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

Options

A. Both belong to same group
B. Diagonal relationship
C. Lanthanoid contraction
D. Having similar chemical properties

Explanation

To understand why zirconium (Zr) and hafnium (Hf) have similar atomic and ionic radii, we can break down the explanation into several steps:

### Step 1: Identify the Elements
Zirconium (Zr) has an atomic number of 40, while hafnium (Hf) has an atomic number of 72. Both elements are located in the d-block of the periodic table.

### Step 2: Understand Atomic and Ionic Radii
Atomic radius refers to the size of an atom, while ionic radius refers to the size of an ion. These sizes can be influenced by various factors, including the number of electron shells and the effective nuclear charge experienced by the electrons.

### Step 3: Introduce Lanthanide Contraction
The key concept that explains the similarity in the atomic and ionic radii of Zr and Hf is the phenomenon known as "lanthanide contraction." This contraction occurs due to the filling of the 4f orbitals in the lanthanide series.

### Step 4: Explain Lanthanide Contraction
As we move across the lanthanide series from cerium (Ce) to lutetium (Lu), the 4f electrons do not shield the nuclear charge effectively. This poor shielding leads to a greater effective nuclear charge felt by the outer electrons, causing a decrease in size (contraction) of the atomic and ionic radii.

### Step 5: Relate to Zr and Hf
Zirconium (4d series) and hafnium (5d series) are positioned in such a way that the effects of lanthanide contraction make their atomic and ionic radii similar. The 5d electrons in hafnium experience a similar effective nuclear charge due to the presence of the 4f electrons in the lanthanides, leading to a contraction that makes hafnium's size comparable to that of zirconium.

### Step 6: Conclusion
Thus, the similarity in atomic and ionic radii of zirconium and hafnium can be attributed to the lanthanide contraction, which affects the size of the 5d elements in a way that is comparable to the 4d elements.

### Final Answer
The similar atomic and ionic radii of Zr and Hf are due to **lanthanide contraction**.

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