There is a unique spectrum for each element, reflecting its unique electron orbital structure. Isotopes show the same lines, but slightly shifted in wavelength. Every element has its own, distinctive spectral signature. Compounds of two or more atoms, often of different elements. Share some electrons in common orbitals.
Bands often span large wavelength regions.
Often get strong lines at infrared, microwave, and radio wavelengths. If an atom or molecule absorbs enough energy from a photon or a collision, an electron can be ejected . Diferent spectral line signatures. Different chemical properties. Which elements are ionized, in whole or in part.