Filling electron shells5.2.5 D and F orbitals
There are also D and F orbitals. D orbitals are present in transition metals. Sulphur and
phosphorus have empty D orbitals. Compounds involving atoms with D orbitals do come
into play, but are rarely part of an organic molecule. F are present in the elements of the
lanthanide and actinide series. Lanthanides and actinides are mostly irrelevant to organic
chemistry.
5.3 Filling electron shells.
When an atom or ion receives electrons into its orbitals, the orbitals and shells fill up in a
particular manner.
There are three principles that govern this process:
1.the Pauli exclusion principle,
2.the Aufbau (build-up) principle, and
3.Hund’s rule.5.3.1 Pauli exclusion principle
No two electrons in an atom can have all four quantum numbers the same. What this
translates to in terms of our pictures of orbitals is that each orbital can only hold two
electrons, one ”spin up” and one ”spin down”.
5.3.2 Hund’s rule
This states that filled and half-filled shells tend to have additional stability. In some in-
stances, then, for example, the 4s orbitals will be filled before the 3d orbitals.
This rule is applicable only for those elements that have d electrons, and so is less important
in organic chemistry (though it is important in organometallic chemistry).
5.4 Octet rule.
Theoctet rulestates that atoms tend to prefer to have eight electrons in their valence
shell, so will tend tocombinein such a way that each atom can have eight electrons in
its valence shell, similar to the electronic configuration of a noble gas. In simple terms,
molecules are more stable when the outer shells of their constituent atoms are empty, full,
or have eight electrons in the outer shell.
The main exception to the rule is helium, which is at lowest energy when it has two electrons
in its valence shell.
Other notable exceptions are aluminum and boron, which can function well with six valence
electrons; and some atoms beyond group three on the periodic table that can have over