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Isotopes of Potassium

List, data and properties of all known isotopes of Potassium.

Content
Atomic properties
Decay properties
NMR active nuclides
Radiation protection
References
Potassium isotopes

The chemical element potassium is found in nature in the form of its stable isotopes potassium-39 and potassium-41, and its long-lived radioisotope potassium-40. The latter decays into the stable argon-40 isotope with a half-life of 1.25 billion years and is therefore the cause of the radioactivity of natural potassium deposits and potassium compounds; the ratio between K-40 and Ar-40 can be used for geological dating (potassium-argon dating).

In addition, 22 potassium isotopes and several nuclear isomers are known, all of which are radioactive and short-lived.

 

Naturally Occurring Potassium Isotopes

Atomic Mass maQuantityHalf-lifeSpin
Potassium
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-

 

Due to variations in the isotopic composition of natural potassium, the mean atomic mass (standard atomic mass) can vary. Reports have also come to light in which significant discrepancies in the isotopic composition of commercially available potassium compounds have been described; this is due to the use of raw materials that have undergone isotopic fractionation, intentionally, accidentally or for unknown reasons.

 

Potassium-40 as a source of natural radioactivity

After thorium and uranium - which only occur locally in larger quantities due to the geological conditions - the widespread and more evenly distributed potassium-40 isotope contributes significantly to natural radioactivity in the form of terrestrial radiation - and is incidentally still responsible for the argon content of our atmosphere. At the same time, potassium is the largest source of natural radioactivity within human and animal organisms. A person with a body mass of 70 kg contains about 0.0164 grams of the radioactive isotope 40K; this results in a rate of about 4300 radioactive decays per second, to which a person is continuously exposed.

 

Isotope Table: Potassium

The two following tables list the most important data and properties of the Potassium isotopes. Further information on the individual Potassium isotopes is listed on separate pages and can be accessed via the link in column 1.

 

Atomic Properties

Isotope
Nuclide		ENAtomic Mass
[Nuclear Mass]
{Mass Excess}		Spin I
(h/2π)		Parent
123456
Potassium-323219K13
Potassium-333319K1433.00756(21) u
[32.9971372 u]
{7.04209 MeV}		3/2+
Potassium-343419K1533.99869(21) u
[33.9882672 u]
{-1.22026 MeV}		(1+)
Potassium-353519K1634.9880054(6) u
[34.9775826 u]
{-11.1729 MeV}		(3/2)+35Ca
Potassium-363619K1735.9813020(4) u
[35.9708792 u]
{-17.41707 MeV}		2+36Ca
Potassium-373719K1836.97337589(10) u
[36.962953 u]
{-24.8002 MeV}		3/2+37Ca
39Ti
Potassium-383819K1937.96908112(21) u
[37.9586583 u]
{-28.80075 MeV}		3+38Ca
Potassium-38m138m119K1937.96908112(21) u
[37.9586583 u]
{-28.80075 MeV}		0+
Potassium-38m238m219K1937.96908112(21) u
[37.9586583 u]
{-28.80075 MeV}		(7)+
Potassium-393919K2038.96370649(3) u
[38.9532836 u]
{-33.80719 MeV}		3/2+39Ca
39Ar
40Sc
Potassium-404019K2139.9639982(4) u
[39.9535754 u]
{-33.53546 MeV}		4-
Potassium-40m40m19K2139.9639982(4) u
[39.9535754 u]
{-33.53546 MeV}		0+
Potassium-414119K2240.96182526(3) u
[40.9514024 u]
{-35.55954 MeV}		3/2+41Ca
41Ar
Potassium-424219K2341.96240231(11) u
[41.9519795 u]
{-35.02202 MeV}		2-42Ar
Potassium-434319K2442.9607347(4) u
[42.9503119 u]
{-36.57539 MeV}		3/2+43Ar
Potassium-43m43m19K2442.9607347(4) u
[42.9503119 u]
{-36.57539 MeV}		7/2-
Potassium-444419K2543.9615870(5) u
[43.9511642 u]
{-35.78148 MeV}		2-44Ar
Potassium-454519K2644.9606915(6) u
[44.9502687 u]
{-36.61563 MeV}		3/2+45Ar
Potassium-464619K2745.9619816(8) u
[45.9515588 u]
{-35.41391 MeV}		(2-)46Ar
Potassium-474719K2846.9616616(15) u
[46.9512388 u]
{-35.71199 MeV}		1/2+47Ar
Potassium-484819K2947.9653412(8) u
[47.9549184 u]
{-32.28446 MeV}		(2-)
Potassium-494919K3048.9682108(9) u
[48.957788 u]
{-29.61145 MeV}		(1/2+,3/2+)
Potassium-505019K3149.972380(8) u
[49.9619572 u]
{-25.72786 MeV}		0(-)
Potassium-50m50m19K3149.972380(8) u
[49.9619572 u]
{-25.72786 MeV}		(2-)
Potassium-515119K3250.975828(14) u
[50.9654052 u]
{-22.51607 MeV}		3/2+
Potassium-525219K3351.98160(4) u
[51.9711772 u]
{-17.13949 MeV}		(2-)
Potassium-535319K3452.98680(12) u
[52.9763772 u]
{-12.29572 MeV}		(3/2+)
Potassium-545419K3553.99463(64) u
[53.9842072 u]
{-5.00212 MeV}		2-
Potassium-555519K3655.00076(75) u
[54.9903372 u]
{0.70794 MeV}		3/2+
Potassium-565619K3756.00851(86) u
[55.9980872 u]
{7.92701 MeV}

 

Radioactive Decay Properties

IsotopeRadioactive DecayExtern
Half-lifeDecay ModeProbabilityEnergy
789101112
K-32- ungelistet -AL
K-3325 nsp → 32Ar?AL
K-3425 nsp → 33Ar?AL
K-35178(8) msβ+; → 35Ar
β+, p; → 34Cl		99.63(15) %
0.37(15) %		11.8744(9) MeV
5.982 MeV		AL
K-36341(3) msEC/β+ → 36Ar
EC, p → 35Cl
EC, α → 32S		99.95 %
0.048(14) %
0.0034(13) %		12.8142 MeV
4.298 MeV
6.165 MeV		AL
K-371.225(7) sβ+ → 37Ar100 %6.14748(23) MeVAL
K-387.651(19) minEC/β+ → 38Ar100 %5.91407(28) MeVAL
K-38m1924.4(5) msEC/β+ → 38Ar
Iso → 38K		99.9670(43) %
0.0330(43) %		6.742 MeV
AL
K-38m221.95(11) μsIso → 38K100 %3.458 MeVAL
K-39stableAL
K-401.248(3) × 109 aβ- → 40Ca
EC/β+ → 40Ar		89.28(11) %
10.72(11) %		1.31089(6) MeV
1.50440(6) MeV		AL
K-40m0.336(13) μsIso → 40K
K-41stableAL
K-4212.355(7) hβ- → 42Ca100 %3.52522(18) MeVAL
K-4322.3(1) hβ- → 43Ca100 %1.8334(5) MeVAL
K-43m200(5) nsIso → 43K
K-4422.13(19) minβ- → 44Ca100 %5.6872(5) MeVAL
K-4517.81(61) minβ- → 45Ca100 %4.1965(6) MeVAL
K-46105(10) sβ- → 46Ca100 %7.7254(23) MeVAL
K-4717.50(24) sβ- → 47Ca100 %6.6324(26) MeVAL
K-486.8(2) sβ- → 48Ca
β-, n → 47Ca		98.86(15) %
1.14(15) %		11.9402(8) MeV
1.9886(23) MeV		AL
K-491.26(5) sβ- → 49Ca
β-, n → 48Ca		14(9) %
86(9) %		11.6883(8) MeV
6.5418(8) MeV		AL
K-50472(4) msβ- → 50Ca
β-, n → 49Ca
β-, 2n ? → 48Ca		71(3) %
29(3) %
13.861(8) MeV
7.501(8) MeV
AL
K-50m131(40) nsIso → 40K100 %
K-51365(5) msβ- → 51Ca
β-, n → 50Ca		35(8) %
65(8) %		13.816(13) MeV
9.002(13) MeV		AL
K-52110(6) msβ- → 52Ca
β-, n → 51Ca
β-, 2n → 50Ca		23.7 %
74(9) %
2.3(3) %		17.13(3) MeV
11.12(3) MeV
7.229 MeV		AL
K-5330(5) msβ- → 53Ca
β-, n → 52Ca
β-, 2n → 51Ca		16 %
67 %
17 %		17.09(12) MeV
13.90(11) MeV
7.723 MeV		AL
K-5410(5) msβ- → 54Ca
β-, n → 53Ca
β-, 2n → 52Ca		< 100 %
> 0 %
> 0 %		20.16(60) MeV
16.31(60) MeV
AL
K-55> 360 nsβ- → 55Ca
β-, n → 54Ca		?
?		19.06(76) MeV
17.8(7) MeV		AL
K-56> 620 nsβ- → 56Ca
β-, n → 55Ca
β-, 2n → 54Ca		?

21.82(89) MeV
18.21(85) MeV
AL

 

Notes (related to the columns):

1 - name of the nuclide, isotope.
2 - E: isotope symbol with mass number (superscript; number of nucleons) and Atomic number (subscript; number of protons).
3 - N: number of neutrons.
4 - relative atomic mass of the Potassium isotope (isotopic mass including electrons) and the mass of the atomic nucleus in square brackets (nuclear mass, nuclide mass without electrons), each related to 12C = 12.00000 [2]. In addition, the mass excess is given in MeV.
5 - nuclear spin I, unit: h/2π.
6 - source nuclides: Possible, assumed or actual source nuclides (mother nuclides, parent nuclides). If applicable, the corresponding decay modes can be found in the data for the respective starting nuclide.

7 - isotope notation in short form.
8 - decay: half-live of the Potassium isotope (a = years; ; d = days; h = hours; min = minutes; s = seconds).
9 - decay mode: type of decay into the respective daughter nuclides with n = neutron emission; p = proton emission; α = alpha decay; β- = beta minus decay with electron emission; EC = electron capture; β+ = positron emission; ε = β+ and/or EC; Iso = isomeric transition; CD = cluster decay; SF = spontaneous decay.
10 - decay probability in percent (%).
11 - decay energy; Particle energy related to decay type.
12 - other information and notes: AL = Adopted Levels (link to external data [1]).

Miscellaneous:

()- Numbers in brackets: uncertainty to represent the spread of the reported value.
~ - Theoretical values or systematic trends.
  - unlisted-: Nuclides that have already been mentioned in the literature but for some reason can no longer be found in the current nuclide tables because their discovery e.g. has not confirmed.

 

NMR active Potassium nuclides

Nuclide
quantity 1)
spin		Nuclear
magnetic
moment
μ/μN		Gyromagnetic ratio
{Quadrupole moment}		Resonant
frequency
v0 bei 1 T		Relative
sensitivity
H0 = const.
v0 = const. 3)
39K
93,2581(44) %
3/2+		+0,391470(8)1,2498
{+ 0,0603(6)}		1,98930,00051
0,2336
40K
0,0117(1) %
4-		-1,298100(3)–1,5542854
{-0,073(1)}		2,47370,00523
1,5493
41K
6,7302(44) %
3/2+		+0,214872(5)0,686
{+ 0,0734(7)}		1,09190,00008
0,1282

1) Quantity Percentage of natural occurrence.

2) Gyromagnetic ratio: 107 rad T-1 s-1
    Quadrupole moment: Q [barn] = [100 fm2]

3) Related to 1H = 1,000.

 

Radiation Protection

According to the Radiation Protection Ordinance (StrlSchV 2018, Germany), the following values (columns 1 to 7) apply to the handling of Potassium radionuclides:

NuclideLimit ValueHASS limitSCDaughter NuclidesHalf-life
K-40106 Bq100 Bq/gUnbegrenzt (UL)10 Bq/cm21.3 × 109 a
K-42106 Bq100 Bq/g0,2 TBq10 Bq/cm212.4 h
K-43106 Bq10 Bq/g0,07 TBq1 Bq/cm222.2 h
K-44105 Bq10 Bq/g22.1 min
K-45105 Bq10 Bq/g17.3 min

(HASS = High-Activity Sealed Radioactive Sources; SC = surface contamination)

 

Literature Sources and References

Properties of the Potassium nucleides

[1] - NuDat: National Nuclear Data Center, Brookhaven National Laboratory, based on ENSDF and the Nuclear Wallet Cards.

[2] - G. Audi et. al.: The NUBASE evaluation of nuclear and decay properties. Nuclear Physics, (2003), DOI 10.1016/j.nuclphysa.2003.11.001.

[3] - Live Chart of Nuclides. Nuclear structure and decay data.

Potassium: NMR properties - 39K-NMR, 40K-NMR, 41K-NMR

[4] - N. J. Stone: Table of nuclear magnetic dipole and electric quadrupole moments. Atomic Data and Nuclear Data Tables, (2005), DOI 10.1016/j.adt.2005.04.001.

[5] - Pekka Pyykkö: Year-2008 nuclear quadrupole moments. Molecular Physics, (2008), DOI 10.1080/00268970802018367.

[6] - Pekka Pyykkö: Year-2017 nuclear quadrupole moments. Molecular Physics, (2018), DOI 10.1080/00268976.2018.1426131.

[7] - N. J. Stone: Table of recommended nuclear magnetic dipole moments. IAEA, (2019).

More sources:

[8] - Isotopic abundances, atomic weights and isotopic masses: see respective keyword.

[9] - NN:
Magische Neutronenzahl exotischer Kalium-Kerne bestätigt.
In: Internetchemie News, (2015).

 

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