sodium

The structure of a sodium atom. The nucleus is composed of 11 protons and 12 neutrons. Eleven electrons orbit the nucleus in 3 orbits: 2 in the inner orbit, 8 in the middle, and 1 in the outer.

Soft, waxlike, silver-white, metallic element, atomic number 11, relative atomic mass 22.989. Its chemical symbol comes from the German Natrium, which Arab alchemists derived from Latin nitrium and ancient Egyptian neter. It is one of the alkali metals (in Group 1 of the periodic table) and has a very low density, being light enough to float on water. It is the sixth-most abundant element (the fourth-most abundant metal) in the Earth's crust. Sodium is highly reactive, oxidizing rapidly when exposed to air and reacting violently with water. It is one of the most reactive metals in the reactivity series of metals. Its most familiar compound is sodium chloride (common salt), which occurs naturally in the oceans and in salt deposits left by dried-up ancient seas.

Other sodium compounds are of great industrial importance and thousands of tons are manufactured annually. Sodium functions with potassium on the cellular level to make possible neuronal transmission, and so it is an essential nutrient for animals. It was named in 1807 by Humphry Davy, because he isolated it from caustic soda (sodium hydroxide).