The shell model has electrons residing in various energy levels about the nucleus. This shell is filled first with electrons, thus any atom with greater than 2 protons or atomic number will have this shell filled. The normal Bohr model of the atom would show the electron shells on a plane as similar to a solar system view, but the electron shells were shifted a bit to reflect a more 3 dimensional model. It is the outermost shell of the atom which is used to analyze which atoms will Bond with each other. It will give up, lend, borrow or steal electrons to do this.
Valence electrons are the number of electrons of an atom that are involved in a chemical bond.
The Oxidation Numbers of the atoms are a signed value of valence numbers that reflect the charge of the atom after the chemical bond. Thus the oxidation number of Oxygen is -2 because the atom will have more electrons associated with it after bonding with another atom or atoms.