Which of the following is not a consequence of the Lanthanoid contraction?

Options

A. 5d series elements have a higher `IE_(1)` than 3d or 4d series
B. Zr and Hf have a comparable size
C. Zr and Hf occurs together in the earth crust in their minerals
D. High density of the sixth period elements

Answer

To solve the question "Which of the following is not a consequence of the Lanthanoid contraction?", we need to analyze each option in relation to the concept of lanthanoid contraction.

### Step-by-Step Solution:

1. **Understanding Lanthanoid Contraction**:
- Lanthanoid contraction refers to the gradual decrease in the size of the lanthanide ions as we move from lanthanum (La) to lutetium (Lu) in the periodic table. This occurs due to the filling of the 4f subshell, which increases the effective nuclear charge without a corresponding increase in shielding effect.

2. **Analyzing the Options**:
- **Option A**: "5d series elements have a higher ionization enthalpy first value than 3d or 4d series."
- This is a consequence of lanthanoid contraction. As the size of the 5d elements decreases due to the contraction, the ionization energy increases.

- **Option B**: "Zirconium and hafnium have comparable size."
- This is also a consequence of lanthanoid contraction. Hafnium (Hf) is smaller than expected due to the contraction, making it comparable in size to zirconium (Zr).

- **Option C**: "Zirconium and hafnium together in the earth crust in their minerals."
- This is related to the similar properties and sizes of Zr and Hf due to lanthanoid contraction, thus they are often found together in nature.

- **Option D**: "High density of sixth period elements."
- This is **not** a direct consequence of lanthanoid contraction. Density is influenced by the atomic mass and volume of the elements, but not specifically by lanthanoid contraction.

3. **Conclusion**:
- The option that is **not a consequence of the lanthanoid contraction** is **Option D: High density of sixth period elements**.

### Final Answer:
**Option D: High density of sixth period elements** is not a consequence of the lanthanoid contraction.

---