Electronic Configuration of Elements

The distribution of electrons into different shells, sub shells and orbitals of an atom is called its electronic configuration.

The electronic configuration of any orbital can be represented as:  nl^x

n is the number of principal shell, l = symbol of the sub shell or orbital, x= number of electrons present in the orbital

4p¹ means that p- sub shell of the 4th main shell contain one electron.

 

In certain elements when the two sub shells differ slightly in their energies, an electron may shift from a sub shell of lower energy to a sub shell of higher energy only if such a shift results in the symmetrical distribution of the electrons in the various orbitals of the sub shell of higher energy.

Symmetrical distribution: The electronic configuration in which all the orbitals of the same sub shell  are either completely filled or are exactly half filled are more stable because of symmetrical distribution of electrons.

1) The expected electronic configuration of chromium

If one of the 4s electron shifts to the vacant 3d orbital ,the distribution of the electron will become more symmetrical and this will impart extra stability.

The actual electronic configuration of chromium

2) The expected electronic configuration of copper

If one of the 4s electron shifts to the vacant 3d orbital ,the distribution of the electron will become more symmetrical and this will impart extra stability.

The actual electronic configuration of copper

Exchange energy

The electrons with parallel spins present in the degenerate orbitals tend to exchange their position .The energy released during this exchange is called exchange energy.

The number of exchanges that can take place is maximum when degenerate orbitals are exactly half filled or completely filled.As a result, the exchange is maximum and so is the stability.