Which of the following pairs constitute isotones ?

Options

A. `78overset195Pt` and `76overset190Os`
B. `33overset77As` and `34overset78Se`
C. `47overset108Ag` and `48overset112Cd`
D. `72overset178Hf` and `56overset137Ba`

Explanation

To determine which pairs constitute isotones, we need to identify pairs of atoms that have the same number of neutrons. The number of neutrons can be calculated using the formula:

\[ \text{Number of neutrons} (n) = A - Z \]

where \( A \) is the atomic mass (mass number) and \( Z \) is the atomic number.

Let's analyze the given pairs step by step:

### Step 1: Analyze the first pair - Platinum (Pt) and Osmium (Os)
- **Platinum (Pt)**:
- Atomic number (Z) = 78
- Atomic mass (A) = 195
- Number of neutrons (n) = A - Z = 195 - 78 = 117

- **Osmium (Os)**:
- Atomic number (Z) = 76
- Atomic mass (A) = 190
- Number of neutrons (n) = A - Z = 190 - 76 = 114

**Conclusion**: The number of neutrons is different (117 ≠ 114), so this pair does not constitute isotones.

### Step 2: Analyze the second pair - Arsenic (As) and Selenium (Se)
- **Arsenic (As)**:
- Atomic number (Z) = 33
- Atomic mass (A) = 77
- Number of neutrons (n) = A - Z = 77 - 33 = 44

- **Selenium (Se)**:
- Atomic number (Z) = 34
- Atomic mass (A) = 78
- Number of neutrons (n) = A - Z = 78 - 34 = 44

**Conclusion**: The number of neutrons is the same (44 = 44), so this pair constitutes isotones.

### Step 3: Analyze the third pair - Silver (Ag) and Cadmium (Cd)
- **Silver (Ag)**:
- Atomic number (Z) = 47
- Atomic mass (A) = 108
- Number of neutrons (n) = A - Z = 108 - 47 = 61

- **Cadmium (Cd)**:
- Atomic number (Z) = 48
- Atomic mass (A) = 112
- Number of neutrons (n) = A - Z = 112 - 48 = 64

**Conclusion**: The number of neutrons is different (61 ≠ 64), so this pair does not constitute isotones.

### Step 4: Analyze the fourth pair - Hafnium (Hf) and Barium (Ba)
- **Hafnium (Hf)**:
- Atomic number (Z) = 72
- Atomic mass (A) = 178
- Number of neutrons (n) = A - Z = 178 - 72 = 106

- **Barium (Ba)**:
- Atomic number (Z) = 56
- Atomic mass (A) = 137
- Number of neutrons (n) = A - Z = 137 - 56 = 81

**Conclusion**: The number of neutrons is different (106 ≠ 81), so this pair does not constitute isotones.

### Final Conclusion
The only pair that constitutes isotones is Arsenic (As) and Selenium (Se).