Isotopes of potassium

Potassium (
₁₉K) has 26 known isotopes from ³¹
K to ⁵⁷
K, with the exception of still-unknown ³²
K, as well as an unconfirmed report of ⁵⁹
K.^[3] Three of those isotopes occur naturally: the two stable forms ³⁹
K (93.3%) and ⁴¹
K (6.7%), and a very long-lived radioisotope ⁴⁰
K (0.012%)

Naturally occurring radioactive ⁴⁰
K decays with a half-life of 1.248×10⁹ years. 89% of those decays are to stable ⁴⁰
Ca by beta decay, whilst 11% are to ⁴⁰
Ar by either electron capture or positron emission. This latter decay branch has produced an isotopic abundance of argon on Earth which differs greatly from that seen in gas giants and stellar spectra. ⁴⁰
K has the longest known half-life for any positron-emitter nuclide. The long half-life of this primordial radioisotope is caused by a highly spin-forbidden transition: ⁴⁰
K has a nuclear spin of 4, while both of its decay daughters are even–even isotopes with spins of 0.

⁴⁰
K occurs in natural potassium in sufficient quantity that large bags of potassium chloride commercial salt substitutes can be used as a radioactive source for classroom demonstrations.^{[citation needed] 40}
K is the largest source of natural radioactivity in healthy animals and humans, greater even than ¹⁴
C. In a human body of 70 kg mass, about 4,400 nuclei of ⁴⁰
K decay per second.^[4]

The decay of ⁴⁰
K to ⁴⁰
Ar is used in potassium-argon dating of rocks. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic ⁴⁰
Ar that has accumulated. Typically, the method assumes that the rocks contained no argon at the time of formation and all subsequent radiogenic argon (i.e., ⁴⁰
Ar) was retained.^{[citation needed] 40}
K has also been extensively used as a radioactive tracer in studies of weathering.^{[citation needed]}

All other potassium isotopes have half-lives under a day, most under a minute. The least stable is ³¹
K, a three-proton emitter discovered in 2019; its half-life was measured to be shorter than 10 picoseconds.^[5][6]

Stable potassium isotopes have been used for several nutrient cycling studies since potassium is a macronutrient required for life.^[7]