The mass number of two atoms X and Y having same atomic number are 208 and 210 respectively. The number of neutrons in the nucleus of X is 126. The number of neutrons in the nucleus of Y is :

Options

A. 128
B. 124
C. 120
D. 130

Answer

To find the number of neutrons in the nucleus of atom Y, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the relationship between mass number, protons, and neutrons**:
The mass number (A) of an atom is the sum of the number of protons (P) and the number of neutrons (N). This can be expressed by the formula:
\[ A = P + N \]

2. **Identify the given data for atom X**:
- Mass number of atom X (A_X) = 208
- Number of neutrons in atom X (N_X) = 126

3. **Calculate the number of protons in atom X**:
Using the formula \( A = P + N \):
\[ 208 = P_X + 126 \]
Rearranging the equation to find \( P_X \):
\[ P_X = 208 - 126 = 82 \]
So, the number of protons in atom X is 82.

4. **Identify the given data for atom Y**:
- Mass number of atom Y (A_Y) = 210
- The number of protons in atom Y (P_Y) is the same as in atom X, which is 82.

5. **Calculate the number of neutrons in atom Y**:
Again using the formula \( A = P + N \):
\[ 210 = P_Y + N_Y \]
Substituting \( P_Y = 82 \):
\[ 210 = 82 + N_Y \]
Rearranging to find \( N_Y \):
\[ N_Y = 210 - 82 = 128 \]

6. **Conclusion**:
The number of neutrons in the nucleus of atom Y is 128.

### Final Answer:
The number of neutrons in the nucleus of Y is **128**.

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