Which of the following is not isoelectronic ?

Options

A. `Na^(+)`
B. `Mg^(2+)`
C. `O^(2-)`
D. `Cl^(-)`

Explanation

To determine which of the following species is not isoelectronic, we need to analyze the number of electrons in each species provided in the question.

### Step-by-Step Solution:

1. **Understand Isoelectronic Species**:
- Isoelectronic species are atoms or ions that have the same number of electrons.

2. **Analyze Sodium (Na)**:
- Sodium has an atomic number of 11.
- As a cation (Na⁺), it loses one electron:
- Number of electrons = 11 (atomic number) - 1 (lost electron) = 10 electrons.

3. **Analyze Magnesium (Mg)**:
- Magnesium has an atomic number of 12.
- As a cation (Mg²⁺), it loses two electrons:
- Number of electrons = 12 (atomic number) - 2 (lost electrons) = 10 electrons.

4. **Analyze Oxide Ion (O²⁻)**:
- Oxygen has an atomic number of 8.
- As an anion (O²⁻), it gains two electrons:
- Number of electrons = 8 (atomic number) + 2 (gained electrons) = 10 electrons.

5. **Analyze Chloride Ion (Cl⁻)**:
- Chlorine has an atomic number of 17.
- As an anion (Cl⁻), it gains one electron:
- Number of electrons = 17 (atomic number) + 1 (gained electron) = 18 electrons.

6. **Compare the Number of Electrons**:
- Na⁺: 10 electrons
- Mg²⁺: 10 electrons
- O²⁻: 10 electrons
- Cl⁻: 18 electrons

7. **Conclusion**:
- The species that is not isoelectronic with the others is Cl⁻ because it has 18 electrons, while the others have 10 electrons.

### Final Answer:
- The species that is not isoelectronic is **Chlorine (Cl⁻)**.