Answer – Francium has the largest atomic radius.

Explanation: 

Atomic radius is often defined as the distance between the center of the nucleus and the outermost orbital of electrons in an atom. However, since orbitals only indicate the probability of electrons, a more accurate definition is provided – atomic radius is half the distance between the nuclei of two identical atoms that are bonded together.

Much like with other atomic properties, atomic radius trends can also be observed across periods and down groups in the periodic table. While moving across a period, both protons and electrons are added to the atoms of elements. But since electrons are added to the valence shell (and not the inner energy levels), they experience a high effective nuclear charge. This leads to decreasing atomic size and hence reduced atomic radius as we go from left to right across a period.

While moving down a group, electrons are added to orbitals of consecutively higher energy levels. These are further away from the nucleus and hence experience a lower nuclear attraction, causing them to be larger in size. This results in the greater atomic size and hence increasing atomic radius as we go down a group.

With these trends as the basis, Francium occurring in Group I and Period 7 is the element with the largest atomic radius. Helium in Group 18 and Period 1, on the other hand, has the smallest atomic radius.

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