<P> Diatomic molecules are molecules composed of only two atoms, of the same or different chemical elements . The prefix di - is of Greek origin, meaning "two". If a diatomic molecule consists of two atoms of the same element, such as hydrogen (H) or oxygen (O), then it is said to be homonuclear . Otherwise, if a diatomic molecule consists of two different atoms, such as carbon monoxide (CO) or nitric oxide (NO), the molecule is said to be heteronuclear . </P> <P> The only chemical elements that form stable homonuclear diatomic molecules at standard temperature and pressure (STP) (or typical laboratory conditions of 1 bar and 25 ° C) are the gases hydrogen (H), nitrogen (N), oxygen (O), fluorine (F), and chlorine (Cl). </P> <P> The noble gases (helium, neon, argon, krypton, xenon, and radon) are also gases at STP, but they are monatomic . The homonuclear diatomic gases and noble gases together are called "elemental gases" or "molecular gases", to distinguish them from other gases that are chemical compounds . </P>

When do two atoms represent the same element