Final answer:
Electron affinity is a measure of the energy released when an extra electron is added to an atom. While the general trend is for electron affinities to become more negative from left to right on the periodic table and less negative from top to bottom of a group, there are exceptions. The anomaly in the electron affinity of X2H and X18O occurs when there is a difference in the electron affinity trend between isoelectronic species.
Step-by-step explanation:
Electron affinity is a measure of the energy released when an extra electron is added to an atom. It is the opposite of ionization energy. Generally, electron affinities become more negative as you move from left to right on the periodic table, indicating that more energy is released when an electron is added.
However, there are some exceptions to this trend, including the anomaly mentioned in the electron affinity of X2H and X18O.
Typically, electron affinities become less negative from top to bottom of a group. This means that less energy is released when an electron is added as you move down the group. However, again there can be exceptions to this trend.
The anomaly in the electron affinity of X2H and X18O occurs when there is a difference in the electron affinity trend between isoelectronic species. Isoelectronic species have the same number of electrons but different atomic numbers.
In this case, X2H represents a smaller atom with a higher effective nuclear charge compared to X18O, which represents a larger atom with a lower effective nuclear charge. Due to the smaller size and higher effective nuclear charge of X2H, its electron affinity is greater than that of X18O, contradicting the general trend.