Chlorine

Discovered by: First studied in detail in 1774 by Swedish chemist Carl Wilhelm Scheele, and he is credited with the discovery.

What is it? A yellow-green gas at room temperature.

Why is it important? In the form of chloride ions, chlorine is necessary to all known species of life. The most common compound of chlorine, sodium chloride, has been known since ancient times; archaeologists have found evidence that rock salt was used as early as 3000 BC and brine as early as 6000 BC.