Several questions arise when you look carefully at the electron configuration of an atom. To answer these questions, it is necessary to know the basic principles that govern the distribution of electrons among atomic orbitals. The electron configuration for any atom follows the following three principles.

1. Building-up principle (Aufbau principle)

Aufbau principle is a scheme used to reproduce the electron configurations of the ground states of atoms by successsively filling subshells with electrons in a specific order (the building-up order). Following this principle, you obtain the electron configuration of an atom by successively filling subshells in the following order:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7, 5f.

2. Hund’s Principle.

In about 1927, Friedrich Hund discovered an empirical rule determining the lowest energy arrangement of electrons in a subshell. Hund’s rule states that:

the lowest-energy arrangement of electrons in a subshell is obtained by putting electrons into separate orbitals of the subshell with the same spin before pairing electrons.

Let us see how this would apply to the carbon atom, whose ground-state configuration is 1s² 2s² 2p². The first four electrons go into the 1s and 2s orbitals.

3. Pauli’s Exclusion Principle.

For many-electron atoms we use the Pauli exclusion principle (after the Austrian physicist Wolfgang Pauli) to determine electron configurations. This principle states that:

no two electrons in an atom can have the same set of four quantum numbers.

If two electrons in an atom should have the same n, and m, values (that is, these two electrons are in the same atomic orbital), then they must have different values of m_s.

Electronic configuration using subshell notation

The subshell (sublevel) notation uses numbers to designate the principal energy levels or principal quantum number and the letters s, p, d and f to identify the sublevels or subshells. Where n represents the principal quantum nuber and l represents the sub shell . The electron configurations for hydrogen (H; Z = 1, 1s¹), helium (He; Z = 2, 1s²) and lithium (Li; Z = 3, 1s², 2s¹).

Electronic configuration using noble gas notation

The electronic configuration of sodium (Z = 11) is 1s²2s²2p⁶3s¹. The first ten electrons of the sodium atom are the inner-shell electrons and the configuration of just those ten electrons is exactly the same as the configuration of the element neon (Z =10). So for sodium, we make the substitution of [Ne] for the 1s²2s²2p⁶ part of the configuration. Sodiums noble gas configuration becomes [Ne]3s¹.