The rules governing the distribution of electrons in orbital or sub-shells are given as follows
1. Auf-bau principle
2. Pauli’s exclusion principle
3. Hund’s rule
1. Auf-bau principle:
The Auf-bau principle states that “the electrons
should be filled in energy orbital or sub-shells according to increasing energy
order and the orbital or sub-shells at lower energy level should be filled
first. Which means that filling of electrons should be started from lower
energy sub-shells and then goes to higher energy orbital or sub-shell. For example,
electrons should be first placed in 1S orbital, then 2S, then 2p and so on in
ascending order with respect to energy of sub-shells.
2. Pauli’s exclusion principle:
The Pauli’s exclusion principle states that “the two
electrons in same orbital or degenerate orbital should have opposite spin’. Or it
can be explained as “the two electrons existing in the same orbital or
degenerate orbital can never have the same four quantum numbers. For example,
according to Pauli’s exclusion principle the electronic configuration of Helium
(He) with two electrons in 1S orbital will be with one electron rotating
clockwise while the other with anticlockwise rotation.
3. Hund’s rule:
The Hund’s rule explains the placement of electrons
in degenerate orbital. According to this rule if empty orbital of same energy
or degenerate orbital are available for electrons to be filled, then electrons
should be filled in separate degenerate orbital possessing the same spin rather
than putting these electrons in paired form with opposite spin.
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