Electronic Configuration:

The distribution and arrangement of electrons in various shells, sub-shells and orbitals is called electronic configuration.

Electronic configuration follows three rules.

Auf Bau Principle:

It states:

“Electrons occupy shells in increasing order of their energies”

or

“Electrons occupy sub-shells in increasing order of their energies or n+ℓ values”

First electrons fill the sub-shell with lower energy or n+l value. If two sub-shells have equal n+l values then the one with lower n value is filled first.

1s        2s        2p        3s        3p        3d        4s 4p        4d        4f         5s   5p   5d   5f

n+l →  1          2          3          3          4          5          4         5          6          7          5     6     7     8

e.g (i)  ₁₃Al27 → 1s2, 2s2, 2p6, 3s2, 3p1                       

e.g (ii) ₂₆Fe56 → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6                                     

e.g (iii) ₂₄Cr52 → 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5                 

Pauli Exclusion Principle:

It states:     

     “No two electrons in an atom can show same set of four quantum numbers”

Different electrons in an atom either differ in “n” if they belong to different shells or they differ in “ℓ” if belong to different sub-shells or they differ in “m_ℓ” if they belong to different orbitals. But if they belong to same shell, same sub-shell and same orbital then they have same n, ℓ  and m _ℓ but then they differ in spin. So principle can also be stated as         

“No two electrons in same orbital can show same spin”

When two electrons enter in the same orbital they enter with opposite spins one with clock wise spin and other with anti-clock spin .

Hund’s Rule:

It states: “When orbitals of equal energy (degenerate) are available, electrons are

distributed in them so as to give maximum number of unpaired electrons.” It means electrons first enter in orbitals of equal energy singly until all are occupied by single electrons.                          

• There are three types of electronic configurations i.e. shell-wise, sub shell-wise and orbital-wise electronic configuration.
• The three types of electronic configurations for iron can be written as:
• Sub-shell wise:

₂₆Fe56 → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6                   

    (Auf Bau principle is applied)

• Shell wise:

      ₂₆Fe56 → K-shell = 2,  L-shell = 8,     M-shell = 14,  N-shell = 2

                                                                                          (Auf Bau principle is applied)

Orbital wise:

In orbital wise electronic configuration all the three rules are applied.

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