Answer : Barium has the largest atomic radius.
Explanation :
The periodic trend observed with respect to atomic radius is explained below.
Across a period : As we move from left towards right across a period, atomic radius decreases.
This happens because as we move towards right, an extra electron gets added to the same shell. But as we move towards right, the effective nuclear increases. As a result nucleus pulls the outer electrons more strongly which results in shrinking of the atom.
Down the group : As we move down the group, the atomic radius increases.
Because as we move down, an extra shell gets added to the atom. Due to this size of the atom increases.
If we look at the positions of the given elements , we see that Calcium and Bromine are present in fourth period whereas Barium and Astatine are present in sixth period.
According to periodic trend observed for group, Barium and Astatine would have larger atomic radii than Calcium and Bromine.
If we apply periodic trend observed across a period, we can say that Barium is larger than Astatine because it is more towards left .
Therefore the atom with largest radius is Barium.
