Which pair of elements is chemically most similar?

Options

A. Na, Al
B. Cu, S
C. Ti, Zr
D. Zr, Hf

Explanation

To determine which pair of elements is chemically most similar, we need to analyze the given options based on their positions in the periodic table and their chemical properties.

### Step-by-Step Solution:

1. **Identify the Elements in Each Option:**
- Option A: Sodium (Na) and Aluminum (Al)
- Option B: Copper (Cu) and Sulfur (S)
- Option C: Zirconium (Zr) and Hafnium (Hf)

2. **Classify the Elements:**
- Sodium (Na) is an S-block element (Group 1).
- Aluminum (Al) is also an S-block element (Group 13).
- Copper (Cu) is a D-block element (Transition metal).
- Sulfur (S) is a P-block element (Group 16).
- Zirconium (Zr) is a D-block element (Group 4).
- Hafnium (Hf) is also a D-block element (Group 4).

3. **Analyze Chemical Similarity:**
- **Sodium and Aluminum:** These elements belong to different groups (Group 1 and Group 13), and their chemical properties differ significantly.
- **Copper and Sulfur:** These elements belong to different blocks (D-block and P-block) and have different chemical properties.
- **Zirconium and Hafnium:** These elements are in the same group (Group 4) and are known to exhibit similar chemical properties. Their similarity is further enhanced by the phenomenon of lanthanide contraction, which makes their sizes comparable and leads to similar reactivity.

4. **Conclusion:**
- The pair of elements that is chemically most similar is **Zirconium (Zr) and Hafnium (Hf)** due to their similar properties resulting from their placement in the same group and the effects of lanthanide contraction.

### Final Answer:
The pair of elements that is chemically most similar is **Zirconium (Zr) and Hafnium (Hf)**.

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