Define ionisation enthalpy. Discuss the factors affecting ionisation enthalpy of the elements and its trends in teh periodic table.

Explanation

### Step-by-Step Solution

**Step 1: Define Ionization Enthalpy**
Ionization enthalpy (or ionization energy) is defined as the amount of energy required to remove an electron from an isolated gaseous atom. This process can be represented by the following equation:

\[ \text{M(g)} \rightarrow \text{M}^+(g) + e^- \]

where M is the atom and \( e^- \) is the electron being removed.

**Step 2: Discuss Factors Affecting Ionization Enthalpy**
Several factors influence the ionization enthalpy of elements:

1. **Effective Nuclear Charge (Z_eff)**:
- The effective nuclear charge is the net positive charge experienced by an electron in a multi-electron atom. A higher Z_eff means that electrons are held more tightly by the nucleus, requiring more energy to remove them. Thus, as Z_eff increases, ionization enthalpy increases.

2. **Atomic Size**:
- The size of the atom plays a crucial role. Larger atoms have their outermost electrons farther from the nucleus, which results in a weaker attraction between the nucleus and the electrons. Therefore, as atomic size increases, ionization enthalpy decreases.

3. **Electronic Configuration**:
- The initial electronic configuration of an atom also affects its ionization enthalpy. Atoms with a stable electronic configuration (like noble gases) have higher ionization enthalpy because it requires more energy to disrupt a stable arrangement. Similarly, half-filled and fully filled subshells are more stable and thus have higher ionization enthalpy.

**Step 3: Discuss Trends in the Periodic Table**
The trends in ionization enthalpy can be observed in two main directions in the periodic table:

1. **Down a Group**:
- As you move down a group in the periodic table, the atomic size increases due to the addition of electron shells. This increase in size leads to a decrease in Z_eff, making it easier to remove the outermost electron. Therefore, ionization enthalpy decreases as you move down a group.

2. **Across a Period**:
- As you move from left to right across a period, the atomic size decreases because the number of protons in the nucleus increases, leading to a greater Z_eff. This increased attraction between the nucleus and the outermost electrons makes it harder to remove an electron, resulting in an increase in ionization enthalpy.

### Summary
- **Ionization Enthalpy**: Energy required to remove an electron from an isolated gaseous atom.
- **Factors Affecting Ionization Enthalpy**:
1. Effective Nuclear Charge (Z_eff)
2. Atomic Size
3. Electronic Configuration
- **Trends**:
- Decreases down a group
- Increases across a period