Alkali Metals

Alkali Metals, series of six chemical elements in group 1 (alternatively called Ia) of the periodic table. They are, in order of increasing atomic number: lithium, Li; sodium, Na; potassium, K; rubidium, Rb; caesium, Cs; and francium, Fr. Francium exists in radioactive form only and is extremely rare. They are soft compared with other metals, have low melting points, and are so reactive that they are never found in nature uncombined with other elements. They all react in the same way. They are powerful reducing agents—that is, they give up an electron easily. Each reacts violently with water to form hydrogen gas and a solution of the metal’s hydroxide, which in each case is a strong base. These metals react vigorously with a variety of other substances, including, for example, oxygen (in the air) and chlorine (and other halogens). The reactions for sodium (using chlorine as an example of a halogen) are as follows (“s”, “l”, and “g” indicate solid, liquid, and gas, respectively):

Sodium + oxygen → sodium oxide

4Na(s) + O2(g) → 2Na2O(s)

Sodium + water → sodium hydroxide + hydrogen

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Sodium + chlorine → sodium chloride

2Na(s) + Cl2(g) → 2NaCl(s)

The equations for the reactions of the other metals are analogous.

Descending the group in the periodic table (that is, from lithium to francium), the metals react more rapidly and vigorously. In water, for example, lithium merely bubbles gently, while potassium gets hot enough to ignite the hydrogen in the water, and caesium is explosive.

A number of other trends in the physical properties of the alkali metals are evident in descending the group. The elements become softer, and their melting points decrease—lithium melts at 181° C (358° F) while caesium’s melting point is 29° C (84° F) so that it is liquid at body temperature, 37° C (98° F).

The similarity of the reactions of the alkali metals is caused by their electron arrangements. Each has one electron in its outer shell and invariably reacts so as to lose this, forming compounds in which the alkali metal exists as an ion with a single positive charge.

Because of their great reactivity, the alkali metals have few uses as uncombined elements. Lithium is used in some low-density alloys for aircraft construction, sodium vapour is used in orange street lamps, and liquid sodium is used as a coolant in some nuclear power stations.