Question

The ionic conductance is least for:
(A) $C{s^ + }$
(B) $R{b^ + }$
(C) ${K^ + }$
(D) $N{a^ + }$

Answer 1

Hint: Ionic conductance is the ability of the given ion to conduct the electric charge. The ionic conductance of the metal ion is dependent upon the extent of its hydration in aqueous solutions.

Complete step by step solution:
We are given an ionic form of four metals and need to find which one of them has highest ionic conductivity. Let’s see what is meant by ionic conductivity.
- Ionic conductivity of an ion is the ability of the ion towards conduction of electricity through the crystal (if in solid state) or in solution.
- For the ionic forms of the metals, the ion which is more mobile will be able to conduct more charge in the aqueous solution.
- We know that in aqueous solution, the ions are surrounded by the water molecules which are solvent molecules. So, we say that the ions are hydrated.
- Now, the ions which are more hydrated, will have more water molecules surrounded by it and ionic conductance of them will be less.
- The ions which are smaller in size, will have more charge to size ratio and they will be more surrounded by the water molecules.
- Na is in the second period, K, Rb and Cs are in the third, fourth and fifth period of the periodic table. So, it is evident that the size of the ions increases as we go down in the periodic table.
So, we can say that the size of $N{a^ + }$ ions will be the least amongst the other given ions. So, it will be most hydrated and as a result, it will be the least conductive.
- Thus we can say that ionic conductance of $N{a^ + }$ will be the least.

So, the correct answer is (D).

Note: Remember that this theory of extent of hydration applies only in the aqueous solutions of the given ions. It is not applicable to the conductance of metal in its pure metallic form.