Contents

Isotopes of potassium

Potassium (
₁₉K) has 24 known isotopes from ³⁵
Kto⁵⁷
K as well as ³¹
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
Cabybeta decay, whilst 11% are to ⁴⁰
Ar by either electron captureorpositron 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
Kto⁴⁰
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]

List of isotopes[edit]

See also[edit]

• Banana equivalent dose

References[edit]