Which of the following is/are Doeberiners triad
(i) P, As, Sb `" "` (ii) Cu, Ag, Au
(iii) Fe, Co, Ni `" " ` (iv) S, Se, Te
Correct answer is

Options

A. (i) and (ii)
B. (ii) and (iii)
C. (i) and (iv)
D. All

Answer

To determine which of the given sets of elements are Doeberiner's triads, we need to apply the principle of Doeberiner's triads. According to this principle, if three elements can be grouped together, the atomic weight of the middle element should be 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 triad (P, As, Sb)
1. **Elements**: Phosphorus (P), Arsenic (As), Antimony (Sb)
2. **Atomic weights**:
- P = 30.97
- As = 74.92
- Sb = 121.76
3. **Calculate the average of P and Sb**:
\[ \text{Average} = \frac{(30.97 + 121.76)}{2} = \frac{152.73}{2} = 76.365 \]
4. **Comparison**:
- The average (76.365) is approximately equal to the atomic weight of As (74.92).
5. **Conclusion**: This is a Doeberiner's triad.

### Step 2: Analyze the second triad (Cu, Ag, Au)
1. **Elements**: Copper (Cu), Silver (Ag), Gold (Au)
2. **Atomic weights**:
- Cu = 63.56
- Ag = 107.87
- Au = 196.97
3. **Calculate the average of Cu and Au**:
\[ \text{Average} = \frac{(63.56 + 196.97)}{2} = \frac{260.53}{2} = 130.265 \]
4. **Comparison**:
- The average (130.265) is not equal to the atomic weight of Ag (107.87).
5. **Conclusion**: This is not a Doeberiner's triad.

### Step 3: Analyze the third triad (Fe, Co, Ni)
1. **Elements**: Iron (Fe), Cobalt (Co), Nickel (Ni)
2. **Atomic weights**:
- Fe = 55.84
- Co = 58.93
- Ni = 58.69
3. **Calculate the average of Fe and Ni**:
\[ \text{Average} = \frac{(55.84 + 58.69)}{2} = \frac{114.53}{2} = 57.265 \]
4. **Comparison**:
- The average (57.265) is not equal to the atomic weight of Co (58.93).
5. **Conclusion**: This is not a Doeberiner's triad.

### Step 4: Analyze the fourth triad (S, Se, Te)
1. **Elements**: Sulfur (S), Selenium (Se), Tellurium (Te)
2. **Atomic weights**:
- S = 32.06
- Se = 78.96
- Te = 127.60
3. **Calculate the average of S and Te**:
\[ \text{Average} = \frac{(32.06 + 127.60)}{2} = \frac{159.66}{2} = 79.83 \]
4. **Comparison**:
- The average (79.83) is approximately equal to the atomic weight of Se (78.96).
5. **Conclusion**: This is a Doeberiner's triad.

### Final Answer:
The correct Doeberiner's triads from the options given are:
- (i) P, As, Sb
- (iv) S, Se, Te