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 for Ce, La, Pm, and Yb in the +3 oxidation state, we will follow these steps:

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

### Step 2: Understand the Concept of Ionic Radii
Ionic radii refer to the size of an ion in a crystal lattice. For cations (positively charged ions), the ionic radius generally decreases as the charge increases due to the increased attraction between the nucleus and the electrons.

### Step 3: Recognize the Trend in Ionic Radii
As we move from left to right in the lanthanide series, the ionic radii decrease. This trend is primarily due to **lanthanoid contraction**, which occurs because of the poor shielding effect of the f-electrons. This results in a stronger effective nuclear charge acting on the outer electrons, pulling them closer to the nucleus and reducing the ionic size.

### Step 4: Apply the Trend to the Given Elements
- **Yb³⁺**: Being the furthest to the right in the series, it will have the smallest ionic radius due to the highest effective nuclear charge.
- **Pm³⁺**: Next in size, as it is to the left of Yb.
- **Ce³⁺**: Larger than Pm³⁺.
- **La³⁺**: The largest among the four due to its position at the beginning of the series.

### Step 5: Write the Correct Order of Ionic Radii
Based on the above analysis, the correct order of ionic radii in the +3 oxidation state is:
**Yb³⁺ < Pm³⁺ < Ce³⁺ < La³⁺**

### Conclusion
Thus, the correct order of ionic radii for Ce, La, Pm, and Yb in the +3 oxidation state is:
**Yb³⁺ < Pm³⁺ < Ce³⁺ < La³⁺**