Atom with the largest atomic radius in group 18

Radon

Explanation:

Group eighteen elements are called noble gases. This group includes helium, neon, argon, krypton, xenon and radon.

Radon is present at the last of this column.

Atomic radii trend along group:

As we move down the group atomic radii increased with increase of atomic number. The addition of electron in next level cause the atomic radii to increased. The hold of nucleus on valance shell become weaker because of shielding of electrons thus size of atom increased.

As the size of atom increases the ionization energy from top to bottom also decreases because it becomes easier to remove the electron because of less nuclear attraction and as more electrons are added the outer electrons becomes more shielded and away from nucleus.

Thus radon has largest atomic radius.