What is the chemical formula for table salt?

NaCl - one sodium atom to one chlorine - and the parts are startling on their own. Sodium is a metal soft enough to cut with a knife that explodes on contact with water; chlorine is a green gas used as a weapon in the First World War. Combine them and the sodium gives up an electron to the chlorine, and the result is something you cannot live without.

That is the point of the formula: NaCl is not sodium plus chlorine mixed, it is a lattice in which every sodium ion is surrounded by six chlorides and vice versa, held by electrical attraction. There is no such thing as a molecule of salt - a grain is one enormous repeating crystal, which is why it cleaves into cubes.

It has been worth fighting over for most of history. Roman soldiers may have received an allowance connected with it, which is where salary comes from; salt taxes helped provoke the French Revolution and Gandhi's Salt March in 1930; and Venice, Liverpool and a good deal of the Sahara trade were built on it. Adding iodine to it from the 1920s, to prevent goitre and the developmental damage of iodine deficiency, is one of the cheapest and most effective public health measures ever undertaken.