The first ionisation energies of magnesium and aluminium are respectively given by

Options

A. 7.64, 5.98
B. 7.64, 7.64
C. 5.98, 7.64
D. 5.98, 5.98

Answer

To solve the question regarding the first ionization energies of magnesium and aluminum, we can follow these steps:

### Step 1: Determine the electronic configurations
- **Magnesium (Mg)** has an atomic number of 12. Its electronic configuration is:
\[ \text{Mg: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \]
- **Aluminum (Al)** has an atomic number of 13. Its electronic configuration is:
\[ \text{Al: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^1 \]

### Step 2: Analyze the position of the outermost electrons
- In magnesium, the outermost electrons are in the 3s subshell (3s²).
- In aluminum, the outermost electron is in the 3p subshell (3s² 3p¹).

### Step 3: Consider the effective nuclear charge and shielding effect
- The 3s electrons in magnesium are closer to the nucleus and experience a stronger effective nuclear charge compared to the 3p electron in aluminum.
- The shielding effect follows the order: s > p > d > f. This means that the 3s electrons are more tightly held by the nucleus than the 3p electrons.

### Step 4: Compare the first ionization energies
- Since the 3s electrons in magnesium are held more tightly due to their proximity to the nucleus, magnesium will have a higher first ionization energy compared to aluminum.
- Therefore, we conclude that:
\[ \text{IE}_1(\text{Mg}) > \text{IE}_1(\text{Al}) \]

### Step 5: Evaluate the options provided
- Based on our analysis, the first ionization energy of magnesium is greater than that of aluminum. Thus, the correct option is the one that states that the first ionization energy of magnesium is higher than that of aluminum.

### Final Answer
The first ionization energies of magnesium and aluminum are such that:
\[ \text{IE}_1(\text{Mg}) > \text{IE}_1(\text{Al}) \]

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