Potassium is a Block S, Group 1, Period 4 element. The electronic configuration is [Ar] 4s¹. In its elemental form potassium's CAS number is 7440-09-7. The potassium atom has a radius of 227.2 .pm and it's Van der Waals radius is 275.pm. Potassium is the seventh most abundant element on earth. In its metallic form it is one of the most reactive and electropositive of all metals and rapidly oxidizes. As with other metals of the alkali group, the metal decomposes in water with the evolution of hydrogen. It catches fire spontaneously in water. In high purity, potassium compounds have numerous pharmacological, medical, and electronics applications. Important compounds include the hydroxide, nitrate, carbonate, chloride, bromide, iodide, and sulfate. Lower purity compounds are used in pyrotechnics for its violet color on ignition and in glass and ceramic glazes to produce this color. Potassium also used as a nutrient in plant growth. Potassium was first discovered by Sir Humphrey Davy in 1807.