Helium is the second element on the Periodic Table of the Elements. It has an atomic weight of 4.003 amu and an atomic number of 2. Helium is a colorless, odorless gas with two protons and two electrons.
This element follows the duet rule, which states that hydrogen and helium may have no more than two electrons in their outer shells. These shells are also called valence shells. The Boise State University website says helium is very stable because it has two electrons in its s orbital.Learn more about Atoms & Molecules