The periodic table is the arrangement of elements according to their atomic numbers [modern periodic law]
A) The elements which have similar properties are placed in same group.
The periodic table is arranged in 18 different groups based on the elements with similar properties
B) the elements of a group have similar properties due to similar outer electronic configuration or same valence electrons.
For example
C)
a) Alkali metals: they all have one electron in their outermost shell or they have one valence electron. due to this they can easily form cations by losing one electron and gain stable noble gas configuration.
Lithium, sodium and potassium
b) noble gases: they have completely filled outer shell or valence shell. So they are not much reactive and do not form compounds or molecules easily
Helium, neon, argon
