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Elements belonging to the same group have the same valency because:

⇒ Valency of an element can be determined by counting the number of electrons present in the outermost shell.

⇒ We can also determine the group number of any element by knowing its valency.

⇒ If the valency of an element is one, it means the element belongs to first group. If the valency of an element is two, this means the element belongs to second group and so on.

⇒ For example:

As you can see in the table, all the alkaline earth metals have valency 2, thus they all belong to second group.