Orbital diagrams show e− pairing in atoms:
An orbital diagram consists of a box representing each orbital and a half arrow representing each electron.
The orbital diagram below is for sodium (atomic number 11)
A half arrow pointing up, , represents an electron with ms = + 1/2
A half arrow pointing down,, represents an electron with ms = −1/2
The spin of an electron is often referred to as either "spin up" or "spin down".
Electrons are said to be paired when they are in the same orbital. An unpaired electron is one not accompanied by a partner of opposite spin.
Hund's rule states that for degenerate orbitals, the lowest energy is attained when the number of electrons with the same spin is maximized.
Simply stated - electrons will occupy orbitals singly, if possible, and single electrons in the same subshell will all have the same spin.
The orbital diagram below represents nitrogen, with unpaired 2p electrons
Electrons arranged in this way (unpaired electrons in degenerate orbitals) are said to have parallel spins.
Hund's rule is based in part on the fact that electrons repel one another. By occupying different orbitals, the electrons remain as far as possible from one another, minimizing electron-electron repulsions.
Note: while orbital diagrams can be drawn to show all the electrons in any atom, they are most often used to show only the valence electrons of an atom. Since valence electrons are the ones involved in chemical reactions, they are of most interest to chemists.