Homonuclear

Homonuclear describes a molecule that contains only atoms of the same chemical element. The term is used to contrast with heteronuclear molecules, which contain two or more different elements. Homonuclear species can be diatomic, triatomic, or larger and are common across the periodic table.

Common diatomic examples include hydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2), and chlorine (Cl2). Polyatomic homonuclear molecules include ozone (O3), the sulfur ring S8, and phosphorus clusters such as P4. In each case all constituent atoms are identical.

Bonding in homonuclear molecules is described by molecular orbital theory. For diatomics, bond order roughly follows observed bonding: H2 has a single bond, N2 a triple bond, O2 a double bond, and F2 a single bond. Oxygen is paramagnetic due to unpaired electrons, in contrast to the closed-shell N2 or F2. The electronic structure of homonuclear species strongly influences their stability and reactivity.

Homonuclear diatomic molecules often lack a permanent dipole moment, making them infrared inactive but Raman active, so they are typically studied by Raman spectroscopy. Exceptions exist among larger homonuclear molecules where geometry can introduce a dipole moment, as in ozone.

In chemistry and atmospheric science, homonuclear species help illustrate principles of bonding, symmetry, and spectroscopy. Their study contributes to understanding energy transfer, reaction dynamics, and the conceptual framework used to model molecular behavior across different elements and molecular sizes.