Explain why elements in the same group of the periodic table have similar chemical properties

Elements in same group have same chemical properties due to same number of valence electrons in valence shell.

Explanation:

Chemical properties are those properties which results in transforming the starting material to a entirely new substance with different chemical and physical properties. This transformation takes place after a chemical reaction. Hence, the chemical reactivity of some atoms is greater than the others and vice versa.

In periodic table it is found that the elements in the same group have almost same chemical behaviour. For example, elements of group 8 (or 18) are almost non reactive, while that of group 1 and group 7 are found very reactive and show same chemical behaviour like group 1 elements forms + 1 ion while group 7 elements form - 1 ions as the group 1 metals have single valence shell electron which upon losing allow the atom to attain stability and the halogen having 7 electrons in valence shell upon accepting one electron allow them to attain a octet of stability respectively.