which among the following species has the same number of electrons in its outermost as well as penultimate shell ?

Options

A. `Mg^(2+)`
B. `O^(2-)`
C. `F^(-)`
D. `Ca^(2+)`

Answer

To solve the question of which species has the same number of electrons in its outermost and penultimate shell, we will analyze each of the given species: Mg²⁺, O²⁻, F⁻, and Ca²⁺.

### Step 1: Understand the Definitions
- **Outermost Shell**: This is the shell with the highest principal quantum number (n). It contains the valence electrons.
- **Penultimate Shell**: This is the shell just before the outermost shell (n-1).

### Step 2: Analyze Each Species

#### Species 1: Mg²⁺
- Atomic number (Z) = 12
- Electronic configuration:
- Neutral Mg: 1s² 2s² 2p⁶ 3s²
- For Mg²⁺: 1s² 2s² 2p⁶ (2 electrons removed from 3s)
- **Outermost Shell (n=2)**: 2s² + 2p⁶ = 8 electrons
- **Penultimate Shell (n=1)**: 1s² = 2 electrons
- **Conclusion**: Not the same.

#### Species 2: O²⁻
- Atomic number (Z) = 8
- Electronic configuration:
- Neutral O: 1s² 2s² 2p⁴
- For O²⁻: 1s² 2s² 2p⁶ (2 electrons added)
- **Outermost Shell (n=2)**: 2s² + 2p⁶ = 8 electrons
- **Penultimate Shell (n=1)**: 1s² = 2 electrons
- **Conclusion**: Not the same.

#### Species 3: F⁻
- Atomic number (Z) = 9
- Electronic configuration:
- Neutral F: 1s² 2s² 2p⁵
- For F⁻: 1s² 2s² 2p⁶ (1 electron added)
- **Outermost Shell (n=2)**: 2s² + 2p⁶ = 8 electrons
- **Penultimate Shell (n=1)**: 1s² = 2 electrons
- **Conclusion**: Not the same.

#### Species 4: Ca²⁺
- Atomic number (Z) = 20
- Electronic configuration:
- Neutral Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
- For Ca²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ (2 electrons removed from 4s)
- **Outermost Shell (n=3)**: 3s² + 3p⁶ = 8 electrons
- **Penultimate Shell (n=2)**: 2s² + 2p⁶ = 8 electrons
- **Conclusion**: Both shells have 8 electrons. This is the correct species.

### Final Answer
The species that has the same number of electrons in its outermost and penultimate shell is **Ca²⁺**.

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