What do elements in a group have?

Elements in a group have very similar properties and characteristics. Every element in a group has the same amount of electrons in their outer shell. For Example, all elements in Group 2 - Alkaline Earth Metals have 2 electrons in their outer shell. Elements in the same group also have very similar characteristics such as what they react with etc. For Example:

Group 2 - Alkali Metals - All react vigourously in Water.

Group 18(0) - Inert Gases - All are highly unreactive.