Dobereiner triad is

Options

A. Na, K, Rb
B. Mg, S, As
C. Cl, Br, I
D. P, S, As

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Dobereiner's Triads**:
Dobereiner's triads are groups of three elements where the atomic mass of the middle element is approximately the average of the atomic masses of the other two elements. This concept was one of the early attempts to classify elements based on their properties.

2. **Analyzing the Options**:
We need to evaluate the given options to see which one fits the criteria of Dobereiner's triads.

3. **Option A - Sodium, Potassium, and Rubidium**:
- Atomic mass of Sodium (Na) = 23
- Atomic mass of Potassium (K) = 39
- Atomic mass of Rubidium (Rb) = 85.4
- Calculate the average of Sodium and Rubidium:
\[ \text{Average} = \frac{23 + 85.4}{2} = \frac{108.4}{2} = 54.2 \]
- Since the atomic mass of Potassium (39) is not equal to 54.2, this triad does not satisfy Dobereiner's rule.

4. **Option B - Magnesium, Sulfur, and Arsenic**:
- Atomic mass of Magnesium (Mg) = 24
- Atomic mass of Sulfur (S) = 32
- Atomic mass of Arsenic (As) = 74.9
- Calculate the average of Magnesium and Arsenic:
\[ \text{Average} = \frac{24 + 74.9}{2} = \frac{98.9}{2} = 49.45 \]
- Since the atomic mass of Sulfur (32) is not equal to 49.45, this triad also does not satisfy Dobereiner's rule.

5. **Option C - Chlorine, Bromine, and Iodine**:
- Atomic mass of Chlorine (Cl) = 35.5
- Atomic mass of Bromine (Br) = 79.9
- Atomic mass of Iodine (I) = 127
- Calculate the average of Chlorine and Iodine:
\[ \text{Average} = \frac{35.5 + 127}{2} = \frac{162.5}{2} = 81.25 \]
- Since the atomic mass of Bromine (79.9) is approximately equal to 81.25, this triad satisfies Dobereiner's rule.

6. **Conclusion**:
The correct option that represents Dobereiner's triad is **Option C: Chlorine, Bromine, and Iodine**.

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