Hund’s rule states that electrons fill a suborbital singly and with the same spin before they form doubles with opposite spins. The rule gets its name from German physicist Friedrich Hund, who formulated it around 1927.

What Is Hund’s Rule?

Hund’s rule describe the order in which electrons fill subshells and the spin quantum number of each electron:

The orbitals of a subshell fill with single electrons before any subshells get double electrons (with antiparallel spin).
The single electrons in subshells have the same spin, so as to maximize total spin.

Basically, the lowest or most stable atomic state is the one that maximizes the total spin quantum number. Another name for Hund’s rule is the “bus seat rule” because people choose separate seats on a bus before they start pairing up.

Relationship to the Aufbau Principle and Pauli Exclusion Principle

The Aufbau principle states that electrons fill the subshells of the lowest energy orbital before moving on to higher energy subshells. Hund’s rule describes the way these electrons fill the lowest energy subshell, where electrons half-fill subshells with electrons having the same spin before that subshell gets two electrons. Those two electrons have opposite spin values due to the Pauli exclusion principle.

Aufbau Rule Examples

Nitrogen Atom: The electron configuration of a nitrogen atom (Z=7) is 1s²2s²2p³. The 1s and 2s subshells are filled. The 2p subshell is only half-filled. So, the electrons in the 1s and 2s subshells are pairs and antiparallel, while the 3 electrons in the 2p subshell are separate from each other and have the same spin.

Oxygen Atom: Oxygen follows nitrogen on the periodic table (Z=8). Its electron configuration is 1s²2s²2p⁴. The filling of the 1s and and 2s subshells is the same as for a nitrogen, but there is an additional electron in the 2p subshell. First, fill each subshell with a single electron. Add the additional electron to make a pair and make it antiparallel to the first electron.

Importance of Hund’s Rule

Hund’s rule is important because it shows how electrons organize into subshells. This identifies the valence electrons (the unpaired ones), which are the electrons that participate in chemical reactions and account for much of an atom’s chemical properties. The valence shell also indicates the magnetic properties of an atom. If there are unpaired electrons, the atom is paramagnetic and attracted to a magnetic field. If all of the electrons are paired, the atom is diamagnetic and is weakly repelled by a magnetic field.

ChemInteractive

Lesson 6: Electrons and the Quantum Model

The Four Quantum Numbers

Aufbau Principle

Pauli Exclusion Principle

Hund's Rule

Periodic Table Trends