Compton Effect: Electron Ejection & Potential Energy

[vin300]

The text on wikipedia says that due to compton scattering, the electron is ejected from the atom, but if so, the difference in energies of the two photons will not appear entirely as its kinetic energy, some of it must be used to overcome its potential energy, i.e. binding energy must be supplied to the electron.
The other doubt is that the electron is considered to be at rest before the collision, but I cannot imagine why the electron should be at rest within the atom.

 

[vin300]

Why is the potential energy of the electron not taken into account while deriving the compton effect formula?

 

[mfb]

If you consider the compton effect, the photon energy is usually much larger than the binding energy of most of the electrons. While the photon might have something like 100keV, most electrons are bound with ~100eV or less. Therefore, this energy is neglected, and it can be assumed to be at rest.