Potassium
[Ar] 4s¹ · 2 · 8 · 8 · 1
An alkali metal no living cell works without. Potassium is reactive enough to ignite in damp air, and inside a neuron its ions together with sodium's build the action potential.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 19
- Atomic mass
- 39.098 а.е.м.
- Electron configuration
- [Ar] 4s¹
- Electrons per shell
- 2 · 8 · 8 · 1
- Block
- s-block
- Group
- Alkali metals
- Period
- 4
- Electronegativity (Pauling)
- 0.82
- Electronegativity (Allen)
- 0.73
- Atomic radius
- 243 pm
- Covalent radius
- 203 pm
- Van der Waals radius
- 275 pm
- Ionisation energy 1
- 418.8 kJ/mol
- Ionisation energy 2
- 3 052 kJ/mol
- Ionisation energy 3
- 4 420 kJ/mol
- Electron affinity
- 48.4 kJ/mol
- Common oxidation states
- +1
- Oxidation states
- -1, +1
Physical properties
- State at 25 °C
- solid
- Density
- 0.862 g/cm³
- Melting point
- 336.7 K · 63.6 °C
- Boiling point
- 1 032 K · 758.9 °C
- Speed of sound
- 2 000 m/s
Thermal properties
- Heat of fusion
- 2.33 kJ/mol
- Heat of vaporisation
- 76.9 kJ/mol
- Specific heat capacity
- 0.757 J/(g·K)
- Thermal conductivity
- 102.5 W/(m·K)
Mechanical properties
- Young's modulus
- 3.53 GPa
- Shear modulus
- 1.3 GPa
- Bulk modulus
- 3.1 GPa
- Poisson ratio
- no data
- Mohs hardness
- 0.4
- Brinell hardness
- 0.363 MPa
Electrical and magnetic properties
- Electrical resistivity
- 72 nΩ·m
- Magnetic ordering
- paramagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- body-centred cubic (bcc)
- Lattice constants
- a = 532.8 pm
Abundance
- In the crust
- 20 900 mg/kg
- In the ocean
- 399 mg/L
- In the universe
- 3 000 mg/kg
- In the human body
- 2 000 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 39K | 93.258 % | stable | — |
| 40K | 0.0117 % | 1.25 Gyr | β−, ЭЗ |
| 41K | 6.73 % | stable | — |
Potassium-40 is radioactive with a 1.25-billion-year half-life and is present in every organism: a 70 kg human runs about 4400 potassium-40 decays per second.
Discovery
- Year of discovery
- 1807
- Discovered by
- Humphry Davy
- Where
- England
- Origin of the name
- from Arabic al-qalyah (plant ashes); the symbol K is from Latin kalium
History
Humphry Davy isolated potassium on 6 October 1807 by electrolysing molten caustic potash — the first metal ever obtained with an electric current. The symbol K is from Latin kalium, itself from the Arabic for plant ash.
Where it occurs
The seventh most abundant element in the crust at 20 900 mg/kg. The ores are sylvite and carnallite from ancient sea beds; the largest deposits are in Canada, Belarus and Russia.
How it is produced
Potash salts are mined and solution-mined and upgraded by flotation. The metal comes from reducing molten KCl with sodium.
Role in living things
The principal intracellular cation. The potassium gradient across a cell membrane creates the membrane potential; a sharp rise in blood potassium stops the heart.
Safety
The metal ignites in damp air and reacts explosively with water. Potassium salts given intravenously in large doses are lethal, which is one component of the American execution protocol.
Uses
- Potash fertiliser — over 90 % of all production
- Caustic potash for liquid soap and biodiesel
- Potassium nitrate in black powder and as a preservative
- Sodium–potassium alloy as a coolant, liquid at room temperature
Curiosities
- A banana is faintly radioactive because of potassium-40 — the origin of the tongue-in-cheek banana equivalent dose.
- Potassium decay supplies nearly all the argon in Earth's atmosphere.
- Potassium burns with a violet flame.
- Potassium was the first metal ever obtained by electrolysis, a week before sodium.