The family of elements with the highest ionisation enthalpy :

Options

A. alkaline earth metals
B. halogens
C. noble gases
D. alkali metals

Answer

### Step-by-Step Solution

1. **Understanding Ionization Enthalpy**:
- Ionization enthalpy (or ionization energy) is the energy required to remove an electron from an isolated gaseous atom. The higher the ionization energy, the more energy is needed to remove an electron.

2. **Factors Affecting Ionization Enthalpy**:
- **Atomic Size**: Smaller atoms have higher ionization enthalpy because the distance between the nucleus and the outermost electron is shorter, resulting in a stronger electrostatic attraction.
- **Nuclear Charge**: Atoms with a higher nuclear charge (more protons in the nucleus) will also have higher ionization enthalpy because the increased positive charge attracts the electrons more strongly.

3. **Trends in the Periodic Table**:
- As you move from left to right across a period in the periodic table, the atomic size decreases and the nuclear charge increases. This results in higher ionization enthalpy.
- Conversely, as you move down a group, the atomic size increases, leading to lower ionization enthalpy.

4. **Identifying the Family with the Highest Ionization Enthalpy**:
- Noble gases (Group 18) have the highest ionization enthalpy. They have a complete outer electron shell (octet), making them very stable and less likely to lose an electron.
- In comparison, alkali metals (Group 1) and alkaline earth metals (Group 2) have lower ionization enthalpies due to their larger atomic sizes and lower effective nuclear charges.

5. **Conclusion**:
- The family of elements with the highest ionization enthalpy is the **noble gases**. They are stable and have a complete outer shell, which makes it difficult to remove an electron.