Why do elements in the same group of the periodic table have similar properties?

Explanation:

Elements present in same group have same number of valence electrons and it is the valence electrons that determine the reactivity of an element.

For example, elements fluorine, chlorine and bromine are all group 17 elements.

Each of them contains 7 valence electrons and hence, all of them needs to gain one electron each in order to complete their octet, respectively.

Hence, we can conclude that elements in the same group of the periodic table have similar properties because they have same number of valence electrons.