Electron Energies

Each isolated atom has only a certain number of orbits available which represent energy levels for the electrons and the electrons can exist at these levels only. greater the distance of the electron from the nucleus, the greater is its total energy. electron near to the nucleus and possesses only a small amount of energy. Hence it is difficult to knock out this electron.  electron  far from the nucleus would have a greater energy and hence it can be easily knocked out of its orbit. This is why' the valence electrons take part in chemical reactions and in bonding the atoms together to form solids.