Which of the following is/are Dobereiner's triad?
(a) Li, Na, K
(b) Cu, Ag, Zn
(c) Fe, Co, Ni
(d) Ca, Sr, Ba Correct answer is

Options

A. (a) & (b)
B. (a) & (d)
C. (b) & (c)
D. (a), (b), (c) & (d)

Answer

To determine which of the given sets of elements are Dobereiner's triads, we need to apply the concept introduced by Johann Wolfgang Döbereiner. According to his theory, a 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 Option (a) Li, Na, K
- Atomic masses:
- Lithium (Li) = 7
- Sodium (Na) = 23
- Potassium (K) = 39
- Calculate the average of Li and K:
\[ \text{Average} = \frac{7 + 39}{2} = \frac{46}{2} = 23 \]
- Since the average atomic mass (23) is equal to the atomic mass of Sodium (Na), this set forms a Dobereiner's triad.

### Step 2: Analyze Option (b) Cu, Ag, Zn
- Atomic masses:
- Copper (Cu) = 63.5
- Silver (Ag) = 107.9
- Zinc (Zn) = 65.4
- Calculate the average of Cu and Zn:
\[ \text{Average} = \frac{63.5 + 65.4}{2} = \frac{128.9}{2} = 64.45 \]
- The average (64.45) is not close to the atomic mass of Silver (Ag = 107.9), so this set does not form a Dobereiner's triad.

### Step 3: Analyze Option (c) Fe, Co, Ni
- Atomic masses:
- Iron (Fe) = 55.8
- Cobalt (Co) = 58.9
- Nickel (Ni) = 58.7
- Calculate the average of Fe and Ni:
\[ \text{Average} = \frac{55.8 + 58.7}{2} = \frac{114.5}{2} = 57.25 \]
- The average (57.25) is not close to the atomic mass of Cobalt (Co = 58.9), so this set does not form a Dobereiner's triad.

### Step 4: Analyze Option (d) Ca, Sr, Ba
- Atomic masses:
- Calcium (Ca) = 40
- Strontium (Sr) = 87.6
- Barium (Ba) = 137
- Calculate the average of Ca and Ba:
\[ \text{Average} = \frac{40 + 137}{2} = \frac{177}{2} = 88.5 \]
- The average (88.5) is close to the atomic mass of Strontium (Sr = 87.6), so this set forms a Dobereiner's triad.

### Conclusion
The sets that form Dobereiner's triads are:
- (a) Li, Na, K
- (d) Ca, Sr, Ba

Thus, the correct answer is **(a) and (d)**.