Homonucleares

Homonucleares are diatomic molecules composed of two identical atoms bonded by covalent bonds. They form a distinct class of diatomic species and are contrasted with heteronucleares, in which the two atoms differ (for example, CO or NO).

Common examples include H2, N2, O2, F2, Cl2, Br2, and I2. In their elemental form, these molecules exist as discrete diatomic units and account for much of the physical behavior of their constituent elements. At room temperature, H2, N2, and O2 are gases; Cl2 is a greenish gas, while Br2 is a liquid and I2 is a solid, with color changes reflecting halogen identity.

Bonding and structure are explained by covalent bonding between identical atoms. The bond order varies among homonucleares: H2 and F2 have a single bond, O2 a double bond, and N2 a triple bond, corresponding to increasing bond strength and decreasing bond length. Molecular orbital theory accounts for these trends by the distribution of bonding and antibonding electrons; for example, N2 has a high bond order (three) and a very strong bond, while O2 contains unpaired electrons in π orbitals, giving paramagnetism.

A notable property of homonuclears is the absence of a permanent dipole moment, since the two atoms are identical. This affects their infrared activity, often making pure rotational transitions weak or forbidden in IR spectroscopy, though Raman spectroscopy can reveal their vibrational modes.

Homonucleares play a foundational role in chemistry and are key to understanding elemental behavior, atmospheric composition (N2 and O2), and the reactivity of halogens (F2, Cl2, Br2, I2).