Q.21:- Would you expect the second electron gain enthalpy of O as positive, more negative or less negative than the first? Justify your answer.
The second electron gain enthalpy of O is positive, when an electron is added to neutral O atom, a monovalent anion (O−) is formed and energy is released, i.e., the first electron gain enthalpy is negative. When second electron is added to form O2−anion, there is a lot of electrostatic repulsion as both O− anion and electron have negative charge. To overcome force of repulsion, energy is absorbed. Thus, the second electron gain enthalpy of 0 is positive. O(g)+e→O−(g);ΔegH1=−141kJmol−1