The only chemical elements that are stable diatomic homonuclear molecular gases at STP are hydrogen (H), nitrogen (N), oxygen (O), and two halogens: fluorine (F) and chlorine (Cl). When grouped with the monatomic noble gases &ndash; helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) &ndash; these gases are referred to as "elemental gases".