A Photon Is A Particle

In the per particle view, a given density is shared by $Z$ particles.
In the per atom view, a given density has $Z$ particles.

In the per particle view, the whole atom is moving (with a given $KE$) and carries with it $Z$ particles.
In the per atom view, the atom is bombarded with photons, each atom has $Z$ particles that can interact with the photons.

In,

$E_{input\,atom}=E_{photon}=...density*Z...$ 

photons interacts with an atom with $Z$ particles.  More energy is needed to interact with an atom, where $Z$ particle coalesce into one as a quasi-nucleus.  The atom is stationary, photons move and collide with it.

In,

$E_{input\,particle}=E_{particle}=...\cfrac{density}{Z}...$

particles interacts with particles.  Less energy is needed to interact at individual particle level compared to the interaction with a quasi-nucleus.  Relatively, the atom is moving in space with $Z$ number of particles.  A gas.

Maybe in a solid, we should have taken the per atom perspective too.  In this case free particles move within the confines of the solid.

Good morning!