Why is potassium radioactive?
Natural potassium is a mixture of three isotopes. Potassium-39 accounts for about 93.3%, potassium-41 for about 6.7%, and potassium-40 for roughly 0.012%.
That last fraction is tiny, and it is radioactive, with a half-life of about 1.25 billion years. Because the half-life is comparable to the age of the Earth, a meaningful fraction of the potassium-40 present when the planet formed is still here — unlike shorter-lived isotopes, which have long since decayed away.
Potassium-40 is unusual in decaying two different ways. About 89% of decays are beta-minus emission to calcium-40. The remaining 11% or so are electron capture to argon-40, accompanied by a gamma ray at 1.46 MeV. This page shows branching ratios like these for every unstable nuclide, and potassium-40 is one of the clearest examples of a nuclide taking two routes at once.
The consequence is that potassium is the largest source of natural radioactivity in the human body. An adult contains roughly 140 grams of potassium, of which about 17 milligrams is potassium-40, undergoing on the order of 4,000 to 5,000 decays per second.
This is unavoidable and harmless. The body regulates total potassium tightly, so eating more potassium does not raise your potassium-40 content — the excess is excreted. The banana equivalent dose, sometimes used to make radiation figures intuitive, is about 0.1 microsieverts, and the comparison is slightly misleading for exactly this reason.
How does potassium date human evolution?
The electron capture branch turns potassium-40 into argon-40, and that pairing makes an unusually good clock.
Argon is a noble gas. When a volcanic rock is molten, any argon in it escapes, so the rock solidifies with essentially zero argon. From that moment, argon-40 accumulates from potassium-40 decay and is trapped in the crystal lattice. Measure the ratio of argon-40 to potassium-40 and you get the time since the rock last melted.
The billion-year half-life makes the method suited to long timescales, and the zero-argon starting point removes the largest source of uncertainty in most radiometric dating.
Potassium–argon dating and its more precise successor, argon–argon dating, are why the timeline of human origins has firm numbers. Hominin fossils in East Africa are typically found between layers of volcanic ash, and dating the ash brackets the fossil. The age of Lucy, of the Laetoli footprints, and of the oldest stone tools all rest on this method.
The same technique dates the Deccan Traps and the Chicxulub impact glass, and established the timeline around the end-Cretaceous extinction.
Argon-40 produced this way is also the reason argon makes up nearly 1% of Earth’s atmosphere — far more than would be expected for a noble gas of its mass. Almost all atmospheric argon is decayed potassium that leaked out of the crust.
How do you read potassium’s electron configuration?
Potassium’s ground state is [Ar] 4s¹: a filled argon core of 18 electrons, plus one in the 4s subshell. Electrons per shell: 2, 8, 8, 1.
Potassium is where the periodic table’s filling order first becomes visibly non-obvious. After argon’s 3p⁶, the next subshell by principal quantum number would be 3d. The nineteenth electron goes into 4s instead, because at this point 4s lies lower in energy than 3d.
This is the Madelung rule at work: subshells fill in order of increasing n + l, and for ties, increasing n. For 4s, n + l = 4 + 0 = 4. For 3d, n + l = 3 + 2 = 5. So 4s fills first.
Potassium and calcium are the two elements where this ordering holds cleanly. Once 3d begins filling at scandium, the relative energies shift, and by the time you reach iron the 3d level sits below 4s — which is why transition metals lose their s electrons first when they ionise.
The single loosely held 4s electron gives potassium its chemistry. Its first ionization energy of about 419 kJ/mol is lower than sodium’s 496, because the electron is one shell further out and better screened, and this is why potassium reacts with water more violently than sodium does — vigorously enough that the hydrogen produced usually ignites on its own.
The term symbol is ²S₁/₂, the same as every alkali metal: one unpaired s electron, no orbital angular momentum.
What does potassium do in the body?
Potassium is the main positive ion inside cells, as sodium is outside them, and the pairing is the point.
The sodium–potassium pump moves three Na⁺ out and two K⁺ in per ATP consumed, maintaining a gradient in both directions across every cell membrane. That gradient stores energy the cell can spend, and it sets the resting membrane potential of about −70 millivolts in a typical neuron.
Nerve and muscle function depend on it directly. An action potential is sodium rushing in and then potassium rushing out to restore the resting state. Heart muscle is especially sensitive: blood potassium outside a narrow range disturbs the rhythm, and severe deviation in either direction can stop the heart. This is why potassium chloride is a component of lethal injection protocols, and why potassium supplementation is handled cautiously in medicine.
In plants, potassium is one of the three macronutrients — the K in NPK fertiliser. It regulates the opening and closing of stomata and activates many enzymes, and potash, mined as potassium chloride, is a globally traded commodity because of it.
The name records the older source. Potash was made by leaching wood ash with water and evaporating the solution in iron pots — literally pot ash — and it supplied potassium carbonate for soap and glass long before the element was isolated. Davy obtained potassium metal in 1807, by electrolysis, days before he obtained sodium the same way.
Where does potash come from?
Potash — the potassium salts used in fertiliser, mostly potassium chloride — is mined from ancient evaporite deposits, and the geology explains an unusual market structure.
These deposits formed where shallow seas dried out. As seawater evaporates its dissolved salts precipitate in sequence, least soluble first: carbonates, then gypsum, then halite, and finally the potassium and magnesium salts, which are the most soluble and crystallise last. A potash bed therefore represents an almost complete evaporation of a sea, which is a specific and uncommon geological event.
The result is that world potash reserves are concentrated in remarkably few places. Saskatchewan in Canada holds by far the largest, from the Devonian Prairie Evaporite. Belarus and Russia hold most of the remainder, and Germany has smaller deposits worked since the nineteenth century.
That concentration has geopolitical consequences. Potassium has no substitute in agriculture — plants need it, and no other element performs its role — and it cannot be synthesised. Fertiliser supply for a large part of the world depends on a handful of mining regions, and disruptions to any of them affect crop yields globally.
Before these mines, potassium came from wood ash, and demand for it was heavy enough that potash production was a significant driver of deforestation in colonial North America.