The electron configuration of an atom is the distribution among the subshells of all of the electrons in the atom. These are designated using the standard form of notation for subshells:

where x = number of electrons in subshell

Example:

3d ⁸

In the n = 3 level the d orbitals are occupied by 8 electrons.Therefore, atoms can be represented by, for example;

Ar 1s² 2s² 2p⁶ 3s² 3p⁶

showing that the s orbitals have two electrons and the p orbitals have 6 with the total number or atomic number (Z), for this element equals 18.

A good approach to understanding electron configurations is to start with hydrogen and consider how each successive electron is added as the atomic numbers of the elements increase in order. This is known as the Aufbau Principle. So, what laws will define where the next electron will go?

The first is the Pauli exclusion principle (named after Wolfgang Pauli); no two electrons can have the same four quantum numbers, n, l , m_l and m_s. With this rule the spin quantum numbers takes effect because it allows two electrons of opposite spin to occupy the same atomic orbital.

The second law, lowest energy principle, states that electrons will occupy the lowest-energy orbitals available to them.

For example:

The final rule governing the filling of orbitals by electrons is Hund's principle which states that orbitals of equal energy are each occupied by a single electron before a second electron, which will have the opposite spin quantum number, enters any of them.

and so on ...

Information provided by: http://www.bpx.cov.ac.uk