Question

Which has the more negative electron affinity, the oxygen atom or the \(\mathrm{O}^{-}\) ion? Explain your answer.

Short answer

The oxygen atom has a more negative electron affinity compared to the O⁻ ion. This is because when an electron is added to an oxygen atom, there is a stronger attraction between the positively charged protons in the nucleus and the negatively charged electron, causing more energy to be released and increasing electron affinity. In contrast, when adding an electron to the O⁻ ion, the repulsion between the negatively charged electrons in the ion results in less energy being released and therefore a less negative affinity.

Step-by-step solution

Define electron affinity

Electron affinity is the energy change associated with adding an electron to a gaseous atom or ion. When an electron is added, the energy can either be released (negative electron affinity) or absorbed (positive electron affinity). Typically, for most elements, electron affinity is negative, which means energy is released when an electron is added.

Understand the periodic trends in electron affinity

As we move across a period from left to right in the periodic table, the electron affinity generally becomes more negative, because the effective nuclear charge increases with higher atomic numbers. This means that atoms in the same period tend to have a stronger attraction to electrons and release more energy upon gaining an electron. However, electron affinity may not become more negative continuously as exceptions can be found in groups 15 and 18.

Compare electron affinity of O and O⁻ ions

When an electron is added to an oxygen atom, it forms the O⁻ ion. Here, the oxygen atom has a more negative electron affinity, and energy is released due to the increased attraction of the negatively charged electron to the positively charged protons in the nucleus. Now we consider adding an electron to the O⁻ ion to form the \(\mathrm{O}^{2-}\) ion. The energy change that occurs when adding an electron to an ion is no longer called electron affinity, but the concept is analogous. However, this time, there are additional negative charges already present due to the gained electron in the O⁻ ion. Adding another electron leads to electrostatic repulsion among the negatively charged electrons, and less energy will be released as a consequence. Therefore, the oxygen atom has a more negative electron affinity than the O⁻ ion.

Conclusion

The oxygen atom has a more negative electron affinity compared to the O⁻ ion. This is because when an electron is added to an oxygen atom, energy is released, and the increased attraction between the positively charged protons in the nucleus and the negatively charged electron increases electron affinity. However, when an electron is added to the O⁻ ion, the presence of the existing negative charge in the ion leads to less energy being released and therefore a less negative affinity.