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Hund's Rule

Reviewed by Raghvendra GopalCheckmark | Last updated: October 18, 2021

What Does Hund's Rule Mean?

Hund's rule states that a larger total spin state of an atom sometimes makes the atom more stable. This rule is fairly reliable (with occasional failures) for the determination of the state of a given excited electron configuration. It was discovered in the year 1925 by Friedrich Hund. According to Hund's rule:

  • Each orbital in a sublevel is separately occupied before any orbital is doubly occupied.
  • All of the electrons in separately occupied orbitals have an equivalent spin (to maximize total spin).

Hund's rule is also known as the Rule of Maximum Multiplicity.

Electrons always enter an empty orbital before they pair up, according to the first rule. Electrons repel each other as a result of their negative charge. Electrons, rather than sharing an orbital with another electron, will occupy their own in order to minimize repulsion. Quantum-mechanical calculations have shown that electrons in singly occupied orbitals are less effectively shielded from the nucleus.

According to the second rule, electrons in singly occupied orbitals that are unpaired have the same amount of spins. The first electron in a sublevel could either "spin-up" or "spin-down." However, once the spin of the first electron that is in a sublevel has been chosen, the spins of all other electrons in that sublevel are dependent on that first spin.


Corrosionpedia Explains Hund's Rule

An atom consists of a nucleus around which electrons revolve in orbitals of different energy. According to the Aufbau principle, electrons fill the lowest energy level before filling up the higher ones. Thus, electrons are found in discrete atomic orbitals in an arrangement known as electron configuration. However, filling up of orbitals follows a specific set of guidelines known as Hund’s rule.

Hund's Rule can help predict the properties of atoms, as paired and unmated electrons have distinct properties (specifically with interactions with magnetic fields).

When atoms come into contact with each other, the outer electrons of those atoms, or valence shell, initially interact. An associated atom is least stable (and therefore most reactive) when its valence shell is not full. The valence electrons are most responsible for an associate element's chemical behavior. Parts that have an equivalent range of valence electrons typically have similar chemical properties.

An associate atom is most stable (and therefore unreactive) once all its orbitals are full of electrons. These configurations are found in the noble gases, which are extremely stable and do not normally react with each other.



Rule of Maximum Multiplicity

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