Question

rank the atoms br, i, cl, and at by increasing electron affinity.
i < at < cl < br
at < br < i < cl
cl < br < i < at
at < i < br < cl

Explanation:

Electron affinity generally increases up a group in the periodic table (smaller atomic size leads to stronger attraction for an added electron), with a slight exception for astatine (At) due to its very large, diffuse electron cloud that reduces attraction for a new electron compared to iodine (I). Chlorine (Cl) has a higher electron affinity than bromine (Br) as it is smaller; Br has a higher affinity than I, and At has the lowest of the four.

Answer:

At < I < Br < Cl