Show diagrammatically the formation O2- ion giving proper explanation.
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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
−
→O
−
(g);Δ
f
H
⊖
=−141 kJmol
−1
O
−
(g)+e
−
→O
2−
(g);Δ
f
H
⊖
=+780 kJmol
−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.
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