The incorrect statement among the following is

Options

A. The first ionisation potential of Al is less than the first ionisation potential of Mg.
B. The second ionisation potential of Mg is greater than the second ionisation potential of Na.
C. The first ionisation potential of Na is less than the first ionisation potential of Mg.
D. The third ionisation potential of Mg is greater than the third ionisation potential of Al.

Explanation

To determine the incorrect statement among the given options regarding the ionization potentials of sodium, magnesium, and aluminum, we can follow these steps:

### Step 1: Understand the Electronic Configurations
- **Sodium (Na)**: The electronic configuration is \(1s^2 2s^2 2p^6 3s^1\).
- **Magnesium (Mg)**: The electronic configuration is \(1s^2 2s^2 2p^6 3s^2\).
- **Aluminum (Al)**: The electronic configuration is \(1s^2 2s^2 2p^6 3s^2 3p^1\).

### Step 2: Analyze the First Ionization Potential
- The first ionization potential (IP) is the energy required to remove the outermost electron.
- For sodium, the outermost electron is in the 3s orbital, while for magnesium, it is in the 3s orbital as well, but magnesium has a fully filled 3s subshell.
- The stability of a fully filled subshell (magnesium) means that it has a higher first ionization potential than sodium.
- **Conclusion**: The first ionization potential of aluminum is less than that of magnesium (Correct statement).

### Step 3: Analyze the Second Ionization Potential
- The second ionization potential refers to the energy required to remove a second electron.
- For sodium, after losing one electron, the configuration becomes \(1s^2 2s^2 2p^6\) (which is stable like neon).
- For magnesium, after losing one electron, the configuration becomes \(1s^2 2s^2 2p^6 3s^1\).
- The stable noble gas configuration of sodium makes it more stable than magnesium after the first ionization.
- **Conclusion**: The second ionization potential of sodium is greater than that of magnesium (Incorrect statement).

### Step 4: Analyze the Third Ionization Potential
- The third ionization potential involves removing a third electron.
- For magnesium, after losing two electrons, the configuration is \(1s^2 2s^2 2p^6\) (noble gas configuration).
- For aluminum, after losing two electrons, the configuration is \(1s^2 2s^2 2p^6 3s^1\).
- The fully filled configuration of magnesium is more stable than the partially filled configuration of aluminum.
- **Conclusion**: The third ionization potential of magnesium is greater than that of aluminum (Correct statement).

### Final Conclusion
- The incorrect statement among the options is: "The second ionization potential of magnesium is greater than the second ionization potential of sodium."