Periodic table is a table that arranges elements based on physical and chemical properties into groups and periods. Non metals reacts by gaining electrons, the ease of gaining electrons determines the reactivity of a non metal. Argon and helium belong to group eight of the periodic table therefore are unreactive (inert), carbon belongs in group thus reacts by gaining 4 electrons while fluorine is a group 7 element and reacts by gaining 1 electron. Therefore, the answer is fluorine,since it requires to gain only one electron.
