Electron gain enthalpy values of noble gases are positive because they have stable electronic configuration and thus have no tendency to take additional electron.

Introduction:
The electron gain enthalpy of an atom is defined as the energy change that occurs when an atom in its gaseous state gains an electron to form a negative ion. Noble gases are a group of elements that are known for their low reactivity and stable electronic configurations. The electron gain enthalpy values of noble gases are positive, and this can be explained by considering their unique characteristics.

Explanation:

1. Stable electronic configuration:
Noble gases have stable electronic configurations with completely filled valence shells. This makes them highly stable and unreactive. In order to gain an electron, noble gases would have to disturb their stable electronic configurations, which requires energy. Therefore, the electron gain enthalpy values of noble gases are positive.

2. Small size:
Noble gases have the smallest atomic sizes among all elements. This is due to the high effective nuclear charge and low shielding effect of the inner electron shells. The small size of noble gas atoms makes it difficult for them to accommodate an additional electron due to repulsion between the existing electrons and the incoming electron. This repulsion requires energy, resulting in positive electron gain enthalpy values.

3. Monoatomic nature:
Noble gases exist as monoatomic species, meaning they consist of single atoms. Monoatomic noble gases have a stable electronic configuration and do not readily form compounds. The electron gain enthalpy values of noble gases are positive because their monoatomic nature makes it energetically unfavorable for them to gain an electron and form a negative ion.

4. Gaseous state:
Noble gases are all gases at room temperature and pressure. In the gaseous state, the atoms are relatively far apart and experience weak intermolecular forces. When a noble gas atom gains an electron, it forms a negative ion which is larger in size and experiences stronger intermolecular forces. This increase in intermolecular forces requires energy, leading to positive electron gain enthalpy values.

Conclusion:
The electron gain enthalpy values of noble gases are positive due to their stable electronic configurations, small size, monoatomic nature, and gaseous state. These factors contribute to the energetic unfavorability of gaining an electron, resulting in positive electron gain enthalpy values.

If the electron gain enthalpy is positive it means that the process is endothermic i.e it is not thermodynamically feasible process . Now noble gases are stable due to complete octet,hence an addition of electron would not be feasible and an external energy would be required.