Which one of the following groups represent a collection of isolectronic species ? (At.no `Cs = 55, Br = 35)`

Options

A. `N^(3-),F^(-),Na^(+)`
B. `Be,Al^(3+),Cl^(-)`
C. `Ca^(2+),Cs^(+),Br`
D. `Na^(+),Ca^(2+),Mg^(2+)`

Answer

To determine which group represents a collection of isoelectronic species, we need to identify species that have the same number of electrons. Let's analyze the options step by step.

### Step 1: Identify the number of electrons in each species
1. **For N³⁻ (Nitride ion)**:
- Atomic number of nitrogen (N) = 7
- N³⁻ means nitrogen has gained 3 electrons.
- Total electrons = 7 (atomic number) + 3 (gained) = **10 electrons**.

2. **For F⁻ (Fluoride ion)**:
- Atomic number of fluorine (F) = 9
- F⁻ means fluorine has gained 1 electron.
- Total electrons = 9 (atomic number) + 1 (gained) = **10 electrons**.

3. **For Na⁺ (Sodium ion)**:
- Atomic number of sodium (Na) = 11
- Na⁺ means sodium has lost 1 electron.
- Total electrons = 11 (atomic number) - 1 (lost) = **10 electrons**.

### Conclusion for Step 1:
- N³⁻, F⁻, and Na⁺ all have **10 electrons**. Thus, they are isoelectronic species.

### Step 2: Check the other options
1. **Beryllium (Be) and Al³⁺**:
- Be has 4 electrons.
- Al³⁺ has lost 3 electrons from its 13 electrons (atomic number), leaving it with 10 electrons.
- Not isoelectronic since Be has 4 electrons.

2. **Ca²⁺, Cs²⁺, and Br**:
- Ca has 20 electrons, loses 2 to become Ca²⁺ (18 electrons).
- Cs has 55 electrons, loses 2 to become Cs²⁺ (53 electrons).
- Br has 35 electrons, gains 1 to become Br⁻ (36 electrons).
- Not isoelectronic since they all have different numbers of electrons.

3. **Na⁺, Ca²⁺, and K⁺**:
- Na⁺ has 10 electrons.
- Ca²⁺ has 18 electrons.
- K⁺ has 18 electrons.
- Not isoelectronic since Na⁺ has 10 while Ca²⁺ and K⁺ have 18.

### Final Answer:
The only group that represents a collection of isoelectronic species is **N³⁻, F⁻, and Na⁺**.