2.4 understand why elements in the same group of the Period Table have similar chemical properties

Elements in the same group have the same number of elements on their outer shell and therefore have similar chemical properties.

E.g all elements in group 2 have 2 electrons on their outer shell, so they all want to lose 2 electrons. Similarly, all noble gases (group 0) have a full outer shell so are inert as they don't want to lose any electrons.