Animated Solution for Chemistry - s and p-Block Elements: The correct order of conductivity of ions in water is

Imagine you are looking at the alkali metals in their pure, gaseous state. As we travel down Group 1 of the periodic table—from Sodium to Cesium—we are adding more and more electron shells.

Naturally, the size of the bare ions increases. Sodium, sitting near the top, is a tiny, compact sphere of positive charge. Cesium, down at the bottom, is a large, bulky sphere. So, in a vacuum, the order of size is simply:

But the question asks about their behavior in water. This changes everything! Water is a polar molecule, meaning it has a slightly negative oxygen end and slightly positive hydrogen ends. When you drop these positive ions into water, the negative oxygen ends of the water molecules rush towards them. This process is called hydration.

Here is the catch: the strength of this attraction depends on the charge density of the ion. Charge density is the ratio of charge to size. Since all these ions have a $+1$ charge, the smallest ion will have the highest charge density.

Sodium, being tiny, has an intensely concentrated positive charge. It acts like a super-magnet, attracting a massive, thick shell of water molecules around it. Cesium, being large, has its $+1$ charge spread out over a wide area. Its charge density is low, so it only weakly attracts a few water molecules.

Therefore, the effective size of the ion in water—its hydrated radius—is completely reversed!

Now, let's talk about conductivity. Conductivity in a solution depends on how fast these ions can move, which is known as ionic mobility.

Imagine a swimming race. The hydrated sodium ion is like a swimmer wearing a massive, heavy winter coat made of water molecules. It experiences tremendous viscous drag and moves very slowly. On the other hand, the hydrated cesium ion is like a sleek, professional swimmer. With only a thin layer of water molecules, it zips through the solution with ease.

Since conductivity is directly proportional to ionic mobility, the fastest ion will conduct the most electricity. Thus, the correct order of conductivity is:

This perfectly matches option (b). Always remember this beautiful paradox: the smallest ion in the gas phase becomes the largest and slowest in the aqueous phase!