The correct order of ionic radii of Ce, La, Pm and Yb in +3 oxidation state is

Options

A. `La^(3+) lt Pm^(3+) lt Ce^(3+) lt Yb^(3+)`
B. `Yb^(3+) lt Pm^(3+) lt Ce^(3+) lt La^(3+)`
C. `La^(3+) ltCe^(3+) ltPm^(3+)lt Yb^(3+)`
D. `Yb^(3+) lt Ce^(3+) ltPm^(3+) lt La^(3+)`

Explanation

To determine the correct order of ionic radii of Ce (Cerium), La (Lanthanum), Pm (Promethium), and Yb (Ytterbium) in the +3 oxidation state, we can follow these steps:

### Step 1: Identify the Elements and Their Positions
- **Lanthanum (La)**: Atomic number 57
- **Cerium (Ce)**: Atomic number 58
- **Promethium (Pm)**: Atomic number 61
- **Ytterbium (Yb)**: Atomic number 70

### Step 2: Understand the Concept of Lanthanide Contraction
- The lanthanide contraction refers to the gradual decrease in ionic and atomic radii of the lanthanide series elements as the atomic number increases from La to Lu (Lutetium).
- This contraction occurs due to the increasing nuclear charge with the addition of electrons in the 4f subshell, which does not shield the outer electrons effectively.

### Step 3: Analyze the Ionic Radii in +3 Oxidation State
- In the +3 oxidation state, all these elements lose three electrons. The effective nuclear charge increases, leading to a decrease in ionic radii.
- The order of ionic radii will be influenced by the position of the elements in the lanthanide series.

### Step 4: Compare the Ionic Radii
- **Yb (Ytterbium)**: Being further down the series, it will have a smaller ionic radius due to lanthanide contraction.
- **Pm (Promethium)**: Comes next; it will have a larger ionic radius than Yb but smaller than Ce and La.
- **Ce (Cerium)**: Will have a larger ionic radius than both Pm and Yb.
- **La (Lanthanum)**: Will have the largest ionic radius among these four.

### Step 5: Establish the Correct Order
Based on the above analysis, the correct order of ionic radii in +3 oxidation state is:
**La > Ce > Pm > Yb**

### Final Answer
The correct order of ionic radii of Ce, La, Pm, and Yb in +3 oxidation state is:
**La > Ce > Pm > Yb**

---