Which element has the chemical symbol Na?

Sodium, from natrium - the Latin for a mineral salt, itself from Egyptian natron, the substance used for mummification. Sodium is a metal soft enough to cut with a knife that explodes on contact with water, and chlorine is a poisonous green gas, and the compound they form is the one thing you cannot live without.

In the body it does half the work of every nerve impulse. Cells maintain a gradient with sodium high outside and potassium high inside, and a signal travelling along a nerve is that gradient briefly collapsing and being pumped back. Maintaining it consumes a substantial share of your resting energy - by some estimates around a fifth of what your brain uses is spent running sodium pumps.

It was isolated in 1807 by Humphry Davy, days after potassium and by the same method, running electricity through a molten salt. Davy identified six elements in about two years using electrolysis, and also invented the miners' safety lamp, refused to patent it, and was famously described by a contemporary as having made his greatest discovery in Michael Faraday - who began as his assistant and bottle-washer.