This lecture delves into the energy scales characterizing different types of molecular bonds, the degrees of freedom of rotation and vibration in molecules, and the quantification of angular momentum in quantum physics. It explores the absorption and emission of photons in molecular spectra, the effects of vibrations on rotations, and the impact of isotopes on spectral lines. Additionally, it discusses the challenges of solving the n-body problem in quantum physics and the use of approximate approaches in predicting properties of solids and molecules.