Final answer:
Br- is larger than Br because gaining an electron increases the electron-electron repulsion in the electron cloud, leading to a larger radii.
Step-by-step explanation:
The atom or ion that is larger between Br- and Br is Br-. When an atom gains an electron to become an anion, its radius increases due to increased electron-electron repulsions in the electron cloud. This makes the bromide ion (Br-) larger than the neutral bromine atom (Br). On the other hand, atoms tend to be smaller when they lose electrons and become cations, as the loss of an electron can lead to less repulsion among the remaining electrons and a tighter pull from the nucleus.