Content in a Nutshell

From saline solutions to street lamps, from baking powders to bleaches, from fertilizers to fireworks, alkali metal compounds find everyday uses in our lives. Elements necessary for life, yet ones that react violently with water: do these two sound incompatible? In their aqueous ionic forms, sodium and potassium ions (Na+ and K+) are essential ingredients for animal and plant life, yet the elemental atomic forms of these alkali metals react vigorously with water and other compounds.

The alkali metals (Li, Na, K, Rb, Cs, Fr) form a vertical family of elements that begin each horizontal row of the Periodic Table. Francium, element 87, is a rare radioactive decay product of the radioactive element, actinium. Even the most stable isotope of franciumdecays sorapidlythatits chemicalpropertiesarenot wellknown.Theother alkali metals are silvery metallic solids, as soft as cold butter. Lithium, sodium, and potassium are less dense than water; hence they float on it (but don't try it!). Lithium's density (0.53 g/cm3) is just over half that of water. All have low melting points. Cesium melts at 29 °C, a bit above room temperature; Na melts at 98 °C, just below the boiling point of water. The alkali metals are so highly reactive that they never occur free (in their elemental form) in nature, but always in combination with other elements. Since they react rapidly with oxygen in air and violently with water, they must be stored in unreactive oil or kerosene.

All alkali metals react vigorously with halogens to produce alkali halides. Large quantities of alkali metal chlorides are found in the oceans, inland seas, and salt deposits. Fifty million billion (5 x 1016) tons of salt (NaCl) are dissolved in earth's oceans. The sodium ion, Na+, is the principal positive ion in fluids surrounding cells in our bodies, where it is needed for water retention and muscle action. This ion (in the form of saline solution) is often given intravenously to hospital patients. Potassium ions, K+, are also essential to life, both plant and animal. Compounds of this ion, such as KCl, K2SO4, and KNO3, are used extensively as fertilizers (see Industrial Inorganic Chemistry module). Insolublelithiumcarbonate,Li2CO3, isused to treat manic depressives, although its mode of action is not well understood.

Each alkali metal atom has one more electron than the chemically stable noble gas atom just preceding it in the Periodic Table. Each atom has a large relative size (radius), coupled with filled inner energy levels of electrons. Each atom can therefore readily lose this one electron, forming stable +1ions with noble gas electron configurations. Thus they have low ionization energies. Cesium's ionization energy is so low that visible light can ionize it, permitting its use in photoelectric cells, where light energy is converted directly into electricity.

The chemistry of alkali metals provides a fascinating entry into the field of descriptive chemistry and a perfect introduction to the concept of periodicity of the elements (see Periodicity module).

Place in the Curriculum

The similarities in physical and chemical properties of alkali metals reinforce the concept of families of elements and thus serve as an excellent introduction to the Periodic Table (Periodicity module.) Due to their strong tendency to form compounds in which they exhibit only the +1 oxidation state, their chemistry is simple and predictable, yet frequently exciting. The topic could thus serve as an introduction to predicting the products of chemical reactions (Simple Chemical Reactions module). As a consequence of their high solubility in water and their relative abundance in nature, the compounds of sodium and potassium find extensive use. Examples familiar to students include table salt (NaCl), baking soda (NaHCO3), lye (NaOH), potash (K2CO3), soap (C17H35COONa), and detergents (e.g., C12H25OSO3­ Na+). All these reasons suggest introducing the topic of alkali metals early. The extraction of the alkali metals from their naturally occurring compounds provides a simple introduction to the concept of oxidation-reduction reactions (Oxidation-Reduction module), while much of the chemistry of the anions in the alkali metal compounds focuses on acid-base (Acid-Base module), precipitation (Solubility and Precipitation module), and redox reactions. Alkali metals can thus be related to several major areas of the typical high school chemistry curriculum.

Central Concepts