Which of the following pair is iso-electronic : -

Options

A. (a) `CO_2, NO_2`
B. (b) `NO_2^(-), CO_2`
C. (c) `CN^(-), CO`
D. (d) `SO_2 , CO_2`

Answer

To determine which of the given pairs are isoelectronic, we need to calculate the total number of electrons in each species. Isoelectronic species have the same number of electrons.

### Step-by-Step Solution:

1. **Calculate the number of electrons in CO2:**
- Carbon (C) has an atomic number of 6, so it has 6 electrons.
- Oxygen (O) has an atomic number of 8, and there are 2 oxygen atoms in CO2.
- Total electrons in CO2 = 6 (C) + 2 × 8 (O) = 6 + 16 = **22 electrons**.

2. **Calculate the number of electrons in NO2:**
- Nitrogen (N) has an atomic number of 7, so it has 7 electrons.
- There are 2 oxygen atoms, each with 8 electrons.
- Total electrons in NO2 = 7 (N) + 2 × 8 (O) = 7 + 16 = **23 electrons**.

3. **Check if CO2 and NO2 are isoelectronic:**
- CO2 has 22 electrons, and NO2 has 23 electrons.
- **Not isoelectronic.**

4. **Calculate the number of electrons in NO2- (NO2 with a negative charge):**
- For NO2-, we add an extra electron due to the negative charge.
- Total electrons in NO2- = 7 (N) + 2 × 8 (O) + 1 (extra electron) = 7 + 16 + 1 = **24 electrons**.

5. **Check if NO2- and CO2 are isoelectronic:**
- NO2- has 24 electrons, and CO2 has 22 electrons.
- **Not isoelectronic.**

6. **Calculate the number of electrons in CN- (Cyanide ion):**
- Carbon (C) has 6 electrons, and nitrogen (N) has 7 electrons.
- There is a negative charge, so we add one extra electron.
- Total electrons in CN- = 6 (C) + 7 (N) + 1 (extra electron) = 6 + 7 + 1 = **14 electrons**.

7. **Calculate the number of electrons in CO (Carbon monoxide):**
- Carbon (C) has 6 electrons, and oxygen (O) has 8 electrons.
- Total electrons in CO = 6 (C) + 8 (O) = **14 electrons**.

8. **Check if CN- and CO are isoelectronic:**
- Both CN- and CO have 14 electrons.
- **They are isoelectronic.**

9. **Calculate the number of electrons in SO2:**
- Sulfur (S) has an atomic number of 16, so it has 16 electrons.
- There are 2 oxygen atoms, each with 8 electrons.
- Total electrons in SO2 = 16 (S) + 2 × 8 (O) = 16 + 16 = **32 electrons**.

10. **Check if SO2 and CO2 are isoelectronic:**
- SO2 has 32 electrons, and CO2 has 22 electrons.
- **Not isoelectronic.**

### Conclusion:
The only pair that is isoelectronic is **CN- and CO**.

### Final Answer:
**Option C: CN- and CO are isoelectronic.**