Though hydrogen is the smallest atom and is perched at the top left of the periodic table, hydrogen in nature exists as two atoms hooked together.

Hydrogen hit the news this weekend as a leak led NASA to scrub the Space Shuttle Endeavor launch:

Hydrogen is as clean as chemical fuels get: burning it produces water, a useful waste product. But just like a possible leak of other fuels including gasoline, hydrogen is a potential safety concern.

Hydrogen and helium (its sister element on the right side of the periodic table) form the basis of new stars. These chemicals rotate and condense, ultimately integrating their nuclei to form larger elements through nuclear fusion and releasing energy.