Which pair is of isoelectronic species?

Options

A. `K^(+), Cl^(-), Na^(+)`
B. `K^(+), Cl^(-)Ca^(2+)`
C. `Fe^(-), Al^(2+), K^(+)`
D. `Fe^(2+), Cu^(2+), V^(3+)`

Explanation

To determine which pair is of isoelectronic species, we need to identify species that have the same number of electrons. Let's analyze the given options step by step.

### Step 1: Identify the number of electrons in each species
1. **Potassium (K)**:
- Atomic number = 19
- In its neutral state, K has 19 electrons.
- For K+, we remove one electron:
- K+ = 19 - 1 = 18 electrons.

2. **Chlorine (Cl)**:
- Atomic number = 17
- In its neutral state, Cl has 17 electrons.
- For Cl-, we add one electron:
- Cl- = 17 + 1 = 18 electrons.

3. **Sodium (Na)**:
- Atomic number = 11
- In its neutral state, Na has 11 electrons.
- For Na+, we remove one electron:
- Na+ = 11 - 1 = 10 electrons.

### Conclusion for the first option:
- K+ has 18 electrons.
- Cl- has 18 electrons.
- Na+ has 10 electrons.
- **K+ and Cl- are isoelectronic.**

### Step 2: Check the second option
1. **Calcium (Ca)**:
- Atomic number = 20
- For Ca2+, we remove two electrons:
- Ca2+ = 20 - 2 = 18 electrons.

2. **K+**:
- As calculated earlier, K+ = 18 electrons.

3. **CN-**:
- Carbon (C) atomic number = 6 (6 electrons)
- Nitrogen (N) atomic number = 7 (7 electrons)
- CN has a total of 6 + 7 = 13 electrons.
- For CN-, we add one electron:
- CN- = 13 + 1 = 14 electrons.

### Conclusion for the second option:
- K+ has 18 electrons.
- Ca2+ has 18 electrons.
- CN- has 14 electrons.
- **K+ and Ca2+ are isoelectronic.**

### Step 3: Check the third option
1. **Fe-**:
- Iron (Fe) atomic number = 26
- For Fe-, we add one electron:
- Fe- = 26 + 1 = 27 electrons.

2. **Cl2+**:
- Chlorine (Cl) atomic number = 17
- For Cl2+, we remove two electrons:
- Cl2+ = 17 - 2 = 15 electrons.

3. **K+**:
- As calculated earlier, K+ = 18 electrons.

### Conclusion for the third option:
- Fe- has 27 electrons.
- Cl2+ has 15 electrons.
- K+ has 18 electrons.
- **None of these are isoelectronic.**

### Step 4: Check the fourth option
1. **Fe2+**:
- Iron (Fe) atomic number = 26
- For Fe2+, we remove two electrons:
- Fe2+ = 26 - 2 = 24 electrons.

2. **Copper (Cu)**:
- Copper (Cu) atomic number = 29
- For Cu2+, we remove two electrons:
- Cu2+ = 29 - 2 = 27 electrons.

### Conclusion for the fourth option:
- Fe2+ has 24 electrons.
- Cu2+ has 27 electrons.
- **None of these are isoelectronic.**

### Final Answer:
The pairs of isoelectronic species are **K+ and Cl-** from the first option, and **K+ and Ca2+** from the second option.