in an ionic bond the cation tends to polarise the electron cloud of the anion by pulling electron density towards itself This causes development of covalent character in the ionic bond because the electron density gets localised in between the nuclei The tendency of the cation to bring about the polarisation of the anion is expressed as its polasising power The ability of ion to undergo polarisation is called its polarisability The polarising power of a cation or an anion is decided on the basis of `F` ajans' rules as follows .
(i) The smaller the cation, the higher is its polarising power
(ii) Cations with pseudo-noble gas configuration `(ns^(2)np^(6)nd^(10))` have relatively high polarising power than those with noble gas configuration `(ns^(2) np^(6))`
(iii) The larger the size of the anion, the higher is its polarisability
The ionic conductance of which of the following is the highest ? .

Options

A. `Li_((aq))^(o+)`
B. `Na_((ag))^(o+)`
C. `K_((aq))^(o+)`
D. `Cs_((aq))^(o+)`

Explanation

To determine which ion has the highest ionic conductance among the given options (lithium ion, sodium ion, potassium ion, and cesium ion), we can follow these steps:

### Step 1: Understand Ionic Conductance
Ionic conductance is directly proportional to the mobility of the ions in solution. Higher mobility means higher conductance.

**Hint:** Remember that mobility refers to how easily an ion can move through a solution.

### Step 2: Analyze Ion Size
The mobility of ions is inversely proportional to their size. This means that smaller ions will have higher mobility, while larger ions will have lower mobility.

**Hint:** Keep in mind that smaller ions can move more freely compared to larger ions.

### Step 3: Compare the Sizes of the Ions
The ions in question are:
- Lithium ion (Li⁺)
- Sodium ion (Na⁺)
- Potassium ion (K⁺)
- Cesium ion (Cs⁺)

As we move down the group in the periodic table, the size of the ions increases. Therefore, the order of size from smallest to largest is:
- Li⁺ < Na⁺ < K⁺ < Cs⁺

**Hint:** Visualize the periodic table to see how the size of the ions increases down the group.

### Step 4: Relate Size to Mobility
Since smaller ions have higher mobility, we can conclude:
- Li⁺ will have the highest mobility.
- Cs⁺ will have the lowest mobility.

**Hint:** Remember that higher mobility corresponds to higher ionic conductance.

### Step 5: Consider Hydration Effects
In an aqueous medium, the hydration of ions also plays a role. Hydration is inversely proportional to the size of the ion. Smaller ions have greater hydration, which can affect their effective size in solution.

**Hint:** Hydration affects how ions interact with water molecules, influencing their movement.

### Step 6: Determine Ionic Conductance
Given that Li⁺ is the smallest and has the highest mobility, it will have the highest ionic conductance. In contrast, Cs⁺, being the largest, will have the lowest ionic conductance.

**Hint:** Focus on the relationship between size and mobility to determine which ion conducts electricity best.

### Conclusion
Based on the analysis, the ion with the highest ionic conductance in aqueous solution is the lithium ion (Li⁺).

**Final Answer:** Lithium ion (Li⁺) has the highest ionic conductance.