How are the elements arranged into groups and periods in the modern periodic Table ? Elements in a group posses similar properties , but elements in a period do not show similarites in their properties . Why ?

Explanation

The modern periodic table is arranged in group and periods based on the electronic configuration of the atoms of elements .
Physical and chemical properties of elements are realated to their electronic configurations particularly the outershell the outershelll configurations.
The atoms of the elements in a group posses electronic configurations .
The elements in a group should have similar have similar chemical properties and there should be regular gradation in their physical properties from top to bottom .
Across the table from left to right in any period , elements gets and increase in the atomic number by I unit between any two successive elements .
THerefore the electronic configuration of Valence shell of any two elements in a given period is not same .
Due to this reason elements along a period posses different chemical properties with regular gradation in their phy in the physical properties from left to right