Rapid Review

Electronic Structure

Each electron in an atom can be described by four quantum numbers: n, l, m, and s.

n, the principal quantum number, describes the size of the orbital.

l describes the shape, e.g. s signifies a spherical shape, p polar, d cloverleaf.

m describes the orientation, e.g. an s orbital can be oriented in only one way, a p orbital can be oriented along the x axis, y axis, or z axis.

s describes the spin of an electron, which is either clockwise or counterclockwise (+½ or -½).

Orbitals are filled according to the following rules.
Aufbau principle
Atomic orbitals are filled one at a time, starting with the orbital that has the lowest energy.

Hund's rule

Electrons added to degenerate orbitals (i.e. that have the same energy, e.g. 2p) have parallel spins until each of the orbitals has one electron before a second electron is placed in one of these orbitals.

Pauli exclusion principle

No two electrons in an atom can have the same set of values for all four quantum numbers n, l, m, and s.

To determine the sequence in which orbitals are filled, follow the arrows in the diagram below.