Bohr Model
In 1913 Niels Bohr refined the Rutherford model by stating that the electrons existed in discrete quantized states called energy levels. This meant that the electrons could only occupy orbits at certain energies. The laws of quantum physics apply here, and they don't comply with the laws of classical newtonian mechanics.
An electron which is completely free from the atom has an energy of 0 joules (or 0 electronvolts). An electron which is described as being at the 'ground state' has an energy which is equal to the ionisation energy of the atom. The electron will reside in this energy level under normal circumstances.
If a photon of light hits the atom it will be absorbed if, and only if, energy of that photon is equal to the difference between the ground state and another energy level in that atom. This raises the electron to a higher energy level.
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Read more about this topic: Photoinduced Charge Separation
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