Periodic Table Worksheet & Explanation - 7th Grade Science The lightest element is Hydrogen its atomic weight is 1 and that is why it is found in the top left corner of the periodic table. The atomic weight of Uranium is 238 being the heaviest element. The Noble Gases Helium, neon, argon, krypton, xenon, and radon are unreactive. The first man-made element is Technetium.