What are the approximate relative magnitudes of the energies associated with electronic excitation of a molecule, with molecular vibration, and with molecular rotation?

Electronic excitation occurs when an electron within a molecule absorbs energy and transitions from a lower energy orbital, such as the ground state, to a higher energy orbital, which is often referred to as an excited state. The magnitude of energy required for this process is substantial because it must overcome the electrostatic forces keeping the electron close to the nucleus.

When the electron transitions to a higher orbital, the molecule absorbs photons, typically in the ultraviolet or visible range of the electromagnetic spectrum. This absorption of energy results in distinct spectral lines, which can be detected and analyzed in spectroscopy—a commonly used technique to study the electronic structure of molecules. From an educational standpoint, understanding electronic excitation is important as it underpins concepts such as fluorescence, phosphorescence, and photochemical reactions.

Molecular vibration refers to the periodic motion of atoms within a molecule relative to each other.

Energetics of Molecular Vibration

The energies involved in molecular vibration are lower than those required for electronic excitation but higher than those for molecular rotation. Vibrational energy levels are quantized, meaning that only certain discrete energy levels are allowed. During a vibrational transition, a molecule typically absorbs infrared radiation, resulting in a change in its vibrational state.

Types of Vibrational Modes

These modes can be stretching (changing the distance between two atoms) or bending (changing the angle between three atoms). Vibrations play a significant role in the thermal properties of substances and are crucial to understanding phenomena such as IR spectroscopy and the vibrational spectra of molecules.

Molecular rotation is the movement of molecules spinning around their center of mass or an axis. Due to the relatively small moments of inertia of molecules, the energy levels associated with molecular rotation are the smallest among the three phenomenon under discussion.

Understanding Rotational Spectra

The rotational energy of a molecule is also quantized, and transitions between these energy levels result in microwave or far-infrared spectral lines. Rotational spectroscopy is used to study the physical properties of molecules, such as bond lengths and moments of inertia, and to deduce the molecular structure.

Effective teaching of molecular rotation involves helping students visualize the rotation and how it interacts with electromagnetic radiation, providing them with the foundation to explore more complex chemical behavior.