Electron Probability-Density Clouds

In the Bohr model, electrons are found in well-defined circular orbits around the nucleus. Using the quantum mechanical picture of the atom, we now know that there are no such circular orbits - instead we can determine how likely it is that an electron in a particular orbit can be found a particular distance from the nucleus. We draw probability density clouds - the denser the cloud at a particular point the more likely it is that the electron is there.