Answer to Question #147755 in General Chemistry for Isurini Sirisena

Question

The table gives the ionic radii of some ions.
FORMULA OF ION IONIC RADIUS / nm
Li+ 0.076
Na+ 0.102
Mg2+ 0.072
Al3+ 0.054
Arrange the ions in order of their polarising power. Show how you arrived at your answer.

Answer 1

It has been seen that Li differs from other alkali metal in many respects and shows a close resemblance to Mg (diagonal relationship), the second element of group-2 and diagonally opposite to Li in the periodic table. On moving diagonally in the periodic table, the polarising power i.e. ratio of the ionic charge and square of the ionic radius is nearly same for the Lithium and Magnesium (ionic charge and ionic radius of Li and Mg are +1 and +2 and 60pm and 65pm respectively) and accounts for the similarities between the properties of the two elements.

The increasing order of polarising power is K+<Ca2+<Mg2+<Be2+. Charge-size ratio increases and thus polarising power increases.For Be2+,Mg2+,Ca2+, since charge is same and size is in the order of Ca2+>Mg2+>Be2+ , the order of polarising power is Ca2+<Mg2+<Be2+.  Charge of K+ is lowest and size is larger than Ca2+ so its polarising power is lowest.