Diatomics

Diatomics are molecules composed of two atoms. They can be homonuclear, consisting of two atoms of the same element (such as H2, O2, N2, Cl2), or heteronuclear, consisting of two different elements (such as CO, NO, HCl). They are among the simplest and most studied molecular systems, and they occur widely in the gas phase, planetary atmospheres, and interstellar space.

Most diatomic molecules are held together by covalent bonds. The bond order—single, double, or triple—determines bond length and strength: single bonds are longer and weaker, double bonds are shorter and stronger, and triple bonds are very strong and short. Homonuclear diatomics typically have zero permanent dipole moment, while heteronuclear diatomics often possess a dipole moment, influencing their infrared activity and reactivity.

Diatomics are prominent in spectroscopy. Their rotational and vibrational transitions produce characteristic spectra, allowing precise measurements of bond lengths, bond energies, and vibrational frequencies. Simple models treat rotation as a rigid rotor and vibration as a harmonic oscillator, though real molecules show anharmonicity and other refinements. The energy required to dissociate a diatomic molecule—the dissociation energy—varies widely between species.

Occurrence and applications. In Earth's atmosphere, nitrogen (N2) and oxygen (O2) are the dominant diatomics, with others like CO and NO participating in chemical processes. Diatomic molecules are central to industrial chemistry, exemplified by the use of N2 and H2 in ammonia synthesis via the Haber process. They also serve as reactive intermediates and probes in chemical and physical research, including studies of kinetics, reaction dynamics, and atmospheric chemistry.