A group of six highly reactive metals, group IA of the Periodic Table of the Elements including, in increasing order of atomic weight and reactivity, lithium, sodium, potassium, rubidium, cesium, and francium. They are the most reactive metals, and always occur in nature in combination with other elements. The alkali metals are all characterized by having hydroxides that are strong bases. Common uses of the alkali metals include the use of lithium, sodium, and potassium as reducing agents and in heat-transfer coolant. Lithium and sodium are used in metallurgy. Sodium and potassium are used in agricultural chemicals and dyes. Rubidium and cesium are used in photoelectric cells.

The physical properties of the alkali metals vary regularly with an increase or decrease in atomic number. They exhibit all of the physical properties generally associated with metals to a high degree, including silver-like luster, high ductility, and excellent conductivity of electricity and heat. Lithium is the lightest metallic element. The alkali metals have low melting points, ranging from a high of 181°C for lithium to a low of 28°C for cesium, which has the second lowest melting point of all the metals. Alloys of alkali metals exist that melt as low as -78°C. Cesium is the most volatile of the alkali metals, with a boiling point of 671°C. The boiling points of the alkali metals increase in regular fashion as the atomic numbers decrease, the highest, 1,342°C, being that of lithium.

The alkali metals are extremely reactive and combine readily with most of the substances found in the atmosphere. The alkali metals all react vigorously, and often violently, with water, releasing hydrogen and forming strong caustic solutions. Most common nonmetallic substances such as the halogens, halogen acids, sulfur, and phosphorus react with the alkali metals. The alkali metals themselves react with many organic compounds, particularly those containing an active hydrogen atom.

Each of the alkali metals has a valence of one; this property is a consequence of their having only a single electron in the outermost shell of their atoms. In passing from lithium to francium, this single electron tends to be less strongly held. The alkali metals have the lowest ionization energies of their respective periods; the most easily ionizable element is francium, followed closely by cesium. The alkali metals are the least electronegative of the elements, with electronegativities ranging from 0.7 (cesium) to 1.0 (lithium).

Reactions with oxygen
All of the alkali metals react with oxygen--lithium and sodium to form monoxides and the heavier alkali metals to form peroxides. The rate of reaction with oxygen, or with air, depends upon whether the metals are in the solid or liquid state, as well as upon the degree of mixing of the metals with the oxygen, or air. In the liquid state, alkali metals can be ignited in air with ease, generating large amounts of heat and a dense choking smoke of the oxide.

Reactions with water
The alkali metals all react violently with water. The rate of the reaction depends on the degree of metal surface presented to the water. With small metal droplets or thin films of an alkali metal, the reaction can be explosive. The rate of the reaction of water with the alkali metals increases with the atomic weight of the metal. The reaction of an alkali metal (M) with water is as follows: 2M + 2H2O -> 2MOH + H2

Reactions with nonmetals
Of the alkali metals, only lithium reacts with nitrogen. In this respect it is more similar to the alkaline-earth metals than to the alkali metals. Lithium also forms a relatively stable hydride, whereas the other alkali metals form hydrides that can be more readily dissociated. Lithium forms a carbide (Li2C2) similar to that of calcium. The other alkali metals do not form stable carbides, although they do react with the graphite form of carbon to give intercalation compounds. The alkali metals can be burned in atmospheres of the various halogens to form the corresponding halides; the reactions are highly exothermic. The alkali metals also react with nonmetals in Groups VA and VIA of the periodic table. Sulfides can be formed by the direct reaction of the alkali metals with elemental sulfur, furnishing a variety of sulfides. Phosphorus combines with the alkali metals to form phosphides with the general formula M3P.

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