system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What determines the light-absorption properties of atoms and molecules?

I know it has to do with electrons dropping and raising energy levels, but what determines the energy levels that are changed and how does it work in molecules? Would all (outer) electrons in a molecule change their energy level at the same time or is there another thing at work here?
gpt
In a molecule, you have many possible quantum states. There is the lowest energy, the ground state. At fixed energies above this, you have various excited states. When a photon of just the right energy to match one of these gaps hits the molecule, it is absorbed and sends the molecule into a higher energy state. 

So how are these higher energy states described? There are many possible options. As you said, it can be excitation of the outer (valence) or inner shell electrons. It can be a vibrational mode of the molecule, such as bending, stretching, or wagging. All of these will correspond with different energies of light. Generally, molecular vibrations absorb in the IR, valence electrons in the UV-vis, and inner shells in the x-ray. This affects the type of spectroscopy you can use to ID a molecule.