Niels Bohr, proposed his model of the atom to explain how electrons can have stable orbits around the nucleus. Bohr modified the Rutherford atom by saying that the electrons move in orbits of fixed size and energy. These orbits , also called shells, are arranged in circles around the nucleus. The farther a shell is from the nucleus, the higher the energy an electron needs to have to be in this shell. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. Radiation occurs when electrons jump from one orbit to another. (This makes the light you can see to distinquish betwen different atoms.) The atom will be stable when the outer shell is filled up.