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),(c) & (d)

Answer

To determine which of the given sets of elements are Dobereiner's triads, we need to apply the concept of Dobereiner's triads. A triad consists of three elements where the atomic mass of the middle element is approximately the average of the atomic masses of the other two elements.

Let's analyze each option step by step:

### Step 1: Analyze Option (a) Li, Na, K
- Atomic numbers:
- Lithium (Li): 3
- Sodium (Na): 11
- Potassium (K): 19
- Check if the average of Li and K equals Na:
\[ \text{Average} = \frac{3 + 19}{2} = \frac{22}{2} = 11 \]
- Since the average equals the atomic number of Sodium, this is a valid triad.

### Step 2: Analyze Option (b) Cu, Ag, Zn
- Atomic numbers:
- Copper (Cu): 29
- Silver (Ag): 47
- Zinc (Zn): 30
- Check if the average of Cu and Zn equals Ag:
\[ \text{Average} = \frac{29 + 30}{2} = \frac{59}{2} = 29.5 \]
- Since 29.5 does not equal the atomic number of Silver, this is not a valid triad.

### Step 3: Analyze Option (c) Fe, Co, Ni
- Atomic numbers:
- Iron (Fe): 26
- Cobalt (Co): 27
- Nickel (Ni): 28
- Check if the average of Fe and Ni equals Co:
\[ \text{Average} = \frac{26 + 28}{2} = \frac{54}{2} = 27 \]
- Since the average equals the atomic number of Cobalt, this is a valid triad.

### Step 4: Analyze Option (d) Ca, Sr, Ba
- Atomic numbers:
- Calcium (Ca): 20
- Strontium (Sr): 38
- Barium (Ba): 56
- Check if the average of Ca and Ba equals Sr:
\[ \text{Average} = \frac{20 + 56}{2} = \frac{76}{2} = 38 \]
- Since the average equals the atomic number of Strontium, this is a valid triad.

### Conclusion
The valid Dobereiner's triads from the given options are:
- (a) Li, Na, K
- (c) Fe, Co, Ni
- (d) Ca, Sr, Ba

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