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 solve the problem step by step, we need to find the number of neutrons in the nucleus of atom Y, given the mass numbers and the number of neutrons in atom X.

### Step 1: Understand the relationship between mass number, atomic number, protons, and neutrons.
The mass number (A) of an atom is the sum of the number of protons (Z) and the number of neutrons (N) in its nucleus. This can be expressed as:
\[ A = Z + N \]

### Step 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

### Step 3: Calculate the number of protons in atom X.
Using the formula:
\[ A_X = Z + N_X \]
Substituting the known values:
\[ 208 = Z + 126 \]
To find Z (the number of protons):
\[ Z = 208 - 126 \]
\[ Z = 82 \]

### Step 4: Identify the given data for atom Y.
- Mass number of atom Y (A_Y) = 210
- The number of protons in atom Y (Z) is the same as in atom X, which we found to be 82.

### Step 5: Calculate the number of neutrons in atom Y.
Using the same formula for atom Y:
\[ A_Y = Z + N_Y \]
Substituting the known values:
\[ 210 = 82 + N_Y \]
To find N_Y (the number of neutrons in atom Y):
\[ N_Y = 210 - 82 \]
\[ N_Y = 128 \]

### Conclusion
The number of neutrons in the nucleus of atom Y is 128.

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