system
Given the following post, respond as if you're commenting on Reddit:
human
What is the actual difference between scattering and absorption at the atomic level, if both are exciting molecules, albeit at different magnitude?

I understand absorption and scattering is what gives things color, like why Chlorophyll is green (because it absorbs more in the red/blue levels), and water is bluish (since it absorbs more in the red region).  

BUT I don't understand why this difference occurs; Why is resonant frequencies absorbed by molecules, if all it's doing is vibrating the molecules more than other wavelength of light. For example, if red light is more closer to the resonant OH bond stretching vibrations, it get absorbed, but blue light is not, so it gets scattered. Don't both wavelengths wiggle the molecules, therefore transmitting energy to the molecule, albeit at different magnitudes? What makes water molecules "give back" the blue light, but "keep" the red light ?
gpt
Raleigh scattering is a non-resonant phenomena and its intensity is proportional to the 4th power of the incident frequency of light. From a classical perspective, the incident light generates an oscillating dipole of the same frequency as the source and this oscillating dipole produces an EM wave that is the "scattered" light. This is the common explanation for explaining why the sky is blue.

 Using a quantum mechanical perspective, the incident light promotes the atomic electron to a virtual state, one of a continuum of such states that are non-resonant, and short lived (10^-14s -ish). The virtual state, relaxes to the ground state radiating a photon of the same energy as the incident photon. The strength of this interaction increases with the polarizability of the atom/molecule and the 4th power of the frequency of incident light.

Electronic absorption is a different phenomena. In this case the photon is resonant with one of the excited states of the atom/molecule.  These are different than the continuum of virtual states mentioned above. These states are long lived, and represent stable, arrangements of the electron (atomic/molecular) orbitals deduced by solving the time-independent Schoedinger equation for the atom/molecule considered. The excited state lifetime is on the order of 10^-9s. Although this can vary over a wide range. 


 The attenuation in intensity of incident light due to absorption is maximum at the resonant frequency corresponding to the energy difference between the ground and excited states.  The excited state can relax back to the ground state via a variety of pathways that depend on the atom/molecule and the local environment. Electronic absorption is the phenomena responsible for colors in dyes, paints, components etc and is one the basis for how lasers work.