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Name, Symbol, Number potassium, K, 19
Series alkali metals
Group, Period, Block 1(IA), 4 , s
Density, Hardness 856 kg/m3, 0.4
Appearance silvery white
Atomic properties
Atomic weight 39.0983 amu
Atomic radius (calc.) 220 (243) pm
Covalent radius 196 pm
van der Waals radius 275 pm
Electron configuration [Ar]4s1
e- 's per energy level 2, 8, 8, 1
Oxidation states (Oxide) 1 (strong base)
Crystal structure cubic body centered
Physical properties
State of matter solid
Melting point 336.53 K (146.08 F)
Boiling point 1032 K (1398 F)
Molar volume 45.94 ×10-6 m3/mol
Heat of vaporization 79.87 kJ/mol
Heat of fusion 2.334 kJ/mol
Vapor pressure 1.06×10-4Pa at __ K
Speed of sound 2000 m/s at 293.15 K
Electronegativity 0.82 (Pauling scale)
Specific heat capacity 757 J/(kg*K)
Electrical conductivity 13.9 106/(mohm)
Thermal conductivity 102.4 W/(m*K)
1st ionization potential 418.8 kJ/mol
2nd ionization potential 3052 kJ/mol
3rd ionization potential 4420 kJ/mol
4th ionization potential 5877 kJ/mol
5th ionization potential 7975 kJ/mol
6th ionization potential 9590 kJ/mol
7th ionization potential 11343 kJ/mol
8th ionization potential 14944 kJ/mol
9th ionization potential 16963.7 kJ/mol
10th ionization potential 48610 kJ/mol
Most stable isotopes
iso NA half-life DM DE MeV DP
39K 93.26% K is stable with 20 neutrons
40K 0.012% 1.277 E9 y β-
41K 6.73% K is stable with 22 neutrons
SI units & STP are used except where noted.

Potassium is a chemical element in the periodic table that has the symbol K (L. kalium) and atomic number 19. This is a soft, silvery-white metallic alkali metal that occurs naturally bound to other elements in seawater and many minerals. It oxidizes rapidly in air, is very reactive, especially in water, and resembles sodium chemically.


Notable characteristics

With a density less than that of water, potassium is the second lightest metal after lithium. It is a soft solid that easily is cut with a knife and is silvery in color on fresh surfaces. It oxidizes in air rapidly and must be stored in mineral oil for preservation.

Similar to other alkali metals, potassium reacts violently with water producing hydrogen. When in water, it may catch fire spontaneously. Its salts emit a violet color when exposed to a flame.


Many potassium salts are very important, and include, potassium; bromide, carbonate, chlorate, chloride, chromate , cyanide, dichromate, hydroxide, iodide, nitrate, sulfate.


Potassium was discovered in 1807 by Sir Humphry Davy, who derived it from caustic potash (KOH). This alkali metal was the first metal that was isolated by electrolysis.


This element makes up about 2.4% of the weight of the Earth's crust and is the seventh most abundant element in it. Due to its insolubility , it is very difficult to obtain potassium from its minerals.

However, other minerals, such as carnallite , langbeinite , polyhalite , and sylvite are found in ancient lake and sea beds. These minerals form extensive deposits in these environments, making extracting potassium and its salts more economical. The principle source of potassium, potash is mined in California, Germany, New Mexico, Utah, and in other places around the world. At 3000 ft below the surface of Saskatchewan are large deposits of potash which may become important sources of this element and its salts in the future.

The oceans are another source of potassium but the quantity present in a given volume of seawater is relatively low compared to sodium.

Potassium can be isolated through electrolysis of its hydroxide in a process that has changed little since Davy. Thermal methods also are employed in potassium production, using potassium chloride. Potassium is almost never found unbound in nature. However, in living organisms K+ ions are important in the physiology of excitable cells.


There are seventeen isotopes of potassium known to exist. The non-synthetic form of potassium are composed of three isotopes: K-39 (93.3%), K-40 (0.01%) and K-41 (6.7%). Naturally occurring K-40 decays to stable Ar-40 (11.2%) by electron capture and by positron emission, and decays to stable Ca-40 (88.8%) by negatron emission; K-40 has a half-life of 1.250 × 109 years.

The decay of K-40 to Ar-40 is commonly used as a method for dating rocks. The conventional K-Ar dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (i.e., Ar-40) was quantitatively retained, i.e., closed system. Minerals are dated by measurement of the concentration of potassium, and the amount of radiogenic Ar-40 that has accumulated. The minerals that are best suited for dating include biotite, muscovite, and plutonic/high grade metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered.

Outside of dating, potassium isotopes have been used extensively as tracers in studies of weathering. They have also be used for nutrient cycling studies because potassium is a macro-nutrient required for life.

K-40 occurs in natural potassium (and thus in some commercial salt substitutes) in sufficient quantity that large bags of those substitutes can be used as a radioactive source for classroom demonstrations.

Biological Role

Potassium in the body exists as a monovalent positive ion (a cation), K+, concentrated by energy-requiring mechanisms primarily within cells (intracellular), where it comprises the cell's most abundant monovalent inorganic cation. The body regulates the K+ concentration in blood moderately closely, as substantial fluctuations can affect action potentials, causing heart and nervous problems. Many antibiotics, such as the one produced by the bacterium Bacillus brevis, affect cells by setting up positive ion gates, where the K+ and Na+ ions are permitted to cross the membrane, thus disrupting the action potential of the cell membrane.

The body maintains potassium ion concentration relatively low in blood plasma (usually 3.5 - 5.0 mmol/L), but much higher inside cells (about 100 mmol/L). Abnormally low blood levels, hypokalemia, and abnormally high levels, hyperkalemia, both can adversely affect the heart.


Solid potassium reacts violently with water. This metal should therefore be kept under a mineral oil such as kerosene and handled with care.


  • Los Alamos National Laboratory – Potassium

External links

  • – Potassium
  • – Potassium

Last updated: 02-05-2005 14:56:50
Last updated: 02-20-2005 19:53:55