potassium-40

physics

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

July 1, 2026

Quick Summary

Potassium-40 (⁴⁰K) is a radioactive isotope of potassium, a chemical element essential for plant and animal life. While natural potassium consists mainly of stable isotopes potassium-39 and potassium-41, about 0.01 percent is potassium-40.1

Potassium-40 has a long half-life of approximately 1.25 billion years. It decays into two daughter products: argon-40 and calcium-40. This decay process is the basis for potassium-argon dating, a method used to determine the age of rocks and minerals. The ratio of argon-40 to potassium-40 in a sample can indicate how long ago the rock or mineral formed. This dating technique is valuable for understanding Earth's history, with applications ranging from dating meteorites to recent volcanic rocks.123

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

3 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

radioactivity
Calculation and measurement of energy
…of naturally occurring but radioactive potassium-40 is measured to be 39.964008 amu. Potassium-40 decays predominantly by β-emission to calcium-40, having a measured mass 39.962589. Through Einstein’s equation, energy is equal to mass (m) times velocity of light (c) squared, or E = mc², the energy release (Q) and the mass…
Read More
…of naturally occurring but radioactive potassium-40 is measured to be 39.964008 amu. Potassium-40 decays predominantly by β-emission to calcium-40, having a measured mass 39.962589. Through Einstein’s equation, energy is equal to mass (m) times velocity of light (c) squared, or E = mc², the energy release (Q) and the mass…
Read More

argon
…the rare, naturally radioactive isotope potassium-40. The gas slowly leaks into the atmosphere from the rocks in which it is still being formed. The production of argon-40 from potassium-40 decay is utilized as a means of determining Earth’s age (potassium-argon dating).
Read More
…the rare, naturally radioactive isotope potassium-40. The gas slowly leaks into the atmosphere from the rocks in which it is still being formed. The production of argon-40 from potassium-40 decay is utilized as a means of determining Earth’s age (potassium-argon dating).
Read More