The size of an atom is determined by the number of protons in its nucleus. The more protons an atom has, the larger it is. Chlorine has 17 protons, while bromine has 35 protons. This means that a bromine atom is larger than a chlorine atom.

The size of an atom also depends on the number of electrons it has. The more electrons an atom has, the larger it is. Chlorine has 17 electrons, while bromine has 35 electrons. This means that a bromine atom is larger than a chlorine atom.

Finally, the size of an atom also depends on the number of neutrons it has. The more neutrons an atom has, the larger it is. Chlorine has 18 neutrons, while bromine has 45 neutrons. This means that a bromine atom is larger than a chlorine atom.