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 the outermost electron from an isolated gaseous atom in its ground state.

**Step 2: Discuss Factors Affecting Ionization Enthalpy**
1. **Atomic Size**
- As the atomic size increases, the distance between the nucleus and the outermost electron also increases. This results in a weaker nuclear attraction on the outermost electron, making it easier to remove. Therefore, larger atoms have lower ionization enthalpy.
- **Hint:** Remember that larger atoms have their outer electrons further from the nucleus, which reduces the attraction.

2. **Screening Effect**
- Inner electrons can shield the outermost electrons from the full effect of the nuclear charge. The more inner electrons there are, the more they shield the outermost electron, leading to a lower effective nuclear charge felt by the outermost electron. This results in a lower ionization enthalpy.
- **Hint:** Think of the inner electrons as a barrier that reduces the pull of the nucleus on the outermost electron.

3. **Nuclear Charge**
- The nuclear charge refers to the total charge of the nucleus due to protons. A higher nuclear charge means a stronger attraction between the nucleus and the outermost electron. Thus, as the nuclear charge increases, the ionization enthalpy also increases because it is harder to remove the outermost electron.
- **Hint:** More protons in the nucleus mean a stronger pull on the electrons.

**Step 3: Trends in the Periodic Table**
1. **Across a Period (Left to Right)**
- As you move from left to right across a period, the atomic number increases, leading to an increase in nuclear charge. The effective nuclear charge increases while the atomic size decreases. Consequently, the ionization enthalpy increases.
- **Hint:** Remember that increased nuclear charge and decreased size make it harder to remove an electron.

2. **Down a Group**
- As you move down a group in the periodic table, the number of electron shells increases. This results in an increase in atomic size, which means the outermost electrons are further from the nucleus. The increased distance and the shielding effect of inner electrons lead to a decrease in ionization enthalpy.
- **Hint:** More shells mean larger atoms, which makes it easier to remove outer electrons.

### Summary
- Ionization enthalpy is the energy needed to remove an outermost electron from an atom.
- Factors affecting ionization enthalpy include atomic size, screening effect, and nuclear charge.
- Trends show that ionization enthalpy increases across a period and decreases down a group in the periodic table.