The electronic configuration of an ion `X^(3-)` is 2, 8, 8. If the nucleon number of the parent atom X 'is 31, then the number of neutrons present in 'X' is ______.

Options

A. 16
B. 19
C. 20
D. 15

Answer

To solve the problem, we need to determine the number of neutrons in the parent atom \( X \) given the following information:

1. The electronic configuration of the ion \( X^{3-} \) is \( 2, 8, 8 \).
2. The nucleon number (mass number) of the parent atom \( X \) is 31.

### Step 1: Determine the Atomic Number of Ion \( X^{3-} \)

The electronic configuration \( 2, 8, 8 \) indicates that the ion has:
- 2 electrons in the first shell,
- 8 electrons in the second shell,
- 8 electrons in the third shell.

To find the atomic number of the ion \( X^{3-} \), we add the total number of electrons:
\[ 2 + 8 + 8 = 18 \]
Since \( X^{3-} \) has gained 3 electrons compared to the neutral atom \( X \), we can find the atomic number of the neutral atom \( X \):
\[ \text{Atomic Number of } X = 18 - 3 = 15 \]

### Step 2: Calculate the Number of Neutrons in Atom \( X \)

The number of neutrons in an atom can be calculated using the formula:
\[ \text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number} \]
We know that:
- Mass Number (nucleon number) of \( X \) = 31
- Atomic Number of \( X \) = 15

Now, substituting these values into the formula:
\[ \text{Number of Neutrons} = 31 - 15 = 16 \]

### Final Answer

The number of neutrons present in atom \( X \) is **16**.

---