Among the third period elements, the actual ionization enthalpies are in the order `Na lt AI lt Mg lt SI lt S lt Plt CI lt Ar`. Explain why:
(i) `Mg` has higher `Delta_(i)H` than `AI`. (ii) `S` has lower `Delta_(i)H` than `P` and `CI`.

Answer

To explain the order of ionization enthalpies among the third period elements and the specific cases of magnesium (Mg) versus aluminum (Al) and sulfur (S) versus phosphorus (P) and chlorine (Cl), we can break down the reasoning into clear steps.

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy (or ionization enthalpy) is the energy required to remove the most loosely bound electron from a gaseous atom. Higher ionization energy indicates that it is more difficult to remove an electron.

2. **Order of Ionization Energies**:
- The order of ionization enthalpies for the third period elements is:
\[ \text{Na} < \text{Al} < \text{Mg} < \text{Si} < \text{S} < \text{P} < \text{Cl} < \text{Ar} \]

3. **Comparison of Mg and Al**:
- **(i) Why does Mg have a higher ionization enthalpy than Al?**
- **Electronic Configuration**:
- Magnesium (Mg): \( [Ne] 3s^2 \)
- Aluminum (Al): \( [Ne] 3s^2 3p^1 \)
- **Stability**:
- Mg has a stable electron configuration with a filled 3s subshell. Removing an electron from Mg requires breaking this stable configuration, resulting in a higher ionization energy.
- Al has one electron in the 3p subshell, which is less stable compared to Mg's configuration. Thus, it is easier to remove this outer electron from Al, leading to a lower ionization energy.

4. **Comparison of S, P, and Cl**:
- **(ii) Why does S have a lower ionization enthalpy than P and Cl?**
- **Electronic Configuration**:
- Sulfur (S): \( [Ne] 3s^2 3p^4 \)
- Phosphorus (P): \( [Ne] 3s^2 3p^3 \)
- Chlorine (Cl): \( [Ne] 3s^2 3p^5 \)
- **Half-Filled Stability**:
- P has a half-filled 3p subshell (3p^3), which is more stable and requires more energy to remove an electron compared to S.
- Cl has a higher effective nuclear charge (17 protons) compared to S (16 protons), which means that the attraction between the nucleus and the outer electrons is stronger, making it harder to remove an electron from Cl than from S.
- Therefore, S has a lower ionization energy than both P and Cl due to its less stable electron configuration and lower nuclear charge.

### Summary:
- Mg has a higher ionization enthalpy than Al because Mg has a stable filled subshell configuration, while Al has a less stable configuration with an additional electron in the 3p subshell.
- S has a lower ionization enthalpy than both P and Cl because P has a stable half-filled configuration, and Cl has a higher effective nuclear charge, making it harder to remove an electron from these elements compared to S.