|
19
K
39.10
|
| Classification: | Potassium is an alkali metal |
| Atomic weight: | 39.0983 g/mol |
| Density @ 20 oC: | 0.862 g/cm3 |
| Atomic volume: | 45.46 cm3/mol |
Discovery of Potassium
In 1806 Sir Humphry Davy discovered that chemical bonding was electrical in nature and that he could use electricity to split substances into their basic building blocks - the chemical elements.
In 1807 he isolated potassium for the first time. He electrolyzed dried potassium hydroxide (potash) which he had very slightly moistened by exposing it to the moist air in his laboratory. The electrolysis was powered by the combined output of three large batteries he had built.
When he applied a voltage from his batteries to the potassium hydroxide, he found globules "having a high metallic lustre" collected at the negatively charged electrode. (1)
Edmund Davy, who assisted in the experiment, described Sir Humphry Davy's reaction to the production of potassium metal, his Eureka moment:
"... when the minute globules of potassium burst through the crust of potash, and take fire as they entered the atmosphere, he could not contain his joy - he actually danced about the room in ecstatic delight; some little time was required for him to compose himself sufficiently to continue the experiment." (2)
Potassium was the first metal to be isolated by electrolysis.
Davy was astonished at the new metal's low density, observing that it floated on oil - something no other metal would do. He placed a piece of potassium in water and observed that the water, "decomposes it with great violence, an instantaneous explosion with brilliant flame." He also (bravely) added potassium to hydrochloric acid and saw it burn with a bright red flame. (3)
The name potassium is from the English word 'potash', originally meaning an alkali extracted with water in a pot of ash of burned wood or tree leaves.
Potassium's symbol K comes from 'kalium' the name of the element in Germany and Scandinavia. (4)
Just a few days after isolating potassium, Davy isolated sodium for the first time using the same method.
States
Melting point: 336.5 K (63.4 oC)
Boiling point: 1038.7 K (765.6 oC)
Energies
Heat of fusion: 2.334 kJ mol-1
1st ionization energy: 418.8 kJ mol-1
3rd ionization energy: 4411.3 kJ mol-1
Heat of vaporization: 79.870 kJ mol-1
2nd ionization energy: 3051.3 kJ mol-1
Electron affinity: 48.385 kJ mol-1
Oxidation & Electrons
Minimum oxidation number: 0
Min. common oxidation no.: 0
Electronegativity (Pauling Scale): 0.82
Maximum oxidation number: 1
Max. common oxidation no.: 1
Polarizability volume: 43.4 Å3
Appearance & Characteristics
Hardness: 0.4 mohs
Harmful effects:
Potassium is considered to be non-toxic. Due to its highly reactive nature, elemental potassium must be handled with extreme care.
Characteristics:
Potassium is silvery-white, low melting, metal soft enough to be easily cut with a knife. It tarnishes rapidly in air, forming a dull oxide coating.
Potassium burns with a lilac colored flame. It is extremely reactive, reacting violently with water, for example, to produce hydrogen gas and potassium hydroxide.
Potassium is a very light metal (the second least dense metal after lithium) and would float on water if it were not so reactive.
Uses:
Potassium is vital for plant growth. Plants use it, for example, to make proteins, hence the greatest demand for potassium compounds is in fertilizers.
Potassium hydroxide is a strong alkali and an important industrial chemical. It is used in the manufacture of soft soaps and as an electrolyte in alkaline batteries.
Potassium chloride is used as a healthier alternative to table salt.
Toughened glass can be made by immersing glass in molten potassium nitrate.
Potassium nitrate is the main explosive ingredient in gunpowder.
Reactions & Compounds
Reaction with 3 M HNO3: vigorous, ⇒ H2, NOx, KNO3
Oxide(s): K2O
Hydride(s): KH
Reaction with 6 M NaOH: vigorous, ⇒ H2, KOH
Chloride(s): KCl
Radius
Ionic radius (2+ ion): pm
Ionic radius (2- ion): pm
Ionic radius (3+ ion): pm
Ionic radius (1- ion): pm
Conductivity
Abundance & Isotopes
Abundance solar system: 4 parts per million by weight, 100 parts per billion by moles
Cost, pure: $100 per 100g
Cost, bulk: $65 per 100g
Source: Potassium does not occur as a free element in nature; it is too reactive, forming compounds from which it is difficult to separate. Potassium is obtained commercially by electrolysis of potassium hydroxide or potassium chloride
Isotopes: Potassium has 20 isotopes whose half-lives are known, with mass numbers 35 to 54. Of these, two are stable, 39K, and 41K. Over 93.2% of naturally occurring potassium is in the form of 39K.
References
2. John Davy, Memoirs of the Life of Sir Humphry Davy, 1836, p384 Longman.
3. John Davy (Editor), The Collected Works of Sir Humphry Davy, Vol V, 1840, p68 Smith, Elder and Co. Cornhill.
4. Vivi Ringnes, Origin of the Names of Chemical Elements., J. Chem. Educ., 1989, 66 (9), p731.
Cite this Page
<a href="http://www.chemicool.com/elements/potassium.html">Potassium</a>or
<a href="http://www.chemicool.com/elements/potassium.html">Potassium Element Facts</a>To cite this page in an academic document, please use the following MLA compliant citation:
"Potassium." Chemicool Periodic Table. Chemicool.com. 24 Feb. 2011. Web. <http://www.chemicool.com/elements/potassium.html>.