Discuss and compare the trend in ionisation enthalpy of the elements of group 1 with those of group 17 elements.

Explanation

### Step-by-Step Solution

1. **Identify the Groups**:
- Group 1 elements: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs).
- Group 17 elements: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I).

2. **Define Ionization Enthalpy**:
- Ionization enthalpy is the energy required to remove an electron from an isolated gaseous atom. It is a measure of how strongly an atom holds onto its electrons.

3. **Factors Affecting Ionization Enthalpy**:
- The ionization enthalpy depends on the atomic size and the effective nuclear charge (Z_eff) experienced by the outermost electron.
- Smaller atomic size and higher Z_eff lead to higher ionization enthalpy.

4. **Trends in Group 1 Elements**:
- Group 1 elements are characterized by larger atomic sizes as you move down the group (Li < Na < K < Rb < Cs).
- As the atomic size increases, the outermost electron is further from the nucleus, resulting in a lower Z_eff.
- Consequently, the ionization enthalpy decreases down the group.

5. **Trends in Group 17 Elements**:
- Group 17 elements (the halogens) have smaller atomic sizes compared to group 1 elements (F < Cl < Br < I).
- As you move down the group, the atomic size increases, but the halogens still have higher ionization enthalpy values than group 1 elements due to their smaller size and higher Z_eff.

6. **Comparison of Ionization Enthalpy**:
- Group 1 elements have lower ionization enthalpy compared to group 17 elements because:
- Group 1 elements are larger, leading to a lower Z_eff.
- Group 17 elements are smaller and have a higher Z_eff, making it more difficult to remove an electron.
- Thus, the trend shows that ionization enthalpy for group 1 elements is less than that of group 17 elements.

7. **Conclusion**:
- In summary, the ionization enthalpy of group 1 elements decreases down the group, while group 17 elements have higher ionization enthalpy values due to their smaller size and higher effective nuclear charge.