Bohr’s Model of the Atom (1913)
Electrons cannot have just any amount
of energy but can have only certain specified amount; i.e. the energy of an electron is quantized. The specified energy values for an atom are called its energy levels.
As an electron moves instantaneously from one energy level to another, there are no intermediate stages.
Niels Henrik David Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922.
http://www.youtube.com/watch?v=Ic8OnvRonb0