Potassium (K)

Every person is measurably radioactive because of potassium-40, which makes up about 0.012% of natural potassium and has a half-life of 1.25 billion years — an adult body emits roughly 4,000 to 5,000 decays a second from it. That same decay to argon-40 is the basis of potassium-argon dating, which is how the age of most volcanic rock is established. A banana contains enough potassium to be detectable by a sensitive radiation monitor, which is where the informal "banana equivalent dose" comes from.

Properties

Atomic number

19

Symbol

K

Standard atomic weight

39.098 u

Category

Alkali metal

Group

1

Period

4

Block

s-block

Electron configuration

[Ar] 4s¹

Electrons per shell

2, 8, 8, 1

Melting point

336.53 K — 63.4 °C — 146.1 °F

Boiling point

1,032 K — 758.9 °C — 1397.9 °F

Density

0.89 g/cm³

Electronegativity

0.82 (Pauling)

Oxidation states

+1

Discovered

1807 by Humphry Davy

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

Noble gas shorthand

[Ar] 4s¹

Shells

K: 2 · L: 8 · M: 8 · N: 1

What it is used for

• Potash fertiliser — the K in N-P-K
• Potassium hydroxide in soft soaps and alkaline batteries
• Potassium nitrate in gunpowder and in curing salts
• Salt substitutes for people on low-sodium diets
• Potassium chloride solutions in medicine, to correct low blood potassium

Hazards

• Reacts violently with water, usually igniting the hydrogen it releases
• Old potassium metal forms shock-sensitive peroxides on its surface

Neighbours in the table

Group 1

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Potassium sits in group 1 with hydrogen, lithium, sodium, rubidium, cesium, francium.