Answer:
Potassium-40 (40K) is a radioactive isotope of potassium which has a long half-life of 1.251×109 years. It makes up 0.012% (120 ppm) of the total amount of potassium found in nature.
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Potassium-40.
General
Natural abundance 0.0117(1)%
Half-life 1.251(3)×109 y
Parent isotopes Primordial
Decay products 40Ca (β−) 40Ar (EC, γ; β+)
