Fluorine, chlorine, and bromine all have seven valence electrons. which element has the largest atomic radius?

Among these three halogen Bromine has the greatest atomic radius. As Bromine is present in 4rth period it outermost electrons are present in 4rth shell, so greater the number of shells greater is the size of an atom. While, in case of chlorine and fluorine they are present in group 3 and 2 respectively.