Which one of the following statements related to lanthanons is incorrect ?

Options

A. Europium shows `+2` oxidation state.
B. The basicity decreases as the ionic radius decreases from Pr to Lu
C. All the lanthanons are much more reactive than aluminium
D. Ce `(+4)` solution are widely used as oxidising agent in volumetric analysis

Explanation

To solve the question regarding which statement related to lanthanons is incorrect, we will analyze each option provided in the question.

### Step 1: Analyze the first statement
**Statement:** Europium shows a +2 oxidation state.

- **Analysis:** Europium (Eu) has the electronic configuration of [Xe] 6s² 4f⁷. It can lose two electrons from the 6s subshell to exhibit a +2 oxidation state. Therefore, this statement is **correct**.

### Step 2: Analyze the second statement
**Statement:** The basicity decreases as the ionic radius decreases from promethium to lutetium.

- **Analysis:** Due to lanthanide contraction, the ionic radius of lanthanides decreases from promethium (Pm) to lutetium (Lu). According to Fajan's rule, as the size of cations decreases, their polarization power increases, leading to an increase in covalent character and a decrease in basicity. Therefore, this statement is **correct**.

### Step 3: Analyze the third statement
**Statement:** All the lanthanides are much more reactive than aluminum.

- **Analysis:** Lanthanides exhibit poor shielding due to the presence of f-electrons, leading to an inert pair effect. This means that their outermost electrons (6s²) are less available for bonding compared to aluminum, which does not have this effect. Therefore, aluminum is more reactive than lanthanides. Thus, this statement is **incorrect**.

### Step 4: Analyze the fourth statement
**Statement:** Cerium +4 solution is widely used as an oxidizing agent.

- **Analysis:** Cerium in the +4 oxidation state is indeed a strong oxidizing agent and is commonly used in various applications. Therefore, this statement is **correct**.

### Conclusion
The incorrect statement among the options provided is the third one: "All the lanthanides are much more reactive than aluminum."

### Final Answer
The incorrect statement related to lanthanons is: **"All the lanthanides are much more reactive than aluminum."**

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