The electron affinity for sulfur is more negative than that for oxygen. How do you account for this?

First, it is important to understand the periodic trends of electron affinity. Electron affinity generally increases from left to right across a period and decreases from top to bottom within a group. This is because the effective nuclear charge (the net positive charge experienced by an electron) increases from left to right, resulting in a stronger attraction between the electron and the nucleus. Additionally, electron shielding decreases down the group, and the atomic radii increase, reducing the attraction between incoming electrons and the nucleus. Oxygen and sulfur are elements in Group 16, also known as the chalcogens, and are in periods 2 and 3, respectively. We can see that sulfur is below oxygen in this group.

Although the general trend states that electron affinity should decrease as we move down a group, oxygen and sulfur are an exception to this rule. Sulfur has a more negative electron affinity than oxygen. This deviation from the general trend is due to certain factors specific to these elements, which we will discuss in the next step.

When trying to determine why sulfur has a more negative electron affinity compared to oxygen, it is important to take into account their electron configurations. Oxygen has the electron configuration of [He]2s^2 2p^4, while sulfur has the electron configuration of [Ne]3s^2 3p^4. When an electron is added to oxygen, the extra electron will occupy one of the existing half-filled 2p orbitals. On the other hand, when an electron is added to sulfur, it will occupy an empty 3p orbital. In oxygen's case, the negatively charged electron will experience some electron-electron repulsion with the other electrons in its respective orbital. This repulsion reduces the energy that is released when the electron is added.

Due to the electron-electron repulsion experienced in oxygen's case when an electron is added, less energy is released in the process compared to the energy released when an electron is added to sulfur. The electron affinity is the measure of the energy change that occurs after an electron is added to a neutral atom. As the energy released when an electron is added to sulfur is more than the energy released when an electron is added to oxygen, sulfur's electron affinity will be more negative than oxygen's electron affinity. In conclusion, the more negative electron affinity of sulfur compared to oxygen can be explained by the electron-electron repulsion experienced in the case of oxygen, which results in less energy being released when an electron is added to it. This is an exception to the general periodic trend of electron affinity decreasing down a group.