(a) The modern periodic table is arranged in groups and periods based on the electronic configuration of the atoms of elements.
(b) Physical and chemical properties of elements are related to their electronic configurations particularly the outershell configurations.
(c) The atoms of the elements in a group possess similar electronic configurations.
(d) The elements in a group should have similar chemical properties and there should be regular gradation in their physical properties from top to bottom.
(e) Across the table from left to right in any period, elements get an increase in the atomic number by 1 unit between any two successive elements.
(f) Therefore the electronic configuration of valence shell of any two elements in a given period is not same.
(g) Due to this reason elements along a period possess different chemical properties with regular gradation in their physical properties from left to right.
