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Properties and data of the isotope 40K.



Potassium-40 isotope

Potassium-40 - symbol 40 K - is a naturally occurring, primordial, radioactive isotope of the chemical element potassium with a half-life of 1.25 billion years; Earth's potassium consists of about 0.0117% (117 ppm) of this isotope, which is responsible for the weak natural radioactivity of the alkali metal.

The atomic nucleus of the nuclei consists of 40K 21 neutrons and the element-specific 19 protons.

See also: list of Potassium isotopes.


General data

Name of the isotope:Potassium-40; K-40Symbol:40K or 4019KMass number A:40 (= number of nucleons)Atomic number Z:19 (= number of protons)Neutrons N:21Isotopic mass:39.9639982(4) u (atomic weight of Potassium-40)Nuclide mass:39.9535754 u (calculated nuclear mass without electrons)Mass excess:-33.53546 MeVMass defect:0.366640644 u (per nucleus)Nuclear binding energy:341.52356004 MeV (per nucleus)
8.538089 MeV (average binding energy per nucleon)
Separation energy:SN = 7.79962(6) MeV (first neutron)
SP = 7.582(5) MeV (first proton)
Half-life:1.248(3) × 109 aDecay constant λ:1.761182124173 × 10-17 s-1Specific activity α:271950.94440454 Bq g-1
7.35002552444 × 10-6 Ci g-1
Spin and parity:
(nuclear angular momentum)
4-Magnetic dipole moment:μ(μN) = -1.298100(3)Charge radius:3.4381(28) femtometer fmMirror nucleus:Scandium-40Year of discovery:1935


Radioactive Decay

Potassium-40 is a rare example of an isotope that undergoes both types of beta decay, and these major decay channels are well studied and experimentally verified. However, despite decades of research and great interest in this natural radionuclide, there is uncertainty about the details of 40K decay.

Another type of decay that has not been confirmed experimentally and is controversial in science:

The radioactive decay of this potassium isotope explains the large amount of argon of almost one percent in the Earth´s atmosphere as well as the prevalence of 40Ar compared to the other argon isotopes; at the same time it is according to 238Th and 238U the third most common source of radiogenic heat in the Earth´s mantle.

Half-life T½ = 1.248(3) × 109 a respectively 3.9356928 × 1016 seconds s.

Decay modeDaughterProbabilityDecay energyγ energy
β-40Ca89.28(11) %1.31089(6) MeV
EE/β+40Ar10.72(11) %1.50440(6) MeV1.460820(5) MeV
10.66(17) %


Radioactive decay of 40K



The 40K atom nucleus is one of the primordial radionuclides - this means that the terrestrial deposits were already present when they were created and were not replenished in significant quantities through radioactive decay processes or human influences. The potassium-40 therefore comes from stellar processes - particularly supernova explosions - that took place before the Earth was formed. Recent studies of meteorites also indicate that most of the terrestrial potassium (90%) comes from non-carbonaceous matter, the origin of which lies in the interior of the solar system [6].



Potassium-40 is a natural radionuclide found in potassium - but its proportion is only 0.0117%. Since potassium is a mineral found practically everywhere, the radioactive K-40 it contains is the largest source of natural radioactivity in humans, animals and the environment. A human body weighing 70 kg contains approximately 140 g of potassium, which is approximately 0.000117 × 140 g = 0.0164 g of the isotope 40 K; The decay causes approximately 4,300 decays per second (Becquerels) within the body continuously throughout life.

Comparison of the natural Potassium isotopes including isotopic abundance (mole fraction of the isotope mixture in percent):


Atomic Mass maQuantityHalf-lifeSpin
Isotopic mixture
39.0983 u100 %
Isotope 39K38.96370649(3) u93.2581(44) %stable3/2+
Isotope 41K40.96182526(3) u6.7302(44) %stable3/2+
Isotope 40K39.9639982(4) u0.0117(1) %1.248(3) × 109 a4-


NMR data

Nuclear magnetic properties of the NMR active Nuclide 40K

Isotope:40K-NMRQuantity:0.0117(1) %Spin:4-Nuclearmagnetic moment
-1.298100(3)Gyromagnetic ratio γ:–1.5542854 · 107 rad T-1 s-1Nuclear g-factor:gl = -0.324525Quadrupole moment Q:-0.073(1) barn (100 fm2)Line width parameter (factor):l = 5.23 fm4Resonance frequency:v0 = 2.4737 at 1 TFrequency ratio:Ξ(40K) = 5.802018 %Relative Sensitivity:0.00523 (H0 = const.)
1.5493 (v0 = const.)
[related to 1H = 1.000]
Reference compound:
0.1 M potassium chloride solution in Deuterium Oxide (D2O).


Radiation Protection

Potassium compounds and preparations are ubiquitous in nature, technology and everyday life and are not considered radioactive hazardous substances despite the K-40 content; The radiation emitted by it is too weak for this.

Substances enriched with 40K have no practical or technical significance; K-40 is one of the radioactive substances for which even high levels of activity do not lead to classification as a highly radioactive radiation source.


Nuclear Isomers

Nuclear isomers or excited states with the activation energy in keV related to the ground state.

Nuclear IsomerExcitation EnergyHalf-lifeSpin
40mK1643.638(11) keV0.336(13) μs0+


Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 21) and isobaric (same nucleon number A = 40) with Potassium-40. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.


OZIsotone N = 21Isobar A = 40


External data and identifiers

CAS:13966-00-2InChI Key:ZLMJMSJWJFRBEC-OUBTZVSYSA-NInChI Code:[40K]PubChem:ID 6328542Adopted Levels, Gammas:NuDat 40K


Literature and References

[1] - D. W. Engelkemeir, K. F. Flynn, L. E. Glendenin:
Positron Emission in the Decay of 40K.
In: Physical Review, 126,1818, (1962), DOI 10.1103/PhysRev.126.1818.

[2] - H. Leutz, G. Schulz, H. Wenninger:
The decay of potassium-40.
In: Zeitschrift für Physik, (1965), DOI 10.1007/BF01387190.

[3] - A. Ažman, A. Moljk, J. Pahor:
Electron capture in potassium 40.
In: Zeitschrift für Physik A, (1968), DOI 10.1007/BF01379914.

[4] - W. Sahm, A. Schwenk:
39K, 40K and 41K Nuclear Magnetic Resonance Studies.
In: Zeitschrift für Naturforschung A, (2014), DOI 10.1515/zna-1974-1208.

[5] - Jack Carter, Ryan B. Ickert, Darren F. Mark et al.:
Production of 40Ar by an overlooked mode of 40K decay with implications for K-Ar geochronology.
In: Geochronology, 2(2), 355–365, (2020), DOI 10.5194/gchron-2-355-2020.

[6] - Nicole X. Nie et al.:
Meteorites have inherited nucleosynthetic anomalies of potassium-40 produced in supernovae.
In: Science, 379, 6630, 373-376, (2023), DOI 10.1126/science.abn178.


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Last update: 2023-12-04

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