An isostere is

Options

A. `ClO_4^-and OCN^-`
B. `NO_2^- and PO_4^(3-)`
C. `CO_2,N_2O , NO_3^-`
D. `NO_2^- and O_3`

Answer

**Step-by-Step Solution:**

1. **Definition of Isostere:**
An isostere is defined as molecules or ions that have the same number of atoms and the same number of valence electrons.

2. **Identifying the Pairs:**
We need to analyze the given pairs of molecules or ions to determine which pair qualifies as isostere based on the definition.

3. **Counting Atoms and Valence Electrons:**
For each pair, we will count the total number of atoms and the total number of valence electrons.

- **First Pair:**
- Example: Chlorine (Cl) and Oxygen (O)
- Count of atoms: Cl (1) + O (1) = 2 atoms
- Valence electrons: Cl (7) + O (6) = 13 electrons
- Total: 2 atoms, 13 valence electrons (not isostere)

- **Second Pair:**
- Example: Carbon (C) and Nitrogen (N)
- Count of atoms: C (1) + N (1) = 2 atoms
- Valence electrons: C (4) + N (5) = 9 electrons
- Total: 2 atoms, 9 valence electrons (not isostere)

- **Third Pair:**
- Example: NO2- (Nitrite ion) and another ion
- Count of atoms: N (1) + O (2) = 3 atoms
- Valence electrons: N (5) + O (6*2) + 1 (for the negative charge) = 18 electrons
- Total: 3 atoms, 18 valence electrons (not isostere)

- **Fourth Pair:**
- Example: Another molecule with the same count
- Count of atoms: 3 atoms
- Valence electrons: 24 electrons
- Total: 3 atoms, 24 valence electrons (isostere)

4. **Conclusion:**
After analyzing all pairs, the correct option that qualifies as isostere is the one with 3 atoms and 24 valence electrons.

**Final Answer:**
The correct option is the pair that has 3 atoms and the same number of valence electrons.

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