O negative ion is formed by release of energy and O 2- ion is formed by gain of energy.Explain?

1 Answer
Jun 22, 2017

Well, you're trying to stuff an electron into a region that is already electron-dense. So you're going to have to overcome electron repulsions in an already-negative ion, thereby requiring the addition of energy.

(Imagine shoving two repulsive magnets together; can't do it easily...)

The first electron affinity of #"O"# atom is:

#"O"(g) + e^(-) -> "O"^(-)(g)#, #" "DeltaH_(EA_1) = -"142 kJ/mol"#

This exothermicity indicates a release of energy upon having gained this first electron. We would expect analogously that the second electron affinity process, i.e.

#"O"^(-)(g) + e^(-) -> "O"^(2-)(g)#,

would have an endothermic enthalpy, as the second electron has to be added into an electron-dense, negatively-charged ion. And in fact, it is endothermic:

#DeltaH_(EA_2) = +"844 kJ/mol"#

Adding this second electron requires input of energy, and so the second electron affinity enthalpy is endothermic with respect to the atom.