Which of these does not represent Dobereiner's triad?

Options

A. Li, Na, K
B. CI, Br, I
C. Be, Mg, Ca
D. NP As

Explanation

To determine which of the given sets of elements does not represent Dobereiner's triad, we will analyze the atomic masses of the elements in each triad and check if the average of the atomic masses of the first and third elements is approximately equal to the atomic mass of the second element.

### Step-by-Step Solution:

1. **Identify the Elements and Their Atomic Masses:**
- **Triad A:** Lithium (Li), Sodium (Na), Potassium (K)
- Atomic Mass of Li = 7
- Atomic Mass of Na = 23
- Atomic Mass of K = 39

- **Triad B:** Chlorine (Cl), Bromine (Br), Iodine (I)
- Atomic Mass of Cl = 35.5
- Atomic Mass of Br = 79.9
- Atomic Mass of I = 126.9

- **Triad C:** Beryllium (Be), Magnesium (Mg), Calcium (Ca)
- Atomic Mass of Be = 9
- Atomic Mass of Mg = 24
- Atomic Mass of Ca = 40

- **Triad D:** Nitrogen (N), Phosphorus (P), Arsenic (As)
- Atomic Mass of N = 14
- Atomic Mass of P = 30.9
- Atomic Mass of As = 74.9

2. **Calculate the Average for Each Triad:**
- **For Triad A (Li, Na, K):**
- Average = (Atomic Mass of Li + Atomic Mass of K) / 2
- Average = (7 + 39) / 2 = 46 / 2 = 23
- This is equal to the atomic mass of Na (23). So, this is a Dobereiner's triad.

- **For Triad B (Cl, Br, I):**
- Average = (Atomic Mass of Cl + Atomic Mass of I) / 2
- Average = (35.5 + 126.9) / 2 = 162.4 / 2 = 81.2
- This is approximately equal to the atomic mass of Br (79.9). So, this is a Dobereiner's triad.

- **For Triad C (Be, Mg, Ca):**
- Average = (Atomic Mass of Be + Atomic Mass of Ca) / 2
- Average = (9 + 40) / 2 = 49 / 2 = 24.5
- This is approximately equal to the atomic mass of Mg (24). So, this is a Dobereiner's triad.

- **For Triad D (N, P, As):**
- Average = (Atomic Mass of N + Atomic Mass of As) / 2
- Average = (14 + 74.9) / 2 = 88.9 / 2 = 44.45
- This is not approximately equal to the atomic mass of P (30.9). So, this is NOT a Dobereiner's triad.

3. **Conclusion:**
- The set that does not represent Dobereiner's triad is **Triad D: Nitrogen, Phosphorus, and Arsenic**.

### Final Answer:
**Triad D (Nitrogen, Phosphorus, and Arsenic) does not represent Dobereiner's triad.**

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