Once we understand the total number of electrons in an antom, and where most likely they are located, we need a way to understand where the electrons will most likely move to.

The quantum numbers can be defined in three way:

principal quantum number n shell or energy level

orbital quantum number l orbital type

magnetic quantum number m direction of orbital

We use three rules:

• the Pauli exclusion principle

Because s orbitals are spherical in shape it’s not possible to have more than one oreintation. For this reason, if you think of the electron as a particle we had a fourth variable called ‘spin’.

If electrons rotate in the same direction it’s called parallel. If they rotate in opposite directions it’s called

So we label the spin an m label also. This gives us two m labels, with one m desginating the direction of the orbital, and another m label the spin. TO differentiate between the two, use ml for the direction, and ms for the spin.

We use a graphic to represent the direction of spin, with a horzintal line desgnating the orbital, and the direction of the arrow designating the spin.

The Pauli exclusion principle

• the Aufbau principle

The Aufbau principle is that the orbitals of the lowest energy fills first.

• Hund’s rule

Hund’s rule says that when filling what’s called a degenerate set of orbitals, that is orbitals that are of the same type but different direction, the electrons would like to fill each pair ideally unpaired at first. As an example if electrons are going into the p orbital, they would prefer to go into the unoccupied p orbitals first.