Electron affinity is the energy change that occurs when a neutral atom gains an electron.
The electron affinity (Eea) of a neutral atom or molecule is defined as the amount of energy released when an electron is added to it to form a negative ion. This can be shown for the chloride ion formation below:
Cl + e− → Cl− + energy
Generally, nonmetals have a more positive electron affinity than metals. Atoms, such as Group 7 elements, whose anions are more stable than neutral atoms have a higher electron affinity. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative values. Chlorine has the highest electron affinity while mercury has the lowest.
Variation with Group and Row
Electron affinity generally increases across a period (row) in the periodic table, due to the filling of the valence shell of the atom. A trend of decreasing electron affinity down the groups in the periodic table would be expected since the additional electron is entering an orbital farther away from the nucleus. Since this electron is farther away, it should be less attracted to the nucleus and release less energy when added. However, this trend applies only to Group-1 atoms. Electron affinity follows the trend of electronegativity: fluorine (F) has a higher electron affinity than oxygen (O), and so on.
Applications of Hard-Soft Acid-Base theory
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• The electron affinity of an atom or molecule is the propensity for that particle to gain an electron.
• There are general trends in electron affinity across and down the periodic table of elements. Electron affinity generally increases across a period in the periodic table and sometimes decreases down a group.
• The chemical rationale for changes in electron affinity across the periodic table is the increased effective nuclear charge across a period and up a group.
Electron affinity: The electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion.
Electronegativity: The tendency of an atom or molecule to attract electrons to itself.