The formation of the oxide ion `O^(2-)` (g) requires first an exothermic and then an endothermic step as shown below :
`O (g) + e^(-)= O^(-) (g), DeltaH^(@) = - 142 kJ moI^(-1)`
`O(g)+e^(-)toO^(2-)(g),DeltaH^(@)=844kJmol^(-1)`
This is because

Options

A. oxygen is more electronegative
B. oxygen has high electron affinity
C. `O^(-)` ion will tend to resist the addition of another electron
D. `O^(-)` ion has comparatively larger size than oxygen atom.

Answer

To understand why the formation of the oxide ion \( O^{2-} \) requires an exothermic step followed by an endothermic step, we can break it down into two main processes:

### Step 1: Formation of the O\(^-\) Ion
1. **Process**: The first step involves the addition of an electron to a neutral oxygen atom \( O(g) \).
2. **Energy Change**: This process is exothermic, meaning it releases energy. The reaction can be represented as:
\[ O(g) + e^- \rightarrow O^-(g) \]
The enthalpy change \( \Delta H \) for this step is \( -142 \, \text{kJ mol}^{-1} \). The negative sign indicates that energy is released during this process.

### Step 2: Formation of the O\(^2-\) Ion
3. **Process**: The second step involves adding another electron to the negatively charged ion \( O^-(g) \):
\[ O^-(g) + e^- \rightarrow O^{2-}(g) \]
4. **Energy Change**: This step is endothermic, meaning it requires energy to proceed. The enthalpy change \( \Delta H \) for this step is \( +844 \, \text{kJ mol}^{-1} \). The positive sign indicates that energy is absorbed during this process.

### Explanation of the Energy Changes
- **First Step (Exothermic)**: When the first electron is added to the neutral oxygen atom, the attraction between the negatively charged electron and the positively charged nucleus of oxygen results in the release of energy. This is a favorable process because it stabilizes the ion formed.

- **Second Step (Endothermic)**: In the second step, when trying to add another electron to the already negatively charged \( O^- \) ion, there is a significant repulsion between the two negatively charged entities (the incoming electron and the \( O^- \) ion). This repulsion makes it more difficult to add the second electron, thus requiring energy to overcome this repulsion.

### Conclusion
Therefore, the formation of the oxide ion \( O^{2-} \) involves an exothermic step followed by an endothermic step due to the nature of electron interactions and the energy changes associated with these processes.