Chlorine (Cl) has an atomic number of 17, which means it has 17 protons in its nucleus. Bromine (Br) has an atomic number of 35, meaning it has 35 protons. The number of protons in an atom determines the number of electrons it has, and the number of electrons determines the size of the atom.

Because bromine has more protons, it has more electrons and therefore a larger atomic radius than chlorine. In other words, bromine atoms are larger than chlorine atoms.