Which one of the following atoms has the largest radius? Ова Осо OCa

Ba

Explanation:

The elements Calcium (Ca) and Cobalt (Co), are both present in the same period, and across the period the atomic radius decreases from left to right, since, the atomic number increases, so the electrons increases, which in turns increases the effective nuclear charge, and hence the size decreases,

Therefore,

Calcium will have bigger atomic radii as compared to Cobalt.

Now,

The elements Barium (Ba) and Calcium (Ca) are, both are present in the same group, and down the group, atomic radii increases, since, as going down the group, the atomic number increase, so the shells increases, hence the size increases.

Therefore, the atom with the largest radii is Barium.