POTASSIUM SYMBOL:K PERIOD:3 GROUP:1(IA) ATOMICNO:19
POTASSIUM SYMBOL:K PERIOD:3 GROUP:1(IA) ATOMICNO:19
ATOMICMASS:39.0983amu VALENCE:1 OXIDATIONSTATE:+1 NATURALSTATE:Solid ORIGINOFNAME:Itssymbol“K”isderivedfromtheLatinwordforalkali,kalium,butitis commonlycalled“potash”inEnglish. ISOTOPES:Atotalof18isotopesofpotassiumhavebeendiscoveredsofar.Justtwoof themarestable:K-39makesup93.2581%ofpotassiumfoundintheEarth’scrust,and K-41makesup6.7301%oftheremainderofpotassiumfoundonEarth.Alltheother 16potassiumisotopesareunstableandradioactivewithrelativelyshorthalf-lives,andas theydecay,theyproducebetaparticles.TheexceptionisK-40,whichhasahalf-lifeof 1.25×10 9 years.
54 | The History and Use of Our Earth’s Chemical Elements
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=8
s2,p6
4-N=1
s1
Properties Elemental potassium is a soft, butter-like silvery metal whose cut surface oxidizes in dry
air to form a dark gray potassium superoxide (KO 2 ) coating. KO 2 is an unusual compound, in that it reacts with both water and carbon dioxide to produce oxygen gas. It appears more like a hard wax than a metal. Its density (specific gravity) is 0.862 g/cm 3 , its melting point is 63.25°C, and its boiling point is 760°C. It has an oxidation state of +1 and reacts explosively with room temperature air or water to form potassium hydroxide as follows: 2K + 2 H 2 O →∆ 2KOH + H 2 . This is an endothermic reaction, which means the heat generated is great enough to ignite the liberated hydrogen gas. Potassium metal must be stored in a non-oxygen, non-aqueous environment such as kerosene or naphtha.
Characteristics Because its outer valence electrons are at a greater distance from its nuclei, potassium is
more reactive than sodium or lithium. Even so, potassium and sodium are very similar in their chemical reactions. Due to potassium’s high reactivity, it combines with many elements, par- ticularly nonmetals. Like the other alkali metals in group 1, potassium is highly alkaline (caus- tic) with a relatively high pH value. When given the flame test, it produces a violet color.
AbundanceandSource Potassium is the eighth most abundant element in the Earth’s crust, which contains about
2.6% potassium, but not in natural elemental form. Potassium is slightly less abundant than sodium. It is found in almost all solids on Earth, in soil, and in seawater, which contains 380 ppm of potassium in solution. Some of the potassium ores are sylvite, carnallite, and polyha- lite. Ore deposits are found in New Mexico, California, Salt Lake in Utah, Germany, Russia, and Israel. Potassium metal is produced commercially by two processes. One is thermochemi- cal distillation, which uses hot vapors of gaseous NaCl (sodium chloride) and KCl (potassium chloride); the potassium is cooled and drained off as molten potassium, and the sodium chlo- ride is discharged as a slag. The other procedure is an electrolytic process similar to that used to produce lithium and sodium, with the exception that molten potassium chloride (which melts at about 770°C) is used to produce potassium metal at the cathode (see figure 4.1).
Guide to the Elements | 55 History
In 1807 Sir Humphry Davy was the first to isolate potassium metal. He used his famous electrolysis method of passing an electric current through melted potassium chloride. He noticed small globs of silvery metallic potassium formed at the cathode that reacted strongly with water releasing hydrogen gas. The reaction was strong enough to raise the temperature to the degree that the hydrogen was ignited. Through this reaction, Davy recognized that he had produced elemental potassium.
For years, several potassium compounds were collected and identified by leeching water through wood ashes, drying the solutions, and then examining their crystals.
CommonUses As with other alkali metals, potassium compounds have many uses. For example, almost
all of the compound potassium chloride is used in fertilizers. Currently potassium chloride is mined or derived from seawater. Many years ago, potassium was secured for human use by burning wood and plant matter in pots to produce an ash called potash, which was mostly potassium carbonate and used as a caustic, mainly for making soap when mixed with fats.
Several explosives are mixtures of compounds of potassium and other substances. It is an important raw material for making explosives, matches, and fireworks. Liquid potassium, when mixed with liquid sodium (NaK), is an alloy used as a heat- exchange substance to cool nuclear reactors. Potassium is an important reagent (something that is used in chemical reactions to analyze other substances) that forms many compounds used in chemical and industrial laboratories. It is used to manufacture both hard and soft soaps, as a bleaching agent, and where a highly caustic chemical is required. Potassium is essential to all living organisms. It is a trace element required for a healthy diet and is found in many foods. One natural source is bananas.
ExamplesofCompounds Potassium nitrate (KNO 3 ) has been known for hundreds of years and was originally known
as “saltpeter,” a name derived from the Greek word petra, which stands for “rock.” Potassium nitrate was thought to be an element until the 1700s, when it was discovered that several of the alkali metals (including potassium) were combined with nitrogen. All of the early sources of potassium nitrate were found as residues around salty lakes, in decayed matter, in guano (bat dung), or in natural deposits. In the tenth century the Chinese were using natural saltpe- ter mixed with charcoal and sulfur to make fireworks and an early form of gunpowder. It was not until the thirteenth century that this technology arrived in Europe, where the production of gunpowder and firearms soon became common. Finding sources of the scarce saltpeter was a considerable problem that was not solved until the 1700s, when chemists realized that nitrogen was also an element in this compound (sometimes called “niter”). Potassium nitrate can be prepared by treating sodium hydroxide (from leeched wood ashes) with nitric acid:
HNO 3 + KOH → KNO 3 +H 2 O. Other processes are possible as well. In addition to fireworks and fertilizers, potassium nitrate is also used for preserving foods and in match heads, and at one time, it was used as a “sexual depressant.” However, the evidence for effectiveness of the treatment is scanty.