Dear student,

Electronic configuration is the arrangement of electrons in an atom in different shells and sub-shells.After the discovery of the shells and sub-shells,various physical and chemical properties of elements were linked to the electronic configuration.As a result, attempts were made to arrange elements by their  electronic configuration so that their properties would obviously fall in a trend.Subsequently the modern arrangement is evolved wherein elements are organized by atomic number.This is known as modern periodic table.

The modern periodic table has elements arranged according to their electronic configurations.From left to right i.e. in a period,the arrangment is such that each subsequent electron added is to the same shell.

Electronic Configuration is the Period

e.g  Configuration of  Li atomic number. 3,  is  1s^₂ ,2s^₁  

Configuration of  Be atomic no. 4,  is  1s^₂ ,2s^₂        

Configuration of  B atomic no. 5,  is  1s^₂ ,2s²,2p¹     

Configuration of  C atomic no. 6,  is  1s^₂ ,2s^₂ ,2p²      

and so on till Neon,the last element.

This addition of each subsequent electron results in a completely new configuration and as such the chemical properties of  the subsequent element change gradually..At the end of each row, a drastic shift occurs in chemical properties.

Electronic Configuration is the Group

From top to down in a column,i.e.in a group, each subsequent electron is added to the new shell,but in same sub-shell.                       e.g. In the 1st group,

Configuration of  H atomic number.1,is 1s^₁  

Configuration of  Li atomic no. 3,  is  1s^₂ ,2s^₁      

Configuration of  Na atomic no. 11,  is  1s^₂ ,s²,2p⁶ ,3s¹    

Configuration of  K atomic no. 19,  is  1s^₂ ,2s^₂ ,2p⁶,3s^₂ ,3p⁶,4s¹   

and so on till Neon,the last element.

The number of valence shell electrons  in an atom would determine the way it will interact with other  atoms, and therefore determine its chemical properties.

Thus the outermost configuration ,which is responsible for various properties as valency,oxidation states etc. remains same. Hence in a group, the physical and chemical properties are the same. To illustrate,the halogens are strong electronegative elements.

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All the best.

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Sagar Singh

B.Tech, IIT Delhi