Helium is the only element with two electrons in its outer shell. Next to hydrogen, helium is the element with the highest abundance in the entire universe, forming the heart of most stars and star-like celestial bodies.
Under normal conditions of temperature and pressure, helium takes the form of a gas that is lighter than air. This gas can occur as one of two isotope forms: helium-3 with one neutron and helium-4 with two neutrons.
Helium is a member of the family of elements called the noble gases. This family consists of elements like helium, neon, xenon and krypton. These substances do not naturally react with other elements to form new compounds. In this case, this occurs because a helium atom already has a stable configuration with two electrons in its outer energy level. The other noble gases also have a full complement of electrons in their outer energy levels and are usually inert.