Which of the following is not a Doeberiner triad

Options

A. Li, Na, K
B. Mg, Ca, Sr
C. Cl, Br, I
D. S, Se, Te

Answer

To determine which of the following is not a Dobereiner triad, we need to understand what a Dobereiner triad is. A Dobereiner triad consists of three elements that can be arranged in increasing order of their atomic masses, where the atomic mass of the middle element is approximately equal to the arithmetic mean of the atomic masses of the other two elements.

### Step-by-step Solution:

1. **Identify the Elements and Their Atomic Masses**:
- We need to check the given options and their respective atomic masses.
- For example:
- Lithium (Li): 6.9
- Sodium (Na): 23
- Potassium (K): 39.1
- Other options may include:
- Magnesium (Mg): 24.3
- Calcium (Ca): 40
- Strontium (Sr): 87.6
- Chlorine (Cl): 35.5
- Bromine (Br): 81.25
- Iodine (I): 127
- Sulfur (S): 32
- Selenium (Se): 79
- Tellurium (Te): 127.6

2. **Calculate the Arithmetic Mean**:
- For each triad, calculate the arithmetic mean of the first and third elements and compare it to the atomic mass of the middle element.
- For example:
- For Li, Na, K:
\[ \text{Mean} = \frac{6.9 + 39.1}{2} = \frac{46}{2} = 23 \quad (\text{Matches Na}) \]
- For Mg, Ca, Sr:
\[ \text{Mean} = \frac{24.3 + 87.6}{2} = \frac{111.9}{2} = 55.95 \quad (\text{Does not match Ca}) \]
- For Cl, Br, I:
\[ \text{Mean} = \frac{35.5 + 127}{2} = \frac{162.5}{2} = 81.25 \quad (\text{Matches Br}) \]
- For S, Se, Te:
\[ \text{Mean} = \frac{32 + 127.6}{2} = \frac{159.6}{2} = 79.8 \quad (\text{Approximately matches Se}) \]

3. **Identify the Non-Triad**:
- From the calculations:
- The first triad (Li, Na, K) is valid.
- The second triad (Mg, Ca, Sr) is not valid since the mean does not equal the atomic mass of Ca.
- The third triad (Cl, Br, I) is valid.
- The fourth triad (S, Se, Te) is valid.
- Therefore, the option that is not a Dobereiner triad is the second one: **Mg, Ca, Sr**.

### Final Answer:
The element set that is not a Dobereiner triad is **option 2: Mg, Ca, Sr**.