The compound in which cation is isoelectronic with anion is :

Question

Accepted Answer

The compound in which the cation is isoelectronic with the anion is K₂S. Let's explore why Option 3 is correct and why the other options are not.

### Explanation for Option 3 (K₂S):

1. **Isoelectronic Definition**: Two species are isoelectronic if they have the same number of electrons.

2. **Potassium (K) and Sulfur (S) in K₂S**:
   - Potassium (K) loses one electron to form K⁺, resulting in an electron configuration of [Ar], which has 18 electrons.
   - Sulfur (S) gains two electrons to form S²⁻, resulting in an electron configuration of [Ar], which also has 18 electrons.
   - Therefore, in K₂S, the cation K⁺ is isoelectronic with the anion S²⁻, as both have the same number of electrons (18).

### Explanation for why other options are incorrect:

1. **Option 1 (NaCl)**:
   - Sodium (Na) loses one electron to form Na⁺, resulting in an electron configuration of [Ne], which has 10 electrons.
   - Chlorine (Cl) gains one electron to form Cl⁻, resulting in an electron configuration of [Ar], which has 18 electrons.
   - Na⁺ and Cl⁻ are not isoelectronic because they have different numbers of electrons (10 and 18, respectively).

2. **Option 2 (NaI)**:
   - Sodium (Na) loses one electron to form Na⁺, resulting in an electron configuration of [Ne], which has 10 electrons.
   - Iodine (I) gains one electron to form I⁻, resulting in an electron configuration of [Kr], which has 54 electrons.
   - Na⁺ and I⁻ are not isoelectronic because they have different numbers of electrons (10 and 54, respectively).

3. **Option 4 (CsF)**:
   - Cesium (Cs) loses one electron to form Cs⁺, resulting in an electron configuration of [Xe], which has 54 electrons.
   - Fluorine (F) gains one electron to form F⁻, resulting in an electron configuration of [Ne], which has 10 electrons.
   - Cs⁺ and F⁻ are not isoelectronic because they have different numbers of electrons (54 and 10, respectively).

In conclusion, Option 3 (K₂S) is the correct answer because it is the only compound where the cation and anion are isoelectronic, both having 18 electrons.