Can a photon that matches the energy gap pass through a molecule? I've heard that atoms and molecules are made up of mostly empty space. I've heard that electrons exist as a probability cloud around and/or between atoms and molecules.
My question is, is there a probability that a single photon that matches the energy gap of a molecule can pass through the electron probability cloud without being absorbed? or must the photon always be absorbed.
 A: YES. 
There is a probability that a single photon that matches the energy gap of a molecule can pass through the electron probability cloud without being absorbed. But that probability is very low. The higher chances are that the photon will be absorbed.
The reason behind this is that:


*

*Molecules are not isolated. There are innumerable molecules even in a very small mass. So the motion of the photon is hindered and the probability of absorption increases.

*Photon is basically a light particle. But it is unknown whether after entering a molecule , it retains it particle nature or switches to wave form. This does violate Planck's Law for the photon enters as photon and we see that the molecule absorbs the photon. But during absorption,the nature of light is unknown.

*Even if you consider a single isolated molecule then also the photon collides with the large number of electrons and gets absorbed.


All these are probable choices for nobody has ever glimpsed into a molecule to see what is going on. Just as I said earlier that both of the choices are possible but with different probability.
