The behavior of atoms is governed by the attraction between the positively charged nucleus and the negatively charged electrons. When atoms engage in chemical interactions, they often gain or lose electrons, which is accompanied by a change in energy. Understanding these energy dynamics provides a framework for predicting how different elements will react and form chemical bonds.
Defining Electron Affinity
Electron affinity (EA) measures the energy change that occurs when a neutral atom in the gaseous state gains an electron. The process involves one mole of gaseous atoms accepting an electron to become a uninegative ion. Since the incoming electron is attracted by the positive nucleus, this process usually releases energy.
This energy release is typically reported as a negative value. A higher electron affinity corresponds to a greater release of energy and a more negative value, signifying a stronger attraction between the atom and the new electron.
General Periodic Trends Governing Electron Affinity
Electron affinity values follow predictable patterns across the periodic table, driven primarily by nuclear charge and atomic size. Moving from left to right across a period, electron affinity generally increases. This occurs because the increasing number of protons creates a stronger effective nuclear charge, pulling the valence electrons and any incoming electron closer to the nucleus.
Conversely, moving down a group, electron affinity tends to decrease. As atomic number increases, electrons are added to successively larger energy shells, increasing the overall size of the atom. The incoming electron is farther away from the positive pull of the nucleus, resulting in a weaker attraction and less energy being released. These trends suggest that the smallest atoms in the upper right of the table should exhibit the highest electron affinities.
The Highest Electron Affinity Element and the Fluorine Anomaly
The element with the highest electron affinity is Chlorine (\(\text{Cl}\)). This is an exception to the general trend, as the smaller element directly above it, Fluorine (\(\text{F}\)), should theoretically have the highest value. Chlorine’s electron affinity is approximately \(-349\) kilojoules per mole, which is slightly higher than Fluorine’s value of approximately \(-328\) kilojoules per mole.
This unexpected result is known as the Fluorine anomaly, explained by Fluorine’s unusually small atomic size. Fluorine’s valence electrons occupy the compact second energy shell. When a new electron attempts to join this shell, it experiences significant electron-electron repulsion from the electrons already present.
This strong repulsive force diminishes the attraction between the nucleus and the incoming electron, reducing the energy released. Chlorine, located in the third period, is a larger atom with valence electrons in the third energy shell. This larger volume easily accommodates the incoming electron with much less electron-electron repulsion, allowing Chlorine to release a greater amount of energy and achieve the highest electron affinity.