Question

The second electron affinity values for both oxygen and sulfur are unfavorable (endothermic). Explain.

Short answer

The second electron affinities of both oxygen and sulfur are unfavorable (endothermic) because adding a second electron to their completely filled subshells (2p for oxygen and 3p for sulfur) results in strong electron-electron repulsion. Overcoming this repulsion requires the absorption of energy, making the processes unfavorable.

Step-by-step solution

Understanding Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a gaseous atom. It represents the amount of energy released (exothermic) or absorbed (endothermic) when an electron is added to an isolated atom in the gas phase to form a negative ion.

Factors affecting Electron Affinity

The factors that affect electron affinity include atomic size, nuclear charge, and electron-electron repulsion in the atom's electron cloud. Generally, electron affinity increases from left to right across a period and decreases from top to bottom within a group in the periodic table.

Determine the Electron Configurations of Oxygen and Sulfur

To analyze the electron affinity of oxygen and sulfur, we need to determine their electron configurations. Oxygen has an atomic number of 8, and its electron configuration is \(1s^{2}\,2s^{2}\,2p^{4}\). Sulfur has an atomic number of 16, and its electron configuration is \(1s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{4}\).

Analyzing the Second Electron Affinities of Oxygen and Sulfur

When an electron is added to oxygen to form an O^(-) ion, it will have the electron configuration \(1s^{2}\,2s^{2}\,2p^{5}\). The added electron goes to the same energy level (second principal energy level) and occupies one of the available p-orbitals. Since the added electron is relatively closer to the nucleus, the energy change will result in the release of energy (exothermic process). However, when another electron is added to form O^(2-), the electron configuration becomes \(1s^{2}\,2s^{2}\,2p^{6}\). In this case, both the 2p subshell and the second principal energy level are completely filled. The incoming electron will experience strong electron-electron repulsion, which requires energy to overcome, making the process endothermic (unfavorable). Similarly, for sulfur, adding an electron to form S^(-) ion, the electron configuration becomes \(1s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{5}\). In this case, the process is exothermic as the added electron goes to the third principal energy level (3p subshell). However, when another electron is added to form S^(2-), it will have the electron configuration \(1s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{6}\). The electron-electron repulsion from the completely filled 3p subshell will make the process endothermic (unfavorable).

Conclusion

In summary, the second electron affinities of both oxygen and sulfur are unfavorable (endothermic) because of the strong electron-electron repulsion in the completely filled subshells (2p for oxygen and 3p for sulfur). The energy required to overcome this repulsion results in an absorption of energy, making the processes unfavorable.