Example of Dobereiner's triad is

Options

A. Li, Al, Ca
B. Li, Na, K
C. Li, K, Na
D. K, Al, Ca

Explanation

To find an example of Dobereiner's triad, we need to understand the concept of triads as proposed by Johann Wolfgang Döbereiner. He grouped elements into triads based on their similar properties and observed that the atomic mass of the middle element in each triad was approximately equal to the average of the atomic masses of the other two elements.

### Step-by-Step Solution:

1. **Understanding Dobereiner's Triad**:
- A triad consists of three elements (X, Y, Z) where Y is the middle element.
- The atomic mass of Y should be approximately equal to the average of the atomic masses of X and Z.

2. **Evaluating the Options**:
- We will evaluate the given options to find a valid triad.

3. **Option A: Lithium (Li), Aluminium (Al), Calcium (Ca)**:
- Atomic masses: Li = 7, Al = 27, Ca = 40
- Average of Li and Ca: (7 + 40) / 2 = 47 / 2 = 23.5
- Since 23.5 is not equal to 27 (mass of Al), this is not a valid triad.

4. **Option B: Lithium (Li), Sodium (Na), Potassium (K)**:
- Atomic masses: Li = 7, Na = 23, K = 39
- Average of Li and K: (7 + 39) / 2 = 46 / 2 = 23
- Since 23 is equal to the mass of Na, this is a valid triad.

5. **Option C: Lithium (Li), Potassium (K), Sodium (Na)**:
- Atomic masses: Li = 7, K = 39, Na = 23
- Average of Li and Na: (7 + 23) / 2 = 30 / 2 = 15
- Since 15 is not equal to 39 (mass of K), this is not a valid triad.

6. **Option D: Potassium (K), Aluminium (Al), Calcium (Ca)**:
- Atomic masses: K = 39, Al = 27, Ca = 40
- Average of Al and Ca: (27 + 40) / 2 = 67 / 2 = 33.5
- Since 33.5 is not equal to 39 (mass of K), this is not a valid triad.

7. **Conclusion**:
- The only valid example of Dobereiner's triad from the given options is **Option B: Lithium, Sodium, and Potassium**.

### Final Answer:
**Example of Dobereiner's triad is: Lithium (Li), Sodium (Na), and Potassium (K).**