PTL:Potassium
From ChemEd Collaborative
Potassium, a silvery metal, is a member of the alkali metal family (Group IA) and the first member of the 4th period. Other members of the group are lithium (Li), sodium (Na), rubidium (Rb), cesium (Cs), and francium (Fr).
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Discovered
The elemental metal was first prepared in 1807 by Davy, who obtained it from caustic potash (KOH) by electrolysis.
Name
The name is derived from the English, potash (K2CO3 and KOH) or pot ashes, historically a source of the element. The name used in Germany is derived from the Latin word, kalium, which was in turn derived from the Arabic word, al-qali, for alkali. The symbol K is an abbreviation of kalium.
Characteristics
The metal is the eighth most abundant element in the earth's crust and makes up about 2.4% by weight of the earth's crust.
The metal has a body centered cubic structure.
Reactions
Potassium is one of the most reactive and electropositive of metals. It is soft, easily cut with a knife, and is silvery in appearance immediately after a fresh surface is exposed. It rapidly oxidizes in air and must be preserved in a mineral oil, such as kerosene. The metal reacts with O2 to give potassium monoxide (K2O), the peroxide (K2O2),
2 K + O2(g) 
K2O2(s)
and the superoxide (KO2).
K(s) + O2(g) KO2(s)
The formation of superoxides - a class of compounds based on the (O2)- ion - is characteristic of potassium, rubidium, and cesium, but not of lithium or sodium.
As with other metals of the alkali group, potassium reacts vigorously with water with the evolution of hydrogen,
2 K(s) + 2 H2O(liq) 2 KOH(aq) + H2(g)
which catches fire spontaneously. Potassium and its salts impart a violet color to flames. An alloy of sodium and potassium (NaK) is used as a heat transfer medium.
Found
Because it is so reactive, potassium metal is never found free in nature. Rather, it is obtained by electrolysis of the hydroxide, much in the same manner as prepared by Davy.
One of the reasons for the great reactivity of potassium is that it has the lowest ionization energy of the 4th period elements; indeed, it has one of the lowest ionization energies of all of the alkali metals.
Most potassium minerals are insoluble and the metal is obtained from them only with great difficulty. Certain minerals, however, such as sylvite (KCl), carnallite [KMgCl3·6(H2O)], langbeinite [K2Mg2(SO4)3], and polyhalite [K2Ca2Mg(SO4)4·2(H2O)] are found in ancient lake and sea beds and form rather extensive deposits from which potassium salts can readily be obtained. Potash is mined in Germany, New Mexico, California, Utah, and elsewhere. Large deposits of potash have been found at a depth of some 3000 ft. in Saskatchewan. Potassium compounds are also found in the ocean, but potassium is present only in relatively small amounts, compared to sodium.
Uses
The greatest demand for potash has been in its use for [fertilizers]. Potassium salts are also found in low-sodium salt (KCl) and in matches (KClO3).
Emission Spectrum
This is the visible emission spectrum for potassium.
