The formation of oxide ion `O^(2-)(g)` from oxygen atom requires first an exothermic and then an endothermic step as shown below
`O(g)+e^(-) rarr O^(-)(g), DeltaH^(-) = - 141 kj mol^(-1)`
`O^(-)(g) +e^(-) rarr O^(2-) (g), DeltaH^(-) =+ 780 kj mol^(-1)`
Thus, process of formation of `O^(2-)` in gas phase is unfavourable even through `O^(2-)` is isoelectronic with neon. It is due to the fact that m

Options

A. oxygen is more electronegative
B. addition of electron in oxygen results in larger size of the ion
C. electron repulsin outweighs the stability gained by achieving noble gas configuration
D. `O^(-)` ion has comparitively smaller size of the oxygen atom

Explanation

To determine why the formation of the oxide ion \( O^{2-}(g) \) from an oxygen atom is unfavorable, we can analyze the steps involved in the process and the energy changes associated with them.

### Step-by-Step Solution:

1. **Understanding the Formation Steps**:
- The formation of \( O^{-}(g) \) from \( O(g) \) involves the addition of one electron:
\[ O(g) + e^{-} \rightarrow O^{-}(g), \quad \Delta H = -141 \text{ kJ mol}^{-1} \]
This step is exothermic, meaning energy is released when the first electron is added to the oxygen atom.

2. **Formation of Oxide Ion**:
- The next step involves adding another electron to form \( O^{2-}(g) \):
\[ O^{-}(g) + e^{-} \rightarrow O^{2-}(g), \quad \Delta H = +780 \text{ kJ mol}^{-1} \]
This step is endothermic, meaning energy must be supplied to add the second electron.

3. **Net Energy Change**:
- To find the overall energy change for the formation of \( O^{2-}(g) \) from \( O(g) \), we can combine the two steps:
\[ \Delta H_{\text{total}} = -141 \text{ kJ mol}^{-1} + 780 \text{ kJ mol}^{-1} = +639 \text{ kJ mol}^{-1} \]
- Since the total change in enthalpy is positive, the overall process is endothermic and thus unfavorable.

4. **Isoelectronic Configuration**:
- Although \( O^{2-} \) is isoelectronic with neon (both have 10 electrons), the stability gained from achieving a noble gas configuration is not enough to overcome the energy required to add the second electron.

5. **Reason for Unfavorable Formation**:
- The unfavorable nature of forming \( O^{2-} \) can be attributed to the high inter-electronic repulsions that occur when adding the second electron. The small size of the oxygen atom means that the electrons are close together, leading to significant repulsion between them.

6. **Conclusion**:
- Therefore, the process of forming \( O^{2-} \) is unfavorable because the electron repulsion outweighs the stability gained from achieving a noble gas configuration.

### Final Answer:
The process of formation of \( O^{2-} \) in the gas phase is unfavorable due to the fact that **electron repulsion outweighs the stability gained by achieving noble gas configuration**.

---