Alkali metal

Potassium

Z 19A 39mass 39.098

Decay curve

Potassium-40

half-life

1.25 billion years

half-lives elapsed1 half-life

0123456

left

50.0%

real time

1.25 billion years

119 left

Each dot is one nucleus. Decay is random for each one — but the pattern that emerges is the exponential curve on the left.

Half-lives	Time elapsed	Remaining	Of 1000
0	—	100.0%	1000
1	1.25 billion years	50.0%	500
2	2.5 billion years	25.0%	250
3	3.75 billion years	12.5%	125
4	4.99 billion years	6.25%	63
5	6.24 billion years	3.13%	31
6	7.49 billion years	1.56%	16

N = N₀ × (½)ⁿ — never exactly zero

Thorium-23214.05 billion years

Uranium-2384.47 billion years

Potassium-401.25 billion years

Neptunium-2372.14 million years

Plutonium-23924,110 years

Carbon-145,730 years