf, Teodoro Laino:
From Spectra to Structure: AI-Powered 31P NMR Interpretation.
In: Analytical Chemistry, 97, 29, (2025), DOI 10.1021/acs.analchem.5c01460.
Last update: 2025-12-28
Perma link: https://www.chemlin.org/isotope/phosphorus-31
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