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Element Fact
Cesium is used in atomic clock. As in 1999, the best clock is accurate to a second in 20 million years.

Potassium, K

potassium sample

Atomic number: 19
Atomic mass: 39.0983(1)
Natural abundance: 18400 ppm
39K (38.9637068) -93.2581%
40K (39.963998) - 0.0117%
41K (40.9618260) - 6.7302%

History and Production

From Latin kalium or Arab qali, meaning alikali. From English potash, meaning pot ashes. Discovered by H. Davy in 1807 by electrolysis of caustic potash (KOH). Commercial productions of the element involves the reduction of molten KCl with metallic sodium at 850C, or using electrolysis method as was discovered by Davy. The potassium compounds are commercially more important than the elemental form. For example, potassium bromide (KBr) is used in photography and as a sedative; KClO3, in safety matches and explosives; KO2, in breathing apparatus.

Physical Data

It is one of the most electropositive of metals. It is soft, silvery in appearance. It is usually kept in a mineral oil since it easily reacts with air. It reacts violently with water, often catches fire spontaneously. Potassium is seventh most abundant in earth's crust of which important minerals including sylvite (KCl) and sylvinite (mixed NaCl and KCl).

Interatomic distance: 454.4 pm
Melting point: 63.38C
Boiling point: 759C
Thermal conductivity/Wm-1K-1: 102.3 (27C)
Density/kgm-3: 862 (20C), 828 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 89.0 kJ/mol
Gibbs free energy of formation: 60.5 kJ/mol
Entropy: 160.3 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Ar] 4s1
Term symbol: 2S1/2
Electron affinity: 48.3845 kJ/mol   Electronegativity (Pauline): 0.82
Ionization energy (first, second, third): 418.810, 3025.78, 4419.61 kJ/mol

Chemical properties

Potassium reacts violently with water to give hydrogen and potassium hydroxide. The metal may ignite from the heat of reaction with a lilac flame. Potassium also readily reacts with the oxygen in air. For this reason, the metal is usually kept in an inert organic liquid such as paraffin in an air-tight bottle (as shown above). Potassium is a very reactive metal, forming a wide range of compounds with most inorganic and organic materials. Under certain conditions potassium reacts with halogen vapours to produce light (chemiluminescence). The reactivity of potassium is powerful enough to reduce most transition metal oxides to the respective metals.

Potassium (and indeed other alkali metals) dissolves in liquid ammonia to form blue solutions and are believed to contain solvated electrons. A slow reaction, which may take several days, eventually produces an amide (KNH2) and hydrogen. It is therefore a powerful reducing agent and is capable to reduce certain organic aromatic compounds that are otherwise difficult or impossible to achieve.

Test for potassium:
(1) Flame test on potassium salts give a lilac-colored flame.
(2) Sodium perchlorate, NaClO4, precipitates potassium perchlorate slowly as a colorless crystals.
(3) Sodium tetraphenylborate, NaB(C6H5)4, gives a white precipitate of the potassium salt in weakly acid solution. The test is quantitative, of which the precipitate can be filtered off, dried, and weighed to estimate potasium.

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