Chlorine is a chemical element with symbol Cl and atomic number 17. It is the second lightest halogen, following fluorine. The most common compound of chlorine, sodium chloride (common salt), has been known since ancient times. Around 1630, chlorine gas was first synthesized in a chemical reaction, but not recognized as a fundamentally important substance. Carl Wilhelm Scheele in Uppsala Sweden wrote a description of chlorine gas in 1774, supposing it to be an oxide of a new element.