Hydrogen

Discovered by: In 1766–1781, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance and that it produces water when burned, the property for which it was later named: in Greek, hydrogen means "water-former".

What is it? A chemical element; it has symbol H and atomic number 1. It is the lightest element, and at standard conditions it is a gas.

Why is it important? Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all normal matter.