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 points regarding magnesium (Mg), aluminum (Al), sulfur (S), phosphorus (P), and chlorine (Cl), we can break down the reasoning as follows:

### Step-by-Step Solution:

1. **Understanding Ionization Enthalpy**:
- Ionization enthalpy (ΔiH) is the energy required to remove an electron from an atom in the gaseous state. Higher ionization enthalpy indicates that it is more difficult to remove an electron.

2. **Trends in Ionization Enthalpy Across a Period**:
- As we move from left to right across a period, the ionization enthalpy generally increases. This is due to the increase in nuclear charge (more protons) which attracts the electrons more strongly, making it harder to remove them.

3. **Comparison of Mg and Al**:
- **(i) Why does Mg have a higher ΔiH than Al?**
- Magnesium (Mg) has the electronic configuration [Ne]3s², while aluminum (Al) has [Ne]3s²3p¹.
- The outermost electron in Al is in a 3p orbital, which is at a higher energy level than the 3s electrons in Mg.
- Additionally, the 3p electron in Al experiences more shielding from the inner electrons compared to the 3s electrons in Mg. This results in a lower effective nuclear charge felt by the 3p electron in Al, making it easier to remove compared to the 3s electrons in Mg.
- Therefore, Mg has a higher ionization enthalpy than Al.

4. **Comparison of S, P, and Cl**:
- **(ii) Why does S have a lower ΔiH than P and Cl?**
- Sulfur (S) has the electronic configuration [Ne]3s²3p⁴, while phosphorus (P) has [Ne]3s²3p³ and chlorine (Cl) has [Ne]3s²3p⁵.
- The 3p orbitals in S contain four electrons, which leads to increased electron-electron repulsion among these electrons. This repulsion makes it easier to remove one of the outer electrons from sulfur compared to phosphorus and chlorine.
- In phosphorus, the three 3p electrons are more stable due to less repulsion, and in chlorine, the presence of five electrons in the 3p subshell makes it more stable and harder to remove an electron.
- Hence, sulfur has a lower ionization enthalpy than both phosphorus and chlorine.

### Summary of Ionization Enthalpy Order:
- The order of ionization enthalpies among the third period elements is influenced by their electronic configurations and the effective nuclear charge experienced by the outermost electrons.