At and Back of

At noun - A highly unstable radioactive element (the heaviest of the halogen series); a decay product of uranium and thorium.

Back of and at are semantically related. In some cases you can replace phrase "Back of" with "At", this terms are similar.

Back of preposition - At, to, or toward the rear of.

At and back of are semantically related. Sometimes you can use "At" instead a preposition phrase "Back of".