A metal cut with a knife
Sodium has eleven electrons, ten in the closed shells of neon and one, alone, outside. That one is loosely held, and the element's whole behaviour follows: it is a metal that gives its electron to almost anything. Freshly cut it is bright silver; the surface dulls in seconds in air, so it is kept under oil. It is soft enough to cut with a knife, lighter than water, and melts at 98 °C. Dropped into water it floats and fizzes on the hydrogen it releases, and a large piece explodes.
The metal conducts heat so well, and stays liquid over so wide a range, that it is pumped through fast nuclear reactors as a coolant. But nearly all the sodium in the world is not the metal; it is the ion, Na⁺, which the metal so readily becomes.
Salt, sea and rock
Sodium is the sixth most abundant element in the crust, 2.36 % by mass, in the feldspars of granite and basalt, in halite, natron, borax and Chile saltpetre. Rivers leach it from rock into the sea, and there it stays: seawater is a little over 1 % sodium by mass, and the salt of the oceans is mostly its chloride.
Davy got the metal by electrolysis and so does industry, in the Downs cell, where a current passes through molten sodium chloride at about 600 °C and the metal collects at the cathode, chlorine at the anode. The quantities are modest. The compounds are made on another scale: hundreds of millions of tonnes of salt a year, and from salt the caustic soda and chlorine of electrolysis and the soda ash of the Solvay process.
In the body, the kitchen and the lamp
An adult body holds about 100 g of sodium, about half of it in the fluid outside the cells and much of the rest in bone. Every cell pumps sodium out and potassium in, and the imbalance it maintains is a store of energy: when a nerve fires, sodium channels open, the ions rush in, and the wave of charge running along the fibre is the impulse. The heart beats and the brain thinks on the sodium gradient, which is why too little salt, or too much, is dangerous.
In the kitchen sodium is salt and bicarbonate; in the factory it is caustic soda for soap and paper, soda ash for glass, and a long list of sodium salts. In the street it was, for much of the twentieth century, the lamp: sodium vapour in a discharge tube gives the yellow of the D lines and more light per watt than almost any other source, at the cost of a world in one colour.
Discovery and name
Soda was old before sodium was known. The Egyptians dug natron from the dry lakes west of the Nile to dry their dead and to make glass, and that word, through Greek and Latin, and the medieval soda, the alkali from the ash of seashore plants, gave the element its two names. Eighteenth-century chemists knew soda and potash as the “fixed alkalis” and suspected they hid metals, but no fire would reduce them.
Electricity did. In October 1807 Humphry Davy, at the Royal Institution in London, passed the current of a large voltaic pile through slightly moistened caustic potash and saw globules of a bright metal form at the negative wire: potassium. A few days later he did the same with caustic soda and got sodium. He announced both in the Bakerian Lecture of 19 November and named the new metal for the soda it came from. Berzelius, preferring Latin, called it natrium, and his symbol, Na, is the one the world uses, whatever it calls the element.