IMAT 2012 Q65 [Electron Affinities]

Which one of the following is correct about the first and second electron affinities of oxygen?

A. first = slightly exothermic; second = very endothermic
B. first = slightly exothermic; second = very exothermic
C. first = slightly endothermic; second = very exothermic
D. first = slightly endothermic; second = very endothermic
E. first = very exothermic; second = very exothermic

1 Like

The first electron affinity is defined as the enthalpy change for:

X_{(g)} + e \rightarrow X^-

The first electron affinity value is virtually always negative for atoms. When an electron is added to a valence shell of an isolated gaseous Oxygen atom in it’s ground state to form an ion, energy is released. This is because the neutral atom tends to complete it’s octet with electrons.

There is energy released in bringing in the negative electron up to the positive nucleus. So, the first electron affinity of oxygen is an exothermic process and it’s value would be negative.

The answer should be A, B or E.

The second electron affinity is defined as the enthalpy change for:

X^- + e \rightarrow X^{2-}

When an extra electron is added to an oxygen ion, that second electron experiences a force of repulsion exerted by the negative charge of the anion. As more electrons are added without a change in the number of protons, there will be an increase in the repulsion of the negative charge within the atom. In other words, you’re forcing an electron into an already negative ion- it wont go in so willingly.

This pushes the atom into a higher energy situation, there will be repulsion between the ion and the electron and this accounts for the endothermic nature of the second electron affinity and it’s positive value.

Our answer therefore is A.