The nuclei of which one of the following pairs of nuclei are isotons ?

Options

A. `._(34)Se^(74), ._(31)Ga^(71)`
B. `._(42)MO^(92), ._(40)Zr^(92)`
C. `._(38)Sr^(84), ._(38)Sr^(86)`
D. `._(20)Ca^(40), ._(16)S^(32)`

Answer

To determine which pair of nuclei are isotons, we need to find pairs of nuclei that have the same number of neutrons. The number of neutrons in a nucleus can be calculated using the formula:

\[ \text{Number of Neutrons} = A - Z \]

where \( A \) is the mass number (total number of protons and neutrons) and \( Z \) is the atomic number (number of protons).

Let's analyze each option step by step:

### Step 1: Analyze Option A (Selenium and Gallium)
- **Selenium (Se)**:
- Mass number (A) = 74
- Atomic number (Z) = 34
- Number of neutrons = \( 74 - 34 = 40 \)

- **Gallium (Ga)**:
- Mass number (A) = 71
- Atomic number (Z) = 31
- Number of neutrons = \( 71 - 31 = 40 \)

Both have 40 neutrons, so **Se and Ga are isotons**.

### Step 2: Analyze Option B (Molybdenum and Zirconium)
- **Molybdenum (Mo)**:
- Mass number (A) = 92
- Atomic number (Z) = 42
- Number of neutrons = \( 92 - 42 = 50 \)

- **Zirconium (Zr)**:
- Mass number (A) = 92
- Atomic number (Z) = 40
- Number of neutrons = \( 92 - 40 = 52 \)

Molybdenum has 50 neutrons and Zirconium has 52 neutrons, so **Mo and Zr are not isotons**.

### Step 3: Analyze Option C (Strontium and Isotope of Strontium)
- **Strontium (Sr)**:
- Mass number (A) = 84
- Atomic number (Z) = 38
- Number of neutrons = \( 84 - 38 = 46 \)

- **Isotope of Strontium (Sr)**:
- Mass number (A) = 86
- Atomic number (Z) = 38
- Number of neutrons = \( 86 - 38 = 48 \)

Strontium has 46 neutrons and its isotope has 48 neutrons, so **these are not isotons**.

### Step 4: Analyze Option D (Calcium and Sulfur)
- **Calcium (Ca)**:
- Mass number (A) = 40
- Atomic number (Z) = 20
- Number of neutrons = \( 40 - 20 = 20 \)

- **Sulfur (S)**:
- Mass number (A) = 32
- Atomic number (Z) = 16
- Number of neutrons = \( 32 - 16 = 16 \)

Calcium has 20 neutrons and Sulfur has 16 neutrons, so **Ca and S are not isotons**.

### Conclusion
The only pair of nuclei that are isotons is **Option A: Selenium (Se) and Gallium (Ga)**, both having 40 neutrons.

### Final Answer
**The nuclei of which one of the following pairs of nuclei are isotons?**
- **Option A: Selenium (Se) and Gallium (Ga)**