91Ó°ÊÓ

As the effective nuclear charge increases along the period from left to right in the periodic table, the atomic radius of the elements of the same period decreases.

If the elements of the same (column) group are considered in the periodic table, then the new shell will enter the atoms, increasing the atomic radius.

The element Cs belongs to the sixth period. So it has the largest size.

The element Rb belongs to the fifth period. So it has the second largest size.

The element Ca belongs to the fourth period. So it has the third-largest size.

The element Mg belongs to the third period. So it has the least size among the given elements.

Thus, the order is:

\(\;Mg{\rm{ }} < {\rm{ }}Ca{\rm{ }} < {\rm{ }}Rb{\rm{ }} < {\rm{ }}Cs\)