The rightmost column of the Periodic Table of Elements lists the noble gases: helium, neon, argon, krypton, xenon and radon. All of these elements are gaseous at room temperature, colorless, odorless and unreactive with other elements. The noble gases share an electron configuration in which the outer, or valence, atomic orbitals are completely filled.
The number of positively charged protons in the nucleus and a matching number of electrons orbiting around the nucleus identify each element. Quantum physics describes the most probable locations for the orbits. These locations form shells, subshells and atomic orbitals. The smallest atomic orbital, s, can hold two electrons. The next orbital, p, can hold up to six electrons. Helium, the lightest noble gas, has only two electrons, which fill its s orbital. All the remaining noble gases have outer shells in which the s and p orbitals are full. This constitutes the "octet rule" for noble gases; the valance (i.e., outermost) shell of each gas has two s electrons and six p electrons. When a valance shell is full, it will not exchange electrons with other elements, creating gases that are too "noble" to mix with other atoms.