Electronic Configuration of Atoms | Chemistry Class 11 - NEET PDF Download

What is Electronic Configuration?

• The distribution of electrons of an atom in its various shells, subshells, and orbitals is known as electronic configuration.  
• Electronic configurations of atoms follow a standard notation in which all electron-containing atomic subshells (with the number of electrons they hold written in superscript) are placed in a sequence. 

Example of Electronic Configuration

• For example, the electronic configuration of sodium is 1s²2s²2p⁶3s¹.
• Valence shell or outermost shell → n^th shell
• Penultimate shell-Inner to outermost shell → (n -1) shell
• Anti Penultimate shell → (n - 2) shell

Electronic configuration can be expressed in the following ways :

(i) Orbital notation method: nl^x

Li → 3 → 1s² 2s¹

(ii) Orbital diagram method :

Li → 3 →

(iii) Condensed form :

Electronic Configuration of ions :

To write the electronic configuration of ions, first, write the electronic configuration of the neutral atom and then add (for a negative charge) or remove (for a positive charge) electrons in the outer shell according to the nature and magnitude of charge present on the ion. e.g.

Al → [Ne]3s² 3p¹,

,

Similarly, in the case of transition elements, electrons are removed from the n^th shell e.g. 4^th shell in the case of 3 d series.

Cl : [Ne] 3s².3p⁵

Cl^- : [Ne] 3s².3p⁶

Exceptional configuration of Copper and Chromium

Expected configuration Cr - 24 -> [Ar] 4s² 3d⁴ 

Observed configuration

Expected configuration

Observed configuration

The above exceptional configuration can be explained based on the following factors.

Symmetrical Electronic Configuration 

It is a well-known fact that symmetry leads to stability, orbitals of a subshell having a half-filled or full-filled configuration (i.e. symmetrical distribution of electrons) are relatively more stable. This effect is dominating in the d and f subshells.

d⁵, d¹⁰, f⁷, f¹⁴ configurations are relatively more stable.

Exchange Energy 

It is assumed that electrons in degenerate orbitals, do not remain confined to a particular orbital rather they keep on exchanging their positions with electrons having the same spin and the same energy (electrons present in orbitals having the same energy). Energy is released in this process known as exchange energy which imparts stability to the atom. The more the number of exchanges, the more will be energy released, and more the energy released stability will be.

You may note that the exchange energy is at the basis of Hund’s rule that electrons which enter orbitals of equal energy have parallel spins as far as possible. In other words, the extra stability of half-filled and completely filled subshell is due to: 

(i) relatively small shielding, 

(ii) smaller coulombic repulsion energy, and (iii) larger exchange energy. 

Magnetic moment (m):

It is the measure of the magnetic nature of a substance

where n is no. of unpaired electrons.

When m = 0, then there is no unpaired electron in the species and it is known as a diamagnetic species.

These species are repelled by magnetic fields.

When , for paramagnetic species. These species have unpaired electrons and are weakly attracted by the magnetic field.

The formula for the magnetic moment is:

where n= no. of unpaired e^-

Ques. Calculate the magnetic moment of these species

(i) Cr 

Solution: 

(i) Cr: No. of unpaired electrons = 6.

=  BM.

Generally, species having unpaired electrons are coloured or impart colour to the flame. Species having unpaired electrons can be easily excited by the wavelengths corresponding to visible lights and hence they emit radiations having characteristic colour.