Question

Formation of $O^{-}$ is exothermic, whereas formation of $O^{2-}$ is endothermic. Explain.

Answer 1

Electron gain enthalphy for oxygen is negative because oxygen being an electronegative atom can easily accept electron.

But gaining one more electron to $O_2$ will mean much more stability,so it is be an exothermic reaction.This expectation is true if we consider a neutral species but as $O^{-}$ is negligibely charged adding another negativelly charged species will be diffcult as the like charges will repel each other ,so in order to force the electronn to fuse with $O^{-}$ ion we supply energy in form of heat .This males it endothermic.