What happends when photon is replaced by electron in an atom?

1) There are no photons in atoms:

"The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force."
Source and further information:
http://en.wikipedia.org/wiki/Atom


2) Photons are absorbed or emitted by an atomic nucleus or an electron, and this is accompanied by a change of energy level.

"Photons are emitted in many natural processes. For example, when a charge is accelerated it emits synchrotron radiation. During a molecular, atomic or nuclear transition to a lower energy level, photons of various energy will be emitted, from infrared light to gamma rays."

"Rather, the photon seems to be a point-like particle since it is absorbed or emitted as a whole by arbitrarily small systems, systems much smaller than its wavelength, such as an atomic nucleus (≈10–15 m across) or even the point-like electron."
Source and further information:
http://en.wikipedia.org/wiki/Photon


3) If you add an electron to an atom, you get a negative ion. If the ion loses energy, it will emit a photon. But you will not be able to say that the ion has lost a photon that was inside it: the ion has lost some of it energy, and this quantum energy has escaped in the form of a photon.

Photons are units of light energy, not atomic particles. Do you mean "proton"? They're like apples & oranges; protons are in the nucleus, while electrons "orbit" the nucleus. You couldn't replace a proton w/ an electron.