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 and the stability of the resulting ions after the removal of two electrons.

### Step-by-Step Solution:

1. **Identify the 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

2. **Understand the Third Ionization Energy (IE3):**
- IE3 refers to the energy required to remove the third electron after the first two have already been removed.
- For Na, after removing two electrons (from 3s¹), we are left with Na²⁺, which has the configuration of 1s² 2s² 2p⁶ (a noble gas configuration).
- For Mg, after removing two electrons (from 3s²), we are left with Mg²⁺, which also has the configuration of 1s² 2s² 2p⁶ (a noble gas configuration).
- For Al, after removing two electrons (from 3s² and 3p¹), we are left with Al³⁺, which has the configuration of 1s² 2s² 2p⁶ (a noble gas configuration).

3. **Assess the Stability of the Resulting Ions:**
- Na²⁺ and Mg²⁺ both achieve a noble gas configuration, which is very stable.
- However, Mg²⁺ has a higher nuclear charge (12 protons) compared to Na²⁺ (11 protons), leading to a stronger attraction between the nucleus and the remaining electrons in Mg²⁺.
- Al³⁺ has a higher positive charge and thus a stronger attraction to its remaining electrons than Na²⁺, but it has a lower nuclear charge than Mg²⁺.

4. **Rank the Ionization Energies:**
- Since Mg²⁺ is more stable and has a stronger attraction to its electrons due to its higher nuclear charge, it will require the most energy to remove an electron (highest IE3).
- Na²⁺ will require less energy than Mg²⁺ but more than Al³⁺ because it has a lower nuclear charge.
- Al³⁺ will require the least energy to remove an electron because it has the lowest nuclear charge and the electron is in a higher energy level (3s).

5. **Final Order of Third Ionization Energies:**
- Therefore, the correct order of third ionization energies (IE3) is:
**IE3 (Mg) > IE3 (Na) > IE3 (Al)**

### Conclusion:
The correct order of third ionization energies (IE3) of Na, Mg, and Al is:
**Mg > Na > Al**