Among the following, the third ionisation energy is highest for

Options

A. Aluminium
B. Beryllium
C. Boron
D. Magnesium

Answer

To determine which element has the highest third ionization energy among Aluminum, Beryllium, Boron, and Magnesium, we can follow these steps:

### Step 1: Understand Ionization Energy
Ionization energy is the energy required to remove an electron from an atom. The third ionization energy refers to the energy needed to remove the third electron from an atom.

### Step 2: Identify the Electron Configuration
- **Beryllium (Be)**: 1s² 2s² (2 valence electrons)
- **Boron (B)**: 1s² 2s² 2p¹ (3 valence electrons)
- **Aluminum (Al)**: 1s² 2s² 2p⁶ 3s² 3p¹ (3 valence electrons)
- **Magnesium (Mg)**: 1s² 2s² 2p⁶ 3s² (2 valence electrons)

### Step 3: Determine the Number of Valence Electrons
- Beryllium and Magnesium have 2 valence electrons.
- Boron and Aluminum have 3 valence electrons.

### Step 4: Analyze the Third Ionization Energy
- For Beryllium and Magnesium, removing the third electron means removing an electron from a filled shell, which requires significantly more energy.
- For Boron and Aluminum, the third ionization energy involves removing an electron from a p-orbital, which is generally less energy-intensive than removing from a filled shell.

### Step 5: Compare the Sizes of the Atoms
- Beryllium is smaller in size compared to Magnesium, which means that the electrons are held more tightly in Beryllium.
- This results in a higher ionization energy for Beryllium compared to Magnesium.

### Step 6: Conclusion
Since Beryllium has a smaller atomic size and higher effective nuclear charge compared to Magnesium, it will have the highest third ionization energy among the given options.

### Final Answer
**Beryllium has the highest third ionization energy among the given elements.**

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