`overset19 8X` atom is isotonic to `oversety 7Y` atom. The value of y is :

Options

A. 18
B. 11
C. 19
D. 17

Explanation

To solve the problem, we need to find the value of \( y \) for the atom \( \overset{y}{7Y} \) that is isotonic to the atom \( \overset{19}{8X} \). Isotonic atoms have the same number of neutrons.

### Step-by-Step Solution:

1. **Identify the Atomic Mass and Atomic Number of Atom X**:
- Atom \( X \) has an atomic mass \( A_X = 19 \) and atomic number \( Z_X = 8 \).

2. **Calculate the Number of Neutrons in Atom X**:
- The number of neutrons \( N_X \) can be calculated using the formula:
\[ N_X = A_X - Z_X \]
- Substitute the values:
\[ N_X = 19 - 8 = 11 \]

3. **Identify the Atomic Mass and Atomic Number of Atom Y**:
- Atom \( Y \) has an atomic mass \( A_Y = y \) and atomic number \( Z_Y = 7 \).

4. **Set Up the Equation for Atom Y**:
- Since atoms \( X \) and \( Y \) are isotonic, they have the same number of neutrons:
\[ N_Y = N_X \]
- The number of neutrons in atom \( Y \) can be calculated as:
\[ N_Y = A_Y - Z_Y = y - 7 \]
- Therefore, we can set up the equation:
\[ y - 7 = 11 \]

5. **Solve for \( y \)**:
- Rearranging the equation gives:
\[ y = 11 + 7 = 18 \]

6. **Conclusion**:
- The value of \( y \) is \( 18 \).

### Final Answer:
The value of \( y \) is \( 18 \).

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