In an atom, successive electrons are confined to specific regions of space known as orbitals. Two electrons (one of spin up and one spin down) are permitted in a given orbital, but hund's rule states that electrons of the same energy remain unpaired if possible. The Pauli exclusion principle, which applies since electrons are fermions, states that no two electrons can occupy the same quantum state.

The structure of electron orbitals was fairly well known prior to the development of quantum mechanics as a result of the importance of the outermost electron shell for an atom's chemical properties. periodic properties of Elements led to the discovery of the periodic table by Mendeleev and others.

The structure of the periodic table and nature of the spectral lines was later explained mathematically by solving the Schrödinger equation for the hydrogen atom exactly, and subsequently more complicated atoms approximately. Optical spectroscopists also knew that different "types" of electronic transitions existed in atoms because of the different qualitative types of spectral lines they saw. They named these lines according to their appearance: sharp (s), principle (p), diffuse (d), and fine (f). The symbols s, p, d, and f were subsequently taken over and applied to the orbitals themselves once quantum mechanics actually derived the existence of these levels. After f, orbitals are simply labelled alphabetically, so the sequence is s, p, d, f, g, h, i, ....

The rules for electron aufbau, i.e., how electrons are placed in orbitals, are given by the following rough scheme.

1. Electrons are always added in order of increasing energy.

2. The first basis of electron assignment is a combination of energy level and sublevel.

3. A new sublevel may not be started until the current sublevel has a maximum number of electrons.

4. If a sublevel has more than one orbital present, each orbital within a sublevel must have a positive spin (+1/2) electron present before any pairing may begin.

5. As electrons are added to higher sublevels, there is less energy difference between electrons, and variations will sometimes occur to provide the lowest energy level and highest stability arrangement.

Electron orbitals have certain symmetry properties which can be studied and classified using group theory. The following table summarizes the names, spectroscopic, and group theory symbols for electron orbitals.

Orbital	Spectroscopic Name	Symbol
s	sharp
p	principle
, ,	diffuse
,	diffuse
f	fine	&;

d-Orbitals