Which of the following is not a Dobereiner triad?

Options

A. CI, Br, I
B. Ca, Sr, Ba
C. Li, Na, K
D. Fe, Co, Ni

Explanation

To determine which of the following is not a Dobereiner triad, we need to analyze each group of three elements based on the criteria of Dobereiner's triads. A Dobereiner triad consists of three elements where the atomic weight of the middle element is approximately equal to the average of the atomic weights of the other two elements.

Let's analyze each option step by step:

### Step 1: Analyze the first option - Chlorine, Bromine, and Iodine
1. **Atomic weights**:
- Chlorine (Cl): 35.5
- Bromine (Br): 80
- Iodine (I): 126.9
2. **Calculate the average of Cl and I**:
\[ \text{Average} = \frac{(35.5 + 126.9)}{2} = \frac{162.4}{2} = 81.2 \]
3. **Comparison**:
- Average (81.2) is approximately equal to Bromine's atomic weight (80).
4. **Conclusion**: This is a Dobereiner triad.

### Step 2: Analyze the second option - Calcium, Strontium, and Barium
1. **Atomic weights**:
- Calcium (Ca): 40
- Strontium (Sr): 87.7
- Barium (Ba): 137.3
2. **Calculate the average of Ca and Ba**:
\[ \text{Average} = \frac{(40 + 137.3)}{2} = \frac{177.3}{2} = 88.65 \]
3. **Comparison**:
- Average (88.65) is approximately equal to Strontium's atomic weight (87.7).
4. **Conclusion**: This is a Dobereiner triad.

### Step 3: Analyze the third option - Lithium, Sodium, and Potassium
1. **Atomic weights**:
- Lithium (Li): 7
- Sodium (Na): 23
- Potassium (K): 39
2. **Calculate the average of Li and K**:
\[ \text{Average} = \frac{(7 + 39)}{2} = \frac{46}{2} = 23 \]
3. **Comparison**:
- Average (23) is equal to Sodium's atomic weight (23).
4. **Conclusion**: This is a Dobereiner triad.

### Step 4: Analyze the fourth option - Iron, Cobalt, and Nickel
1. **Atomic weights**:
- Iron (Fe): 55.8
- Cobalt (Co): 58.9
- Nickel (Ni): 58.7
2. **Calculate the average of Fe and Ni**:
\[ \text{Average} = \frac{(55.8 + 58.7)}{2} = \frac{114.5}{2} = 57.25 \]
3. **Comparison**:
- Average (57.25) is not equal to Cobalt's atomic weight (58.9).
4. **Conclusion**: This is not a Dobereiner triad.

### Final Answer:
The element group that is not a Dobereiner triad is **Iron, Cobalt, and Nickel**.

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