The correct order of third ionization energies `(IE_3)` of Na, Mg and Al is

Options

A. `Mg gt Na gt Al`
B. `Al gt Mg gt Na`
C. `Na gt Mg gt Al`
D. `Mg gt Al gt Na`

Answer

To determine the correct order of the third ionization energies (IE3) of sodium (Na), magnesium (Mg), and aluminum (Al), we need to analyze the electronic configurations of these elements and the stability of the resulting ions after the removal of electrons.

### Step-by-Step Solution:

1. **Understand Ionization Energy**:
Ionization energy is the energy required to remove an electron from an isolated gaseous atom. The third ionization energy (IE3) refers to the energy required to remove the third electron.

2. **Identify Electronic Configurations**:
- **Sodium (Na)**: Atomic number = 11
- Electronic configuration: 1s² 2s² 2p⁶ 3s¹
- **Magnesium (Mg)**: Atomic number = 12
- Electronic configuration: 1s² 2s² 2p⁶ 3s²
- **Aluminum (Al)**: Atomic number = 13
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p¹

3. **Determine the Process of Ionization**:
- For **Na**:
- IE1: Na → Na⁺ + e⁻ (removal of 3s¹)
- IE2: Na⁺ → Na²⁺ + e⁻ (removal of 2p⁶)
- IE3: Na²⁺ → Na³⁺ + e⁻ (removal of 2s²)
- For **Mg**:
- IE1: Mg → Mg⁺ + e⁻ (removal of 3s²)
- IE2: Mg⁺ → Mg²⁺ + e⁻ (removal of 3s¹)
- IE3: Mg²⁺ → Mg³⁺ + e⁻ (removal of 2p⁶)
- For **Al**:
- IE1: Al → Al⁺ + e⁻ (removal of 3s²)
- IE2: Al⁺ → Al²⁺ + e⁻ (removal of 3p¹)
- IE3: Al²⁺ → Al³⁺ + e⁻ (removal of 2p⁶)

4. **Analyze Stability of Ions**:
- After the third ionization, Na³⁺ has the configuration of 1s² 2s² 2p⁶ (a stable noble gas configuration).
- Mg³⁺ also has the configuration of 1s² 2s² 2p⁶.
- Al³⁺ has the same stable configuration of 1s² 2s² 2p⁶.

5. **Compare the Ionization Energies**:
- The energy required to remove an electron from a stable noble gas configuration (like 1s² 2s² 2p⁶) is higher than from less stable configurations.
- However, since Na³⁺ is formed from a neutral atom with a single electron in the outer shell, it is easier to remove the third electron from Na than from Mg or Al, which have more electrons in their outer shells.

6. **Conclusion**:
The order of third ionization energies (IE3) is:
\[ \text{IE3 (Na)} < \text{IE3 (Mg)} < \text{IE3 (Al)} \]

### Final Answer:
The correct order of third ionization energies (IE3) of Na, Mg, and Al is:
\[ \text{IE3 (Na)} < \text{IE3 (Mg)} < \text{IE3 (Al)} \]