Alkali metal
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The alkali metals are the elements from group 1 in the periodic table, consisting of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr).
Although hydrogen (H) is sometimes placed in the group 1, it is not considered an alkali metal.
Contents
Elements in the alkali metal groups
Lithium
Lithium, atomic number 3, is the lightest metal and has the highest melting point of the alkali metals. It burns with an extremely bright flame.
Sodium
Sodium, atomic number 11, is a reactive metal which burns with a powerful yellow flame.
Potassium
Potassium, atomic number 19, is even more reactive than sodium, and burns with a purple flame.
Rubidium
Rubidium, atomic number 37, is a very reactive metal, which is less often encountered, as it is expensive.
Caesium
Caesium (or cesium), atomic number 55, is the most reactive of the group, and has a slight gold coloration. It also has the lowest melting point of the group (28.5 °C).
Francium
Francium, atomic number 87, is radioactive and due to its very short half-life of its main isotope (22 minutes), cannot be isolated.
Properties
Chemical
All alkali metals readily react with air and violently with water. Potassium will form a dark layer of potassium peroxide, which is known to be shock sensitive.
The alkali metals (along with the alkaline earth metals calcium, strontium and barium) all dissolve in nitrogenous solvents to form deep blue, and, at higher concentrations, golden solutions of the metal and solvated electrons, which are useful in organic chemistry as they are extremely strong reducing agents.
Physical
Alkali metals are silvery-gray in color, except for caesium which has a pale gold coloration.
Safety
All alkali metals react violently with water. Potassium can form an explosive layer of peroxide in contact with air over months. Caesium and rubidium can be pyrophoric and react explosively with water, even in small amounts.
