Potassium-39
Properties and data of the isotope 39K.
Potassium-39 is a stable, naturally occurring isotope of the chemical element potassium, which has 20 neutrons in its atomic nucleus in addition to the element-specific 19 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 39.
The discovery of potassium-39 during the investigation of positive anode rays using a Cavendish mass spectrograph was reported in 1921 [1].
See also: List of individual Potassium isotopes (and general data sources).
General data
Name of the isotope:Potassium-39; K-39Symbol:39K or 3919KMass number A:39 (= number of nucleons)Atomic number Z:19 (= number of protons)Neutrons N:20Nucleon pairing (Z - N):odd - evenNuclear ratio (N/Z ratio):1.0526315789474 (= neutron-proton ratio)Neutron excess (N-Z):1Isotopic mass:38.963706485(5) u (atomic weight of Potassium-39)Nuclide mass:38.9532836 u (calculated nuclear mass without electrons)Mass excess:-33.80719 MeVMass defect:0.358267443 u (per nucleus)Nuclear binding energy:333.72397355 MeV (per nucleus)
8.55702496 MeV (average binding energy per nucleon)Separation energy:SN = 13.07775(20) MeV (first neutron)
SP = 6.38134(19) MeV (first proton)Half-life:stableSpin and parity:
(nuclear angular momentum)3/2+Magnetic dipole moment:μ(μN) = +0.391470(8)Charge radius:3.4349(19) femtometer fmMirror nucleus:Calcium-39Year of discovery:1921
Parent Nuclides
Direct parent isotopes are: 39Ca, 39Ar, 40Sc.
Occurrence
Comparison of the natural Potassium isotopes including isotopic abundance (mole fraction of the isotope mixture in percent):
| | Atomic Mass ma | Quantity | Half-life | Spin |
|---|
Potassium
Isotopic mixture | 39.0983 u | 100 % | | |
| Isotope 39K | 38.963706485(5) u | 93.2581(44) % | stable | 3/2+ |
| Isotope 40K | 39.96399817(6) u | 0.0117(1) % | 1.248(3) × 109 a | 4- |
| Isotope 41K | 40.961825256(4) u | 6.7302(44) % | stable | 3/2+ |
NMR data
Nuclear magnetic properties and parameters of the NMR active Nuclide 39K
Isotope:39K-NMRQuantity:93.2581(44) %Spin:3/2+Nuclearmagnetic moment
μ/μN:+0.391470(8)Gyromagnetic ratio γ:1.2500608 · 107 rad T-1 s-1
1.98953 MHz T-1Nuclear g-factor:gl = 0.26098Quadrupole moment Q:+0.603(6) barn (100 fm2)Line width parameter (factor):l = 4848.12 fm4Resonance frequency:v0 = 1.9893 MHz at 1 TRelative Sensitivity:[related to 1H = 1.000]:
0.00051 (H0 = const.)
0.2336 (v0 = const.)
0.000476 (at natural abundance)
relative receptivity as compared to 13C:
2.79 (at natural abundance)Reference compound:
(conditions)Solution of 0.1 M potassium chloride (KCl) in deuterium oxide (D2O).Frequency ratio:Ξ(39K) = 4.666373 % [based on the reference substance]
Isotones and Isobars
The following table shows the atomic nuclei that are isotonic (same neutron number N = 20) and isobaric (same nucleon number A = 39) with Potassium-39. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.
| Z | Isotone N = 20 | Isobar A = 39 |
|---|
| 8 | 28O | |
| 9 | 29F | |
| 10 | 30Ne | |
| 11 | 31Na | 39Na |
| 12 | 32Mg | 39Mg |
| 13 | 33Al | 39Al |
| 14 | 34Si | 39Si |
| 15 | 35P | 39P |
| 16 | 36S | 39S |
| 17 | 37Cl | 39Cl |
| 18 | 38Ar | 39Ar |
| 19 | 39K | 39K |
| 20 | 40Ca | 39Ca |
| 21 | 41Sc | 39Sc |
| 22 | 42Ti | 39Ti |
| 23 | 43V | |
| 24 | 44Cr | |
| 25 | 45Mn | |
| 26 | 46Fe | |
| 27 | 47Co | |
| 28 | 48Ni | |
External data and identifiers
Literature and References
[1] - F. W. ASTON:
The Constitution of the Alkali Metals.
In: Nature, 107, 72, (1921), DOI 10.1038/107072b0.
Last update: 2026-01-07
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