Physical property data for metals have usually been gathered most extensively on materials that were convenient to study in the laboratory or that were useful for construction purposes. The alkali metals are neither, yet their physical properties have been studied in recent years more extensively than many common metals of construction. The reason for this interest lies primarily in the use of sodium and potassium as heat exchange fluids in fast-breeder nuclear reactors; the heavier members, rubidium and cesium, also have potential utility in various power generating systems.
All of the alkali metals are silvery, low-melting, soft, ductile materials; cesium has a golden tinge and is liquid at ordinary temperatures. Their densities are quite low, and thermal and electrical conductivities are among the highest of all known materials. Ionization potentials are the lowest of all groups in the periodic chart. All of the metals are paramagnetic, due to a single unpaired s electron.
Compounds of these metals are characterized by relatively high melting points and high boiling points. The simple compounds are generally colorless, in keeping with the colorless nature of the cations; thus, colored compounds of sodium, potassium, rubidium and cesium owe their color to the anion, e.g. chromate, permanganate, etc.
