Hund's rule

Themes > Science > Chemistry > General Chemistry > Atomic Structure > Electronic Structures of Atoms > Atomic Structure Index > Hund's rule

When deciding how electrons are arranged in an orbital diagram for a given atom or ion, Hund's rule is used.

When several orbitals of equal energy are available, electrons enter singly with parallel spins

In other words, add one electron to each orbital of equal level (such as the five d orbitals) all with the same spin. Only when you run out of orbitals with equal energy should you pair up electrons.

For example, in an iron atom, the valence level consists of 8 electrons, the 4s orbital, and the five 3d orbitals. (The core is the same as the argon atom.)
[Ar]4s()3d()()()()()
The 4s orbital is lower energy than the 3d, so it fills first: place 1 electron into the orbital

[Ar]4s()3d()()()()()

There is only 1 4s orbital, so the next electron has to go into the single orbital with spin down.

[Ar]4s( )3d()()()()()

We now have six electrons left: according to Hund's rule we should place as many into orbitals singly with aligned spin. Five electrons can go into the 5 3d orbitals

[Ar]4s( ) 3d() () () () ()

We have used up seven electrons: we have only 1 left. It must go into one of the 3d orbitals

[Ar]4s( ) 3d( ) () () () ()

Example: What is the orbital diagram for the Co atom?

Solution: The cobalt atom has 9 valence electrons, a core shell like argon and a 4s and 5 3d orbitals in the valence shell. Fill the s orbitals first
[Ar]4s( )3d()()()()()

We now have 7 electrons left. Start by placing each in a different d orbital with spins aligned.

[Ar]4s( ) 3d() () () () ()

We have two electrons left after filling each 3d orbital once: we have to pair them up.

[Ar]4s( ) 3d( ) ( ) () () ()

This is the orbital diagram for cobalt.

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