Concept of Electronic Configuration

 

Electronic configuration:

                                The shorthand notation consists of  the principle energy level(n value),the letter representing sub-level (l value), and number of  electrons (x) in the sub-shell is written as a superscript as shown below:

          

              For Hydrogen atom (H) having atomic number (Z)=1 ,the number of electrons is one ,then the electronic configuration is:

For-many electrons of atom ,we must know the electronic configurations of atoms.The distribution of electrons in various atomic orbitals provides an understanding of electronic behaviour of the atom and,in turn,its reactivity.Let us consider the Helium atom.

      

To describe the electronic configuration for more than one electron in atom ,we need three principles:

 Those are the:

1. Pauli Exclusion Principle
2. Aufbau principle 
3. Hund's rule                       

Pauli Exclusion principle

                      According to Pauli Exclusion principle , no two electrons of the same atom can have allthe four quantum numbers same.

                    If n,l,ml, are same for two electrons them ms must be different.In Helium atom the spins must be paired.

                  

                  Electrons with paired spins are denoted by 

             The major consequence of exclusion principle involves orbital occupancy. Since only two values of ms are allowed,an orbital can hold only two electrons and they must have opposite spins.

     

             Hence, the electronic configuration of Helium(Z=2) is:

             

Aufbau principle:

         As,we pass from one element to another one of next higher atomic number,one electron is added every time to the atom.

          

               The maximum number of electrons in any shell is "2n^2",where n is the principal quantum number. 

        The maximum number of electrons in a sub-shell (s,p,d and f) is equal to 2(2l+1) where l=0,1,2,3.... Thus these sub shells can have a maximum of 2,6,10,14 elwctrons respectively.

         In ground state the electronic configuration can be built up by placing electrons in the lowest available orbitals until the total number of electrons added in equal to the atomic number.This is called Afbau principle.Thus orbitals are filled in the order of increasing energy.

Two general rules that predict electronic configurations.

1.Electrons are assigned to orbitals in order of increasing value of (n+l).

2.For sub-shells with same value of (n+l), electrons are assigned first to the sub-shell with lower 'n'.

          The following  diagram shows the increasing value of (n+1). Ascending order of energies of various atomic orbitals is given below:

           

                1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p<8s.....

                                            The filling order of atomic orbitals(moellar chart)

                                                           

3.Hund's rule:

               According to this rule electron pairing in orbitals starts only when all available empty orbitals of the same energy(degenerate orbitals) are singly occupied.

                     

              The configuration of Carbon(C) atom(Z=6) is 1s^22s^22p^2.The first four electrons go into the 1s and 2s orbitals.The next two electrons go into separate 2p orbitals, with both electrons having the same spin.

       Note that the unpaired electrons in the 2p orbitals are shown with parallel spins.