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Although hydrogen (H) is sometimes placed in the group 1, it is not considered an alkali metal.
Elements in the alkali metal groups
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, atomic number 11, is a reactive metal which burns with a powerful yellow flame.
Potassium, atomic number 19, is even more reactive than sodium, and burns with a purple flame.
Rubidium, atomic number 37, is a very reactive metal, which is less often encountered, as it is expensive.
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, atomic number 87, is radioactive and due to its very short half-life of its main isotope (22 minutes), cannot be isolated.
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.
Alkali metals are silvery-gray in color, except for caesium which has a pale gold coloration.
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.