Which among the following is not iso-electronic ?

Options

A. `NO^(-)`
B. `CN^(-)`
C. `N_(2)`
D. `CO`

Explanation

To determine which among the given species is not isoelectronic, we need to analyze the number of electrons in each species. Isoelectronic species are those that have the same number of electrons.

### Step-by-step Solution:

1. **Identify the Species**: The species mentioned are:
- NO⁻ (Nitrogen monoxide with a negative charge)
- CN⁻ (Cyanide ion)
- N₂ (Diatomic nitrogen)
- CO (Carbon monoxide)

2. **Calculate the Number of Electrons in Each Species**:
- **NO⁻**:
- Nitrogen (N) has an atomic number of 7, so it has 7 electrons.
- Oxygen (O) has an atomic number of 8, so it has 8 electrons.
- The negative charge indicates an additional electron.
- Total electrons in NO⁻ = 7 (N) + 8 (O) + 1 (extra electron) = 16 electrons.

- **CN⁻**:
- Carbon (C) has an atomic number of 6, so it has 6 electrons.
- Nitrogen (N) has an atomic number of 7, so it has 7 electrons.
- The negative charge indicates an additional electron.
- Total electrons in CN⁻ = 6 (C) + 7 (N) + 1 (extra electron) = 14 electrons.

- **N₂**:
- Each nitrogen atom has 7 electrons.
- Total electrons in N₂ = 7 (N) + 7 (N) = 14 electrons.

- **CO**:
- Carbon (C) has 6 electrons.
- Oxygen (O) has 8 electrons.
- Total electrons in CO = 6 (C) + 8 (O) = 14 electrons.

3. **Compare the Number of Electrons**:
- NO⁻ has 16 electrons.
- CN⁻ has 14 electrons.
- N₂ has 14 electrons.
- CO has 14 electrons.

4. **Conclusion**:
- The species that is not isoelectronic with the others is **NO⁻**, as it has 16 electrons while the others have 14 electrons.

### Final Answer:
**NO⁻ is not isoelectronic with the other species.**

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