The key idea in Bohr's model of the atom is that there are certain orbits
in which an electron can travel around the nucleus without having to radiate any type of energy.
The orbits in Bohr's model are circular and are at a fixed distance from the nucleus.
When an electron is located in a certain orbit, that electron has a specific amount of energy.
The greater the radius of the shell, or the distance of the electron from the
nucleus, the greater the energy of an electron in that shell. Shells are the area that separates the
orbit from the nucleus.