Answer: Their atoms have the same number of valence electrons.
Explanation:
The elements in the periodic table are arranged in order of increasing atomic numbers. The horizontal rows are called as periods and vertical columns are called as groups.
The elements in the groups are arranged in order of increasing atomic numbers and thus contain different number of protons and different number of neutrons. The electrons are added to new energy shells but contain same valence electrons and thus have similar chemical reactivity.
Example: All the alkali metals have 1 valence electron and thus easily lose electrons to form unipositive cations.
