Physics & Chemistry


September 20, 2011, 8:17 pm
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Potassium pearls in oil. Size of the largest pearl is 5 mm. Source:

Potassium is the chemical element with the symbol K, atomic number 19, and atomic mass 39.0983. Potassium was first isolated from the mineral potash. Elemental potassium is a soft silvery-white metallic alkali metal which oxidizes swiftly when exposed to air, and is highly chemically reactive with water, with the result of liberating hydrogen . Elemental potassium does not occur in the natural environment, since it reacts violently with water. As various molecular compounds, potassium makes up about one and one half percent of the mass of the Earth's crust and is the seventh most abundant element in the crust. The element is electropositive and highly reactive.

Previous Element: Argon

Next Element: Calcium


Physical Properties
Color silvery-white
Phase at Room Temp. solid
Density (g/cm3) 0.856
Hardness (Mohs) .5
Melting Point (K) 336.5
Boiling Point (K) 1038.7
Heat of Fusion (kJ/mol) 2.4
Heat of Vaporization (kJ/mol) 88
Heat of Atomization (kJ/mol) 90
Thermal Conductivity (J/m sec K) 102.5
Electrical Conductivity (1/mohm cm) 138.9
Source Sylvite, carnallite
Atomic Properties
Electron Configuration [Ar]4s1
Number of Isotopes 3
Electron Affinity (kJ/mol) 48.385
First Ionization Energy (kJ/mol) 418.8
Second Ionization Energy (kJ/mol) 3051.3
Third Ionization Energy (kJ/mol) 4411.3
Electronegativity 0.82
Polarizability (Å3) 43.4
Atomic Weight 39.0938
Atomic Volume (cm3/mol) 45.7
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) 227
Ionic Radius1+ (pm) 152
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) ---
Common Oxidation Numbers +1
Other Oxid. Numbers ---
In Earth's Crust (mg/kg) 2.09×104
In Earth's Ocean (mg/L) 3.99×102
In Human Body (%) 0.20%
Regulatory / Health
CAS Number 7440-09-7
OSHA Permissible Exposure Limit No limits
OSHA PEL Vacated 1989 No limits
NIOSH Recommended Exposure Limit No limits
Mineral Information Institute
Jefferson Accelerator Laboratory


The term potassium derives from the word potash, after the method by which potash was extracted, viz. the leaching of ash from charred wood and leaves, and thence evaporating the residue in a pot. Potash is principalily a potassium salt, since plants have little sodium content, and the balance of plant mineral content is of calcium salt of low aqueous solubility. While potash has been used since ancient times, it was not understood to be a fundamentally different substance from sodium mineral salts. Georg Ernst Stahl postulated the fundamental distinction between sodium and potassium salts in the year 1702.


  • N.Shi, S.Ye, A.Alam, L.Chen and Y.Jiang. 2006. Atomic structure of a Na+- and K+-conducting channel. Nature 440 (7083): 570–4.
  • V.A.Shaposhnik. 2007. History of the discovery of potassium and sodium (on the 200th anniversary of the discovery of potassium and sodium. Journal of Analytical Chemistry 62 (11): 1100–1102.




Hogan, C. (2011). Potassium. Retrieved from


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