Question

what is the reason behind the similarity in properties along a group in a periodic table​

Answer 1

The elements of same group have similar chemical properties because they have same number of valence electrons.

Answer 2

$Answer$

(i) All the elements of a group have similar chemical properties because they have same no. of valence electrons in their outermost shell.

(ii) All the elements of a period have different chemical properties because they have different no. of valence electrons in their atoms.