Question

Zr (Z=40) and Hf (Z=72) have similar atomic and ionic radii because of

• A. having similar chemical properties
• B. belonging to same group
• C. diagonal relationship
• D. lanthanoid contraction

Answer 1

The Correct Option is D

To understand why Zr (Zirconium) and Hf (Hafnium) have similar atomic and ionic radii, we need to delve into the concept of "lanthanoid contraction".

1. Understanding Lanthanoid Contraction:
   • The lanthanoid contraction refers to the steady decrease in the size of the atoms and ions of the lanthanoids with increasing atomic number, from Lanthanum (La) to Lutetium (Lu).
   • This is because as we move across the lanthanoid series, new electrons are added to the f-orbital, and the increased effective nuclear charge causes a greater attraction between the electrons and the nucleus, resulting in a smaller atomic and ionic size.
2. Impact on Zirconium (Zr) and Hafnium (Hf):
   • Zirconium and Hafnium are placed in the same group (Group 4) of the periodic table, thus having similar electronic configuration in their outermost shells.
   • Despite the difference in atomic numbers (Zr = 40 and Hf = 72), their atomic and ionic radii are strikingly similar. This anomaly arises because of the lanthanoid contraction.
   • The addition of 4f electrons in the lanthanoid series, which occurs just before Hafnium, causes a contraction that offsets the increase in size that would ordinarily be expected from the additional electron shells.
3. Conclusion:
   • Therefore, the similar atomic and ionic radii of Zr and Hf can be attributed directly to the lanthanoid contraction, which is highlighted in the correct option.

Hence, the correct answer is lanthanoid contraction.