================================================================= | | | | | | SYMBOL | ATOMIC | DENSITY | MELTING | FIRST PREPARED | | WEIGHT | | POINT | __________|________|________|_________|_________|________________ | | | | | Lithium | Li | 7.03 | 0.59 | 186. deg. | Davy 1820 Sodium | Na | 23.05 | 0.97 | 97.6 deg. | " 1807 Potassium | K | 39.15 | 0.87 | 62.5 deg. | " 1807 Rubidium | Rb | 85.5 | 1.52 | 38.5 deg. | Bunsen 1861 Caesium | Cs | 132.9 | 1.88 | 26.5 deg. | " 1860 ================================================================= ~The family.~ The metals listed in the above table constitute the even family in Group I in the periodic arrangement of the elements, and therefore form a natural family. The name alkali metals is commonly applied to the family for the reason that the hydroxides of the most familiar members of the family, namely sodium and potassium, have long been called alkalis. 1. _Occurrence._ While none of these metals occur free in nature, their compounds are very widely distributed, being especially abundant in sea and mineral waters, in salt beds, and in many rocks. Only sodium and potassium occur in abundance, the others being rarely found in any considerable quantity. 2. _Preparation._ The metals are most conveniently prepared by the electrolysis of their fused hydroxides or chlorides, though it is possible to prepare them by reducing their oxides or carbonates with carbon. 3. _Properties._ They are soft, light metals, having low melting points and small densities, as is indicated in the table. Their melting points vary inversely with their atomic weights, while their densities (sodium excepted) vary directly with these. The pure metals have a silvery luster but tarnish at once when exposed to the air, owing to the formation of a film of oxide upon the surface of the metal. They are therefore preserved in some liquid, such as coal oil, which contains no oxygen. Because of their strong affinity for oxygen they decompose water with great ease, forming hydroxides and liberating hydrogen in accordance with the equation M + H_{2}O = MOH + H, where M stands for any one of these metals. These hydroxides are white solids; they are readily soluble in water and possess very strong basic properties. These bases are nearly equal in strength, that is, they all dissociate in water to about the same extent.