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In general, when an electron is added to a neutral atom, energy is released, and the process is exothermic. However, the second electron affinity can be endothermic if it is energetically unfavorable for the atom to gain an additional electron. In the case of oxygen and sulfur, their second electron affinities are endothermic, which we need to explain.

Recall the electronic configurations of oxygen and sulfur: Oxygen: \(1s^2 2s^2 2p^4\) (8 electrons) Sulfur: \(1s^2 2s^2 2p^6 3s^2 3p^4\) (16 electrons) Both oxygen and sulfur have six valence electrons.

When the first electron is added, both oxygen and sulfur form negatively charged ions that have stable configurations with filled p orbitals: Oxygen: \(1s^2 2s^2 2p^6\) (9 electrons; O^- ion) Sulfur: \(1s^2 2s^2 2p^6 3s^2 3p^6\) (17 electrons; S^- ion) However, the addition of a second electron to these ions becomes energetically unfavorable. This is because, the incoming electron faces repulsion from the other electrons already in the ion, and there is no significant stabilization by achieving a filled subshell or lower energy orbital. The negatively charged ions will repel the incoming electrons more strongly than neutral atoms, which require additional energy to overcome the electrostatic repulsion between the negatively charged ion and the incoming electron. That's why the second electron affinity value for both oxygen and sulfur is endothermic, as energy is absorbed, making it unfavorable.