Homonuclear Diatomic Molecules


Learning Objective

  • Recognize the properties of homonuclear diatomic molecules.

Key Points

    • Diatomic molecules are always linear.
    • Diatomic molecules have quantized energy levels for rotation and vibration.
    • The halogen series contains many homonuclear diatomic molecules.
    • Hydrogen, nitrogen, and oxygen are stable homonuclear diatomic molecules.


  • diatomicconsisting of two atoms
  • homonuclearhaving atoms of only one element, especially elements of only a single isotope

Diatomic molecules are composed of only two atoms, of either the same or different chemical elements. Common diatomic molecules include hydrogen (H2), nitrogen (N2), oxygen (O2), and carbon monoxide (CO). Seven elements exist as homonuclear diatomic molecules at room temperature: H2, N2, O2, F2, Cl2, Br2, and I2. The bond in a homonuclear diatomic molecule is non-polar due to the electronegativity difference of zero.


All diatomic molecules are linear, which is the simplest spatial arrangement of atoms.

NitrogenA space-filling model of the homonuclear diatomic molecule nitrogen. Note the inevitable linear geometry.

Energy Levels

It is convenient and common to represent a diatomic molecule as two point masses (the two atoms) connected by a massless spring. The energies involved in the molecule’s various motions can then be broken down into three categories:

  • Translational energies (the molecule moving from point A to point B)
  • Rotational energies (the molecule spinning about its axis)
  • Vibrational energies (the molecules vibrating in a variety of ways)