The formation of the oxide ion, \\\\(O^{2-}(g)\\\\), from oxygen atom requires first an exothermic and then an endothermic step as shown below:\\\\(O(g)+e^-\\rightarrow O^-(g); \\Delta_fH^{\\ominus}=-141\\ kJ mol^{-1}\\\\)\\\\(O^-(g)+e^-\\rightarrow O^{2-}(g); \\Delta_fH^{\\ominus}= +780\\ kJ mol^{-1}\\\\)Thus, the process of formation of \\\\(O^{2-}\\\\) in the gas phase is unfavorable even though \\\\(O^{2-}\\\\) is isoelectronic with neon. It is due to the fact that electron repulsion outweighs the stability gained by achieving noble gas configuration. When an electron is added to \\\\(O^-\\\\) anion, there is strong electrostatic repulsion between the two negative charges. Due to this, the second electron gain enthalpy of oxygen is positive.