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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.

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.

When an electron is added to an oxygen atom, it forms the O鈦� 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鈦� 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鈦� 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鈦� ion.

The oxygen atom has a more negative electron affinity compared to the O鈦� 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鈦� ion, the presence of the existing negative charge in the ion leads to less energy being released and therefore a less negative affinity.