The Bohr model is a very simple solar-system model of the atom in which electrons are in well-defined circular orbits. It is important to understand that this is not the way we think of the atom now. Instead, we picture the electrons as being smeared out into a charge cloud around the nucleus, with the density of the cloud at a given point telling us the probability of finding the electron at that point.

Some of the results of Bohr's analysis are still valid, however. These include:

• the angular momentum of the electron is quantized, which leads directly to:
  
  
• the energy of the electrons is also quantized, with:
  
  
• the energies of the different energy levels being given roughly by the equation:

En = -

2p2mk2Z2e4 n2h2

This can be written in a more compact form:

En = -13.6 eV

Z2 n2

There is just the one quantum number, n, in the Bohr model. In our more realistic model of the atom we now use four quantum numbers to describe the electron energy states.