Class 11 Chemistry Structure of Atom Rules Of Writing Electronic Configuration


          “It is the number of electrons in different orbitals. “

There are certain set of rules that are followed:

Aufbau principle: It states that in ground state of atom, the orbitals are filled in order of their increasing energies. That is electrons first occupy the lower energy orbital then higher energy orbitals are filled.

The orbitals are filled in order:




To know the energy of orbital with lower energy the n+l rule is followed:

According to it:

  1. The lower the value of n+l for an orbital, the lower is its energy and is filled first.

For example: Out of 2s and 2p ,2s is filled first as n+l for 2s, is 2+0=2 and for 2p it is 2+1=3. Therefore, 2s is filled first.

  1. If two orbitals have same value of n+l , than the orbital with lower value of n is filled first.

For example: Out of 3p and 4s, n+l for both is 4 . Therefore, 3p is filled first as it possess lesser value of n.

  1.  Pauli’s exclusion principle: According to it, an orbital can accommodate maximum of two electros and that must be of opposite spin.

For example: If orbital has s2, then the orbital has the arrangement as given below.



       Correct                   incorrect

  1. Hund’s rule of maximum multiplicity: According to it, electron pairing in degenerate orbital will not take place until and unless, each orbital is singly filled.

For example,if the d orbital has d6 ,then the filling is not:



  • Sodium (Na)

Atomic number=11

Electronic configuration = 1s2,2s2,2p6,3s1


  • For Potassium(k) :Atomic number =19

Electronic configuration = 1s2,2s2,2p6,3s2,3p6,4s1


For Aluminum ion

Atomic number of Aluminum =13

Aluminum ion (Al3+)=1s2,2s2,2p



For Copper (Cu)= [Ar]18,4s23d9

But actually, it has : [Ar]18,4s13d10

Similarly, for Chromium it should be: [Ar]18,4s23d4

But in actual it is:=[Ar]18,4s13d5

The reason behind is:

  1. Half-filled and fully filled orbital are more stable

If configuration is 4s2 3d9 , then the  d orbital is not fully filled.  If configuration is 4s1 3d10 , then the d orbital is completely filled. That means it becomes more stable.

  1. The more is the exchange energy more stable is the orbital as shown below:

If configuration was 4s23d4 the exchange energy is:


Total number of exchanges = 3 + 2 + 1 = 6

 If configuration is 4s13d5the exchange energy is:


Total number of exchanges = 4 + 3 + 2 + 1 = 10

So, more is the exchange energy -more is the stability.

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