Sodium

Humphry Davy produced sodium in 1807 by passing current from a voltaic pile through molten caustic soda, one of the first elements ever obtained by electricity — he isolated potassium the same week and calcium, barium, magnesium and strontium within the year. Sodium is mononuclidic, every atom on Earth being ²³Na, so its atomic weight is known to eleven figures with no isotopic variation to blur it. Its emission doublet at 589 nm is the yellow of a street lamp and the orange flare when soup boils over on a gas ring, and it is the sharpest line in the solar spectrum Fraunhofer catalogued as D. Biologically it is indispensable: the sodium-potassium pump burns something like a fifth of your resting energy budget keeping Na⁺ outside cells so that nerves have a gradient to discharge.