Beryllium, Atomic Number: 4
With an atomic number of four, beryllium feels like the odd one out on this list. But the fourth element gets lost between its more famous single-digit companions like oxygen or helium. NASA often uses beryllium because it's a light metal and non-magnetic, and the element will soon accompany a prestige mission: The James Webb Space Telescope, the Hubble telescope's successor-to-be, will carry 18 mirrors made of beryllium when it heads to space in a launch scheduled for 2013.
Image: European Space Agency