Atomic structure
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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