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

Options

A. diagonal relationship
B. lanthanoid contraction
C. actinoid contraction
D. belonging to the same group.

Answer

To answer the question of why zirconium (Z=40) and hafnium (Z=72) have similar atomic and ionic radii, we can break down the explanation step-by-step:

### Step 1: Understanding the Elements
- **Zirconium (Z=40)** and **Hafnium (Z=72)** are both transition metals located in the d-block of the periodic table.
- They are in the same group (Group 4) of the periodic table, which typically suggests that they would have similar properties, including atomic and ionic radii.

### Step 2: Lanthanide Contraction
- The key reason for the similarity in atomic and ionic radii between Zr and Hf is the phenomenon known as **lanthanide contraction**.
- Lanthanide contraction refers to the gradual decrease in the size of the lanthanide ions (elements with atomic numbers 57 to 71) as the atomic number increases.

### Step 3: Poor Shielding Effect
- In the lanthanide series, the 4f electrons are added. These electrons have a poor shielding effect on the outer electrons (in this case, the 5s and 5p electrons).
- This poor shielding means that the effective nuclear charge experienced by the outer electrons increases as you move from lanthanum (La, Z=57) to lutetium (Lu, Z=71).

### Step 4: Impact on Atomic Size
- As the effective nuclear charge increases due to the poor shielding by the 4f electrons, the outer electrons are pulled closer to the nucleus.
- This results in a decrease in atomic size, which is observed as a contraction in the size of the atoms.

### Step 5: Conclusion
- Since hafnium (Hf) follows zirconium (Zr) in the periodic table and is influenced by the lanthanide contraction, both Zr and Hf exhibit similar atomic and ionic radii despite the significant difference in their atomic numbers.

### Final Answer
Zirconium (Z=40) and Hafnium (Z=72) have similar atomic and ionic radii because of **lanthanide contraction**.

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