Potassium

19 argon ← potassium → calcium Na ↑ K ↓ Rb periodic table
General
Name, Symbol, Number	potassium, K, 19
Chemical series	alkali metals
Group, Period, Block	1, 4, s
Appearance	silvery white
Atomic mass	39.0983(1) g/mol
Electron configuration	[Ar] 4s1
Electrons per shell	2, 8, 8, 1
Physical properties
Phase	solid
Density (near r.t.)	0.89 g/cm³
Liquid density at m.p.	0.828 g/cm³
Melting point	336.53 K (63.38 °C, 146.08 °F)
Boiling point	1032 K (759 °C, 1398 °F)
Critical point	(extrapolated) 2223 K, 16 MPa
Heat of fusion	2.321 kJ/mol
Heat of vaporization	76.90 kJ/mol
Heat capacity	(25 °C) 29.600 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 473 530 601 697 832 1029
Atomic properties
Crystal structure	cubic body centered
Oxidation states	1 (strongly basic oxide)
Electronegativity	0.82 (Pauling scale)
Ionization energies (more)	1st: 418.8 kJ/mol
	2nd: 3052 kJ/mol
	3rd: 4420 kJ/mol
Atomic radius	220 pm
Atomic radius (calc.)	243 pm
Covalent radius	196 pm
Van der Waals radius	275 pm
Miscellaneous
Magnetic ordering	?
Electrical resistivity	(20 °C) 72.0 nΩ·m
Thermal conductivity	(300 K) 102.5 W/(m·K)
Thermal expansion	(25 °C) 83.3 µm/(m·K)
Speed of sound (thin rod)	(20 °C) 2000 m/s
Shear modulus	1.3 GPa
Bulk modulus	3.1 GPa
Mohs hardness	0.4
Brinell hardness	0.363 MPa
CAS registry number	7440-09-7
Notable isotopes
Main article: Isotopes of potassium iso NA half-life DM DE (MeV) DP 39K 93.26% K is stable with 20 neutrons 40K 0.012% 1.277×109 y β- 1.311 40Ca ε 1.505 40Ar β+ 1.505 40Ar 41K 6.73% K is stable with 22 neutrons

Potassium (chemical symbol K, atomic number 19) is a member of a group of chemical elements known as alkali metals.

• silvery white metal.

• is the chemical element in the periodic table.
• Potassium 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 and is very reactive, especially toward water. In many respects, potassium and sodium are chemically similar.

Discovery and etymology

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

Potassium was not known to the ancient Romans, and its names are neo-Latin, not Classical Latin. The Latin name kalium (from which potassium gets its symbol, K) was taken from the word "alkali", which came from Arabic al qalīy, meaning "the calcined ashes." The name potassium was made from the English word "potash," which originally meant an alkali extracted in water in a pot of ash of burnt wood or tree leaves.

Occurrence

Potassium makes up about 2.4% of the weight of the Earth's crust and is the seventh most abundant element in it. As it is very electropositive, potassium is never found as the free element in nature. Rather, it forms compounds from which it is difficult to separate.

Potassium minerals such as carnallite, langbeinite, polyhalite, and sylvite form extensive deposits in the beds of ancient lakes and seas. The principal source of potassium, potash (impure form of potassium carbonate), is mined in California, New Mexico, Utah, Germany, and in other places around the world. Large deposits of potash have been found about 3,000 feet below the surface of Saskatchewan, and several large mines have been in operation since the 1960s. Saskatchewan pioneered the method of freezing wet sands (the Blairmore formation) as a way to drive mine shafts through them. The oceans are another source of potassium, but the quantity present in a given volume of seawater is relatively low, compared to the concentration of sodium.

Potassium can be isolated through the electrolysis of its hydroxide, by a process that has changed little since the time of Davy. Thermal methods are also employed to obtain potassium from potassium chloride.

Notable characteristics

As an alkali metal, potassium is part of group 1 of the periodic table, between sodium and rubidium, and it lies in period 4, just before calcium. It is a soft, low-melting solid that can be easily cut with a knife. With a density less than that of water, potassium is the second least dense metal after lithium.

Like other alkali metals, potassium has a single electron in its outermost shell, and it can readily lose this electron to become a positive ion. For this reason, although freshly cut potassium is silvery in appearance, it oxidizes and tarnishes rapidly when exposed to the air. In addition, it reacts violently with water, even more so than lithium or sodium, producing hydrogen. The reaction produces so much heat that the evolved hydrogen gas ignites. To prevent this from happening, samples of potassium are stored in an inert medium such as kerosene.

Potassium and its compounds emit a violet color in a flame. This fact is the basis of the flame test for the presence of potassium in a sample.

Isotopes

Potassium in feldspar

There are 17 known isotopes of potassium, three of which occur naturally: ³⁹K (93.3%), ⁴⁰K (0.012%), and ⁴¹K (6.7%). The isotope ⁴⁰K is radioactive and decays to stable ⁴⁰Ar (argon-40) (11.2%) by electron capture and positron emission, and it decays to stable ⁴⁰Ca (calcium-40) (88.8%) by beta decay. The half-life of ⁴⁰K is 1.250×10⁹ years.

The decay of ⁴⁰K to ⁴⁰Ar is the basis for a commonly used method for dating rocks. The conventional potassium-argon dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent ⁴⁰Ar was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic (radioactively produced) ⁴⁰Ar that has accumulated. Minerals that are best suited for dating include biotite, muscovite, 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.

Besides their use for dating, potassium isotopes have been used extensively as radioactive tracers in studies of weathering. They have also been used for nutrient cycling studies because potassium is a macronutrient required for living organisms.

Given that radioactive ⁴⁰K occurs in nature, it becomes part of some commercially available salt substitutes in sufficient quantity for large bags of those substitutes to be used as a radioactive source for classroom demonstrations. In healthy animals and people, ⁴⁰K represents the largest source of radioactivity, greater even then ¹⁴C (carbon-14). In a person who weighs 70 kilograms (kg), about 4,000 nuclei of ⁴⁰K decay per second. [1]

Applications

• It is primarily used in fertilizer as either the chloride, sulfate or carbonate - not as the oxide.
• Potassium hydroxide is an important industrial chemical used as a strong base.
• Potassium nitrate is used in gunpowder (black powder). An older term for KNO₃ is saltpeter.
• Potassium carbonate, known as potash, is used in glass manufacture.
• Glass treated with liquid potassium is much stronger than regular glass.
• NaK, an alloy of sodium and potassium, is used as a heat-transfer medium.
• Potassium is an essential component needed in plant growth and is found in most soil types.
• In animal cells potassium ions are vital to keeping cells alive (see Na-K pump)
• Potassium chloride is used as a substitute for table salt and is also used to stop the heart, e.g. in cardiac surgery and in executions by lethal injection in solution.
• The superoxide KO₂ is used as a portable source of oxygen and as a carbon dioxide absorber. It is useful in portable respiration systems.

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

Precautions

Solid potassium reacts violently with water, and should therefore be kept under a mineral oil such as kerosene and handled with care. Unlike lithium and sodium however, potassium cannot be stored under oil indefinitely. If stored longer than 6 months to a year, dangerous shock-sensitive peroxides can form on the metal and under the lid of the container that can detonate upon opening. DO NOT store potassium, rubidium or cesium for longer than a year unless stored in an inert (argon) atmosphere or in a vacuum.

Potassium in diet

Potassium is an essential mineral micronutrient in human nutrition; it assists in muscle contraction and in maintaining fluid and electrolyte balance in body cells. Potassium is also important in sending nerve impulses as well as releasing energy from protein, fat, and carbohydrates during metabolism.

A shortage of potassium can cause a potentially fatal condition known as hypokalemia, typically resulting from diarrhea, increased diuresis and vomiting. Deficiency symptoms include, muscle weakness, paralytic ileus, ECG abnormalities, decreased reflex response, and in severe form respiratory paralysis and alkalosis and arrhythmia.

Eating a variety of foods that contain potassium is the best way to get an adequate amount. Healthy individuals who eat a balanced diet rarely need supplements. Foods with high sources of potassium include orange juice, avocados, celery, parsnips and turnips, although many other fruits, vegetables, and meats contain potassium. Research has indicated that diets high in potassium can reduce the risk of hypertension.

The 2004 guidelines of the Institute of Medicine specify an RDA of 4,700 mg of potassium. However, it is thought that most Americans consume only half that amount per day ([2]). Similarly, in the European Union, particularly in Germany and Italy, insufficient potassium intake is widespread ([3]).

Some people with kidney disease are advised to avoid large quantities of dietary potassium.

See also

• Potassium compounds
• Potassium in biology

References

ISBN links support NWE through referral fees

• Los Alamos National Laboratory – Potassium

External links

• WebElements.com – Potassium

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

• Potassium  ^history

The history of this article since it was imported to New World Encyclopedia:

• History of "Potassium"

Note: Some restrictions may apply to use of individual images which are separately licensed.