Potassium Facts 1

Potassium is the 19th element of the periodic table. These potassium facts contain chemical and physical data along with general information and history.

Element cell for Potassium

Potassium periodic table cell

Basic Potassium Facts

Name: Potassium

Atomic Number: 19

Element Symbol: K

Group: 1

Period: 4

Block: s

Element Family: Alkali Metal

Atomic Mass: 39.0983(1)

Electron Configuration: [Ar]4s1
Full: 1s22s22p63s23p64s(full)

Discovery: Sir Humphry Davy in 1807

Davy is well known for his electrolysis experiments. He would place his instrument in samples of various common chemicals and observe what collects at the electrodes. He isolated pure potassium metal from potash (potassium hydroxide).

Name Origin: Potassium was named from its source: potash. Potash was formed from wood ashes soaked in water.

Element Symbol Origin: The K symbol for potassium comes from the Latin name for potash, kalium. German and Scandinavian chemists used the name kalium for potassium in their texts and journals.


Natural potassium is comprised of three isotopes: 39K, 40K and 41K.
Two stable and twenty two radioactive isotopes exist ranging from 32K to 56K.

Potassium-39 is a stable isotope containing 20 neutrons. 93.2581% of natural potassium is potassium-39.

Potassium-40 is a radioactive isotope containing 21 neutrons. Most of the time, potassium-40 decays by β- decay into 40Ca, but it can also decay β+ decay or electron capture into 40Ar with a half-life of 1.2 billion years. Potassium-40 accounts for 0.117% of natural potassium.

Potassium-41 is a stable isotope containing 22 neutrons. 6.7302% of natural potassium is potassium-39.

Potassium Metal

Small sample of potassium metal under oil. Credit: Creative Commons

Physical Data

Density:  0.89 g/cm3

Melting Point: 336.7 K ​(63.5 °C, ​146.3 °F)

Boiling Point: 1032 K ​(759 °C, ​1398 °F)

Critical Point: 2223 K at 16 MPa

State at 20ºC: Solid

Heat of Fusion: 2.33 kJ/mol

Heat of Vaporization: 76.9 kJ/mol

Molar Heat Capacity: 29.6 J/mol·K

Potassium atom

Electron shell configuration for potassium.

Atomic Data

Atomic Radius: 2.27 Å (empirical)

Covalent Radius: 2.03 Å

Van der Waals Radius:  2.75 Å

Electron Affinity: 48.385 kJ/mol

Electronegativity: 0.82

1st Ionization Energy: 418.81 kJ/mol

2nd Ionization Energy: 3051.83 kJ/mol

3rd Ionization Energy: 4419.607 kJ/mol

4th Ionization Energy: 5876.92 kJ/mol

5th Ionization Energy: 7975.48 kJ/mol

6th Ionization Energy: 9590.6 kJ/mol

7th Ionization Energy: 11995.347 kJ/mol

8th Ionization Energy: 13841.79 kJ/mol

Oxidation States: +1 (common), -2 (uncommon)

Potassium Flame Test

Potassium flame test. Potassium burns with a bright red light. Credit: Herge/Public Domain

Fun Potassium Facts

  • Potassium is a shiny, lustrous metal at room temperature. When exposed to air, an oxidizing layer forms quickly, turning its appearance to a dull gray.
  • Potassium vigorously reacts with water to form hydrogen gas. This gas can ignite from the energy released from the reaction, giving the impression the potassium burns in water.
  • Potassium was the first metal to be discovered by electrolysis.
  • Potassium has a low density for a metal. Pure potassium metal will float on water.
  • Potassium burns with a bright red in a flame test. When in water, the flame takes on a lilac colored hue.
  • Potassium is the seventh most abundant element in the human body.
  • Potassium is the seventh most abundant element in the Earth’s crust, accounting for 4.2% by mass.
  • The largest industrial use of potassium is for fertilizer.
  • Potassium is used in soaps, gunpowder, bleaching agents and glass making.
  • Potassium-40 is used much like carbon-14 as a radioactive dating marker. K-40 is used to determine the age of rock formations.

Learn more about elements on the periodic table.

About Todd Helmenstine

Todd Helmenstine is the physicist/mathematician who creates most of the images and PDF files found on sciencenotes.org. Nearly all of the graphics are created in Adobe Illustrator, Fireworks and Photoshop. Todd also writes many of the example problems and general news articles found on the site.

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