Alkali metal

The alkali metals are a series of chemical elements forming Group 1 (IUPAC style) of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). (Hydrogen, although nominally also a member of Group 1, very rarely exhibits behavior comparable to the alkali metals). The alkali metals provide one of the best examples of group trends in properties in the periodic table, with well characterized homologous behavior down the group.

Properties

The alkali metals are all highly reactive and are never found in elemental forms in nature. Because of this, they are usually stored immersed in mineral oil or kerosene (paraffin oil). They also tarnish easily and have low melting points and densities.

Physically, the alkali metals are mostly silver-colored, except for metallic caesium, which has a golden tint. These elements are all soft metals of low density. Chemically, all of the alkali metals react aggressively with the halogens to form ionic salts. They all react with water to form strongly alkaline hydroxides. The vigor of reaction increases down the group. All of the atoms of alkali metals have one electron in their outmost electron shells, hence their only way for achieving the equivalent of filled outmost electron shells is to give up one electron to an element with high electronegativity, and hence to become singly charged positive ions, i.e. cations.

When it comes to their nuclear physics, the elements potassium and rubidium are naturally weakly radioactive because they each contain a long half-life radioactive isotope:

The element hydrogen, with its solitary one electron per atom, is usually placed at the top of Group 1 of the periodic table, for convenience, but hydrogen is not counted as an alkali metal. Under typical conditions, pure hydrogen exists as a diatomic gas consisting of two atoms per molecule.

The removal of the single electron of hydrogen requires considerably more energy than removal of the outer electron from the atoms of the alkali metals. As in the halogens, only one additional electron is required to fill in the outermost shell of the hydrogen atom, so hydrogen can in some circumstances behave like a halogen, forming the negative hydride ion. Binary compounds of hydrogen with the alkali metals and some transition metals have been produced in the laboratory, but these are only laboratory curiosities without much practical use. Under extremely high pressures and low temperatures, such as those found at the cores of the planets Jupiter and Saturn, hydrogen does become a metallic element, and it behaves like an alkali metal. (See metallic hydrogen.)

The alkali metals have the lowest ionization potentials in their periods of the periodic table, because the removal of their single electrons from their outmost electron shells gives them the stable electron configuration of inert gases. Another way of stating this is that they all have a high electropositivity. The "second ionization potential" of all of the alkali metals is very high, since removing any electron from an atom having a noble gas configuration is difficult to do.

Series of alkali metals, stored in mineral oil ("natrium" is sodium.)

All of the alkali metals are notable for their vigorous reactions with water, and these reactions become increasingly vigorous when going down their column in the periodic table towards the heaviest alkali metals, such as caesium. Their chemical reactions with water are as follows:

Alkali metal + water → Alkali metal hydroxide + hydrogen gas

For a specific example with potassium (K):

2 K (s) + 2 H₂O (l) → 2 KOH (aq) + H₂ (g)

Trends

Like in other columns of the periodic table, the members of the alkali metal family show patterns in their electron configurations, especially their outmost electron shells. This causes similar patterns in their chemical properties:

The alkali metals show a number of trends when moving down the group - for instance: decreasing electronegativity, increasing reactivity, and decreasing melting and boiling point. Their densities generally increase, with the notable exception that potassium is less dense than sodium, and the possible exception of francium being less dense than caesium. (The highly radioactive element francium only exists in microscopic quantities.)

Compounds

Alkali metals form a very wide range of amalgams.^[1] They tend to form ionically bonded salts with most electronegative elements on the periodic table, like cesium fluoride and sodium chloride.

References

• Campbell, Linda M., Aaron T. Fisk, Xianowa Wang, Gunter Kock, and Derek C. Muir (2005). "Evidence for Biomagnification of Rubidium in Freshwater and Marine Food Webs". Canadian Journal of Fisheries and Aquatic Sciences 62: 1161–1167. doi:10.1139/f05-027. 
• Chang, Cheng-Hung, and Tian Y. Tsong (2005). "Stochastic Resonance of Na, K-Ion Pumps on the Red Cell Membrane". AIP Conference Proceedings: 18th International Conference on Noise and Fluctuations. 780. American Institute of Physics. pp. 587–590. doi:10.1063/1.2036821. ISBN 0-7354-0267-1. 
• Joffe, Russell T., Stephen T. Sokolov and Anthony J. Levitt (2006). "Lithium and Triiodothyronine Augmentation of Antidepressants". Canadian Journal of Psychiatry 51 (12): 791–793. PMID 17168254. 
• Bauer, Brent A., Robert Houlihan, Michael J. Ackerman, Katya Johnson, and Himeshkumar Vyas (2006). "Acquired Long QT Syndrome Secondary to Cesium Chloride Supplement". Journal of Alternative and Complementary Medicine 12 (10): 1011–1014. doi:10.1089/acm.2006.12.1011. PMID 17212573. 
• Erermis, Serpil, Muge Tamar, Hatice Karasoy, Tezan Bildik, Eyup S. Ercan, and Ahmet Gockay (1997). "Double-Blind Randomised Trial of Modest Salt Restriction in Older People". Lancet 350: 850–854. doi:10.1016/S0140-6736(97)02264-2. 
• Krachler, M, and E Rossipal (1999). "Trace Elements Transfer From Mother to the Newborn - Investigations on Triplets of Colostrum, Maternal and Umbilical Sera". European Journal of Clinical Nutrition 53 (6): 486–494. doi:10.1038/sj.ejcn.1600781. PMID 10403586. 
• Stein, Benjamin P., Stephen G. Benka, Phillip F. Schewe, and Bertram Schwarzhild (1996). "Physics Update". Physics Today 49 (6): 9. doi:10.1063/1.2807642. 
• "Group 1: The Alkali Metals". Visual Elements. Royal Society of Chemistry. http://www.chemsoc.org/Viselements/pages/data/intro_groupi_data.html. Retrieved 2009-12-08. 

See also

• alkaline earth metal
• Lithium
• Sodium
• Potassium
• Rubidium
• Caesium
• Francium

External links

• Science aid:Alkali metals A simple look at alkali metals
• Atomic and Physical Properties of the Group 1 Elements An in-depth look at alkali metals

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