homonucleaire

Homonucleaire is a term used in chemistry to describe molecules whose atoms (or nuclei) are identical. It is most often applied to homonuclear diatomic molecules, in which two atoms of the same element form a bond, such as H2, N2, O2, F2, Cl2, Br2, and I2. The contrast is heteronucleaire, where the two atoms are different elements.

Because the atoms are identical, homonuclear diatomics generally lack a permanent dipole moment and are nonpolar. This has consequences for spectroscopy: many fundamental vibrations are infrared-inactive, while changes in molecular polarizability make them Raman-active. The vibrational frequencies, bond orders, and rotational spectra depend on the specific molecule; for example, N2 has a strong triple bond, while H2 and Cl2 have single bonds.

Quantum-mechanical symmetry and identical nuclei can lead to special phenomena. For H2, nuclear spin statistics produce two forms, ortho and para hydrogen, with different rotational state populations that affect low-temperature spectra and thermodynamic properties.

Homonucleaire species are common in nature and industry. O2 and N2 dominate Earth's atmosphere; H2 is a major energy carrier in research and some industry; halogen homonuclears (F2, Cl2, Br2, I2) are reactive and used in chemistry and materials processing. While the term is general, in spectroscopy and molecular physics it highlights how identical nuclei shape physical behavior and selection rules.

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