Which of the following set have almost similar atomic radii ?

Options

A. `Hf, Zr`
B. `K, Rb`
C. `Be, Al`
D. `B, Al`

Explanation

To determine which of the following sets of elements have almost similar atomic radii, we will analyze each set one by one.

### Step 1: Analyze the first set - Hafnium (Hf) and Zirconium (Zr)
- Hafnium (Hf) is a 5d transition metal, while Zirconium (Zr) is a 4d transition metal.
- The atomic radii of these two elements are similar due to the phenomenon known as **lanthanide contraction**.
- The presence of 14 lanthanide elements between Zr and Hf results in a significant increase in the effective nuclear charge experienced by the outer electrons, leading to a decrease in atomic size.
- **Conclusion**: Hafnium and Zirconium have almost similar atomic radii.

### Step 2: Analyze the second set - Potassium (K) and Rubidium (Rb)
- Potassium (K) and Rubidium (Rb) are both alkali metals located in Group 1 of the periodic table.
- While they are in the same group, Rb is below K in the periodic table, which means Rb has a larger atomic radius than K due to the addition of a new electron shell.
- **Conclusion**: Potassium and Rubidium do not have similar atomic radii.

### Step 3: Analyze the third set - Beryllium (Be) and Aluminum (Al)
- Beryllium (Be) is in Group 2 and Aluminum (Al) is in Group 13 of the periodic table.
- Beryllium and Aluminum exhibit a **diagonal relationship** in the periodic table, which means their atomic sizes are comparable.
- This relationship arises because of the similar charge-to-radius ratio, leading to similar atomic sizes.
- **Conclusion**: Beryllium and Aluminum have almost similar atomic radii.

### Step 4: Analyze the fourth set - Boron (B) and Aluminum (Al)
- Boron (B) is in Group 13 and Aluminum (Al) is also in Group 13 but in a different period.
- However, Boron has a significantly smaller atomic radius than Aluminum due to its position in the periodic table.
- **Conclusion**: Boron and Aluminum do not have similar atomic radii.

### Final Answer
Based on the analysis:
- The sets with almost similar atomic radii are:
1. Hafnium (Hf) and Zirconium (Zr)
2. Beryllium (Be) and Aluminum (Al)

Thus, the correct options are **1 and 3**.

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