Which of the following facts are not explainable by lanthanoid contraction

Options

A. I.E. of Ag `lt` Au
B. `I.E._(I)` of Hg `gt` Cd
C. Size of `Hf ~= Zr`
D. Size of Y `lt` La

Answer

To solve the question of which fact is not explainable by lanthanoid contraction, we will analyze each option provided and determine if it can be explained by the concept of lanthanoid contraction.

### Step-by-Step Solution:

1. **Understanding Lanthanoid Contraction**:
- Lanthanoid contraction refers to the gradual decrease in ionic radii of the lanthanide series (elements 57 to 71) due to the increasing nuclear charge without a corresponding increase in shielding effect. This results in a smaller than expected ionic radius for these elements.

2. **Analyzing Each Option**:
- **Option A**: "The ionization enthalpy of silver (Ag) is less than that of gold (Au)."
- This statement is correct and can be explained by lanthanoid contraction. The increased nuclear charge in gold leads to a higher ionization enthalpy compared to silver.

- **Option B**: "The first ionization enthalpy of mercury (Hg) is greater than that of cadmium (Cd)."
- This statement is also correct. As we move from cadmium to mercury, the size decreases due to lanthanoid contraction, leading to a higher ionization enthalpy for mercury.

- **Option C**: "The size of hafnium (Hf) is nearly equal to the size of zirconium (Zr)."
- This statement is correct as well. The sizes of Hf and Zr are similar due to the effects of lanthanoid contraction, which affects the transition metals in the same group.

- **Option D**: "The size of yttrium (Y) is less than that of lanthanum (La)."
- This statement is incorrect. The decrease in size of yttrium compared to lanthanum cannot be explained by lanthanoid contraction, as yttrium is not a lanthanide and does not experience the same effects.

3. **Conclusion**:
- The correct answer is **Option D**, as it is the only statement that cannot be explained by lanthanoid contraction.

### Final Answer:
The fact that is not explainable by lanthanoid contraction is **Option D**: "The size of yttrium (Y) is less than that of lanthanum (La)."