In the given set of elements: Arsenic - Bromine - Cesium. which would be the smallest?

I assume you mean in terms of atomic radius. Bromine would be smallest. Cesium has electrons in six energy levels, whereas bromine and arsenic have electrons only in the first four. The sixth energy level certainly has a larger radius than the fourth.

A little less obvious is the fact that bromine has a smaller atomic radius than arsenic. But as you go across the periodic table from left to right, the atoms shrink. Bromine has electrons in the first four energy levels, and they are attracted to a nucleus containing 35 protons. Arsenic has electrons in the first four energy levels and they are attracted to a nucleus containing 33 protons. The 35 protons of bromine will pull the electrons in closer than the 33 protons of arsenic; therefore bromine is the smaller atom in terms of atomic radius.

If smallest refers to at. radius then the Br will have smallest radius.

Their radii are

Br =1.12 A

As = 1.39 A

Cs = 2.67A

The smallest? you mean by mass right?

well...As=74.92

Br=79.9

cesium=132.9

:. The smallest element by mass would obviously be Arsenic because it's molar mass is 74.92 g/mol

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Hope this helps!! =)