Given below are two statements :
One is labelled as Assertion (A) and the other is labelled as Reaso

Q: Doesn't high polarising power mean it can polarise water molecules better?

While it's true that $Li^+$ interacts strongly with water, 'polarising power' in chemistry specifically refers to Fajan's rules—the ability of a cation to distort the electron cloud of an anion to introduce covalent character. Hydration is primarily an ion-dipole electrostatic attraction.

Q: Why is Lithium the only alkali metal that forms hydrated salts?

Lithium has the smallest ionic radius among alkali metals, giving it the highest charge density. This allows it to attract water molecules strongly enough to overcome the lattice energy, trapping them in the crystal structure (e.g., $LiCl \cdot 2H_2O$).

Q: Are Assertion-Reason questions always this tricky?

Yes! The classic trap is giving two true statements that share the same root cause (here, small size and high charge density) but aren't directly causally linked to each other. Always ask: 'Does R directly cause A?'

Q: What is the formula for polarising power?

Polarising power is qualitatively proportional to the charge of the cation divided by its radius (Ionic Potential or $\phi = \frac{q}{r}$).

Q: Does hydration energy increase or decrease down the group?

Hydration energy decreases down the group. As the ionic size increases from $Li^+$ to $Cs^+$, the charge density drops, leading to weaker attractions with water molecules.

Question

Given below are two statements :
One is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) Lithium salts are hydrated.
Reason (R) Lithium has higher polarising power than other alkali metal group members.
In the light of the above statements, choose the most appropriate answer from the options given below

Options:

(A) Both (A) and (R) are true but (R) is not the correct explanation of (A).

(B) (A) is true but (R) is not true

(C) (A) is false but (R) is ture.

(D) Both (A) and (R) are true (R) is the correct explanation of (A).

Sigma Percentile · Accepted Answer

### The Tiny Giant: Lithium

When we talk about the alkali metals, Lithium is the odd one out. Despite being the first member of its group, it behaves quite differently from Sodium, Potassium, and the rest. This anomalous behavior is primarily due to its exceptionally small ionic size. In this problem, we are presented with two statements about Lithium, and we need to determine not only if they are true, but if one causes the other.

### Analyzing the Assertion: Hydration

The assertion states that **Lithium salts are hydrated**. Is this true? Absolutely. A classic example is Lithium Chloride, which crystallizes as a dihydrate, $LiCl \cdot 2H_2O$.

But why does this happen? The answer lies in the concept of **hydration enthalpy**. Because the $Li^+$ ion is so small, its positive charge is highly concentrated in a tiny volume. This creates a very high **charge density**. When placed in water, this intense electric field strongly attracts the negative ends (the oxygen atoms) of the polar water molecules. This strong **ion-dipole attraction** forms a thick hydration shell around the Lithium ion, releasing a large amount of hydration energy. This energy is sufficient to overcome the lattice energy, allowing the salt to crystallize with water molecules trapped in its structure.

### Analyzing the Reason: Polarising Power

Now, let's look at the reason statement: **Lithium has higher polarising power than other alkali metal group members**.

According to Fajan's rules, **polarising power** is the ability of a cation to distort the electron cloud of a nearby anion. This distortion pulls the electron density into the region between the two nuclei, introducing **covalent character** into an otherwise ionic bond. The polarising power of a cation is directly proportional to its charge and inversely proportional to its size (often measured as the ionic potential, $\phi = \frac{q}{r}$).

Since $Li^+$ has the smallest radius among the alkali metals, it indeed possesses the highest polarising power in its group. Therefore, the reason statement is also factually true.

### The Verdict: Correlation vs Causation

Here is where the trap lies. Both the assertion and the reason are true statements. But does the reason *explain* the assertion?

Let's break it down:
1. **Hydration** is an electrostatic interaction between a cation and polar solvent molecules (ion-dipole attraction).
2. **Polarising Power** is the distortion of an anion's electron cloud by a cation, leading to covalent character.

While both of these phenomena are direct consequences of Lithium's **small size and high charge density**, they are independent properties. Polarising power does not *cause* hydration. They are simply two different effects stemming from the same root cause.

Therefore, both (A) and (R) are true, but (R) is **not** the correct explanation of (A).

Select Answer:

Visualized Solution

$A sser t i o n (A)$

$R e a so n f or H y d r a t i o n$

$R e a so n (R)$

$P o l a r i s in g P o w er v s H y d r a t i o n$

$C o n c l u s i o n$

The Sigma Insight: Alkali Metals

The Tiny Giant

Analyzing the Assertion

Analyzing the Reason

The Verdict

Similar Questions

Among the statements (A)-(D), the correct ones are (A) lithium has the highest hydration enthalpy among alkali metals. (B) lithium chloride is insoluble in pyridine. (C) lithium cannot form ethynide upon its reaction with ethyne. (D) Both lithium and magnesium react slowly with $H_{2} O$ .

The incorrect statement is

Choose the correct statement from the following.

The correct order of hydration enthalpies of alkali metal ions is

Match List I with List II. Choose the correct answer from the options given below.

List-I

List-II

The ionic mobility of alkali metal ions in aqueous soluton is maximum for

The correct order of conductivity of ions in water is

Both lithium and magnesium display several similar properties due to the diagonal relationship; however, the one which is incorrect is