electronic energy level

In molecules, electronic energy levels are more complex than in atoms. The molecular orbital (MO) approach describes how atomic orbitals combine to form molecular orbitals that extend over the entire molecule. Electronic transitions occur when an electron moves from one MO to another.

The lowest energy level is the ground state, while higher energy levels are called excited states. These transitions, particularly involving valence shell MOs, are responsible for visible and ultraviolet absorption spectra. When molecules absorb energy, electrons are promoted to higher energy MOs, leading to characteristic spectral bands.123