Question

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

Short answer

The ${\mathrm{O}}^{-}$ ion has a more negative electron affinity than the neutral oxygen atom. This is because adding an electron to the ${\mathrm{O}}^{-}$ ion results in a more stable, completely-filled outer electron shell, which releases more energy and makes its electron affinity more negative.

Step-by-step solution

Determine the electron configurations for O and ${\mathrm{O}}^{-}$

Oxygen has 8 electrons in its neutral state. The electron configuration of a neutral oxygen atom is 1s²2s²2p⁴, with 2 electrons in the 1s orbital, 2 electrons in the 2s orbital, and 4 electrons in the 2p orbital. The oxygen ion with a -1 charge (${\mathrm{O}}^{-}$) has an additional electron, so its electron configuration becomes 1s²2s²2p⁵.

Analyze the stability of the electron configurations

An element tends to be more stable when its outer electron shell is filled. In the case of oxygen and its ion, neither has a completely filled outer electron shell (which would be 2s²2p⁶). The oxygen atom has 4 electrons in its 2p orbital and ${\mathrm{O}}^{-}$ has 5 electrons in the 2p orbital. When adding an electron to O and ${\mathrm{O}}^{-}$, a neutral O will have its outer electron shell closer to being filled (with 5 electrons in the 2p orbital), while ${\mathrm{O}}^{-}$ will have a completely filled electron shell (with 6 electrons in the 2p orbital).

Compare electron affinities

From the previous step, we know that the neutral oxygen atom has a half-filled electron shell and will become more stable as more electrons are added. When an electron is added to an oxygen atom, energy is released, making its electron affinity negative. Since adding an electron to the ${\mathrm{O}}^{-}$ ion will make it completely stable as it fills the outer electron shell, more energy is released during this process. This means the electron affinity of the ${\mathrm{O}}^{-}$ ion will be more negative than the electron affinity of the oxygen atom.

Conclude which one has more negative electron affinity

Based on the stability analysis and comparison of electron affinities, it can be concluded that the ${\mathrm{O}}^{-}$ ion has the more negative electron affinity than the neutral oxygen atom. This is because adding an electron to the ${\mathrm{O}}^{-}$ ion results in a more stable, completely-filled outer electron shell, thus releasing more energy and making its electron affinity more negative.