Answer:
Argon. If you look at the periodic table, the noble gases (column on the furthest right of the table - labeled column 18) have fully filled electron shells. This is why noble gases (helium, neon, argon, krypton, xenon, and radon) are not very reactive with other elements - they are happy with their fully filled electron shells. So, the third element down on the noble gas column (Argon) has 3 fully filled electron shells. Periodic table courtesy of Wikipedia
(taken from Quora)
