Which of the following ions is not isoelectronic with `O^(2-)`

Options

A. `N^(3-)`
B. `F^(-)`
C. `Ti^(+)`
D. `Na^(+)`

Answer

To determine which of the given ions is not isoelectronic with `O^(2-)`, we need to understand the concept of isoelectronic species. Isoelectronic species are atoms or ions that have the same number of electrons.

### Step-by-Step Solution:

1. **Identify the number of electrons in `O^(2-)`:**
- The atomic number of oxygen (O) is 8, which means it has 8 electrons.
- The `O^(2-)` ion has gained 2 additional electrons, giving it a total of:
\[ 8 + 2 = 10 \text{ electrons} \]

2. **Check each option to see if they have 10 electrons:**

- **Option 1: `N^(3-)`**
- The atomic number of nitrogen (N) is 7, which means it has 7 electrons.
- The `N^(3-)` ion has gained 3 additional electrons, giving it a total of:
\[ 7 + 3 = 10 \text{ electrons} \]
- Therefore, `N^(3-)` is isoelectronic with `O^(2-)`.

- **Option 2: `F^-`**
- The atomic number of fluorine (F) is 9, which means it has 9 electrons.
- The `F^-` ion has gained 1 additional electron, giving it a total of:
\[ 9 + 1 = 10 \text{ electrons} \]
- Therefore, `F^-` is isoelectronic with `O^(2-)`.

- **Option 3: `Ti^(+1)`**
- The atomic number of titanium (Ti) is 21, which means it has 21 electrons.
- The `Ti^(+1)` ion has lost 1 electron, giving it a total of:
\[ 21 - 1 = 20 \text{ electrons} \]
- Therefore, `Ti^(+1)` is **not** isoelectronic with `O^(2-)`.

- **Option 4: `Na^+`**
- The atomic number of sodium (Na) is 11, which means it has 11 electrons.
- The `Na^+` ion has lost 1 electron, giving it a total of:
\[ 11 - 1 = 10 \text{ electrons} \]
- Therefore, `Na^+` is isoelectronic with `O^(2-)`.

3. **Conclusion:**
- The only ion that is not isoelectronic with `O^(2-)` is `Ti^(+1)`.

### Final Answer:
The ion that is not isoelectronic with `O^(2-)` is **Option 3: `Ti^(+1)`**.

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