Electron Affinity (Chemistry Glossary Definition)

Electron affinity reflects the ability of an atom to accept an electron. It is the energy change that occurs when an electron is added to a gaseous atom. Atoms with stronger effective nuclear charge have greater electron affinity. The reaction that occurs when an atom takes an electron may be represented as: X e−  Ã¢â€ â€™ X−   energy Another way to define electron affinity is as the amount of energy needed to remove an electron from a singly charged negative ion: X−  Ã¢â€ â€™ X e− Key Takeaways: Electron Affinity Definition and Trend Electron affinity is the amount of energy required to detach one electron from a negatively charged ion of an atom or molecule.It is indicated using the symbol Ea and is usually expressed in units of kJ/mol.Electron affinity follows a trend on the periodic table. It increases moving down a column or group and also increases moving from left to right across a row or period (except for the noble gases).The value may be either positive or negative. A negative electron affinity means energy must be input in order to attach an electron to the ion. Here, electron capture is an endothermic process. If electron affinity is positive, the process is exothermic and occurs spontaneously. Electron Affinity Trend Electron affinity is one of the trends that can be predicted using the organization of elements in the periodic table. Electron affinity increases moving down an element group (periodic table column).Electron affinity generally increases moving left to right across an element period (periodic table row). The exception is the noble gases, which are in the last column of the table. Each of these elements has a completely filled valence electron shell and an electron affinity approaching zero. Nonmetals typically have higher electron affinity values than metals. Chlorine strongly attracts electrons. Mercury is the element with atoms that most weakly attract an electron. Electron affinity is more difficult to predict in molecules because their electronic structure is more complicated. Uses of Electron Affinity Keep in mind, electron affinity values only apply to gaseous atoms and molecules because the electron energy levels of liquids and solids are altered by interaction with other atoms and molecules. Even so, electron affinity has practical applications. It is used to measure chemical hardness, a measure of how charged and readily polarized Lewis acids and bases are. Its also used to predict electronic chemical potential. The primary use of electron affinity values is to determine whether an atom or molecule will act as an electron acceptor or an electron donor and whether a pair of reactants will participate in charge-transfer reactions. Electron Affinity Sign Convention Electron affinity is most often reported in units of kilojoule per mole (kJ/mol). Sometimes the values are given in terms of magnitudes relative to each other. If the value of electron affinity or Eea is negative, it means energy is required to attach an electron. Negative values are seen for the nitrogen atom and also for most captures of second electrons. It can also be seen for surfaces, such as diamond. For a negative value, the electron capture is an endothermic process: Eea   −ΔE(attach) The same equation applies if Eea  has a positive value. In this situation the change ΔE  has a negative value and indicates an exothermic process. Electron capture for most gas atoms (except noble gases) releases energy and is exothermic. One way to remember capturing an electron has a negative ΔE  is to remember energy is let go or released. Remember: ΔE  and Eea  have opposite signs! Example Electron Affinity Calculation The electron affinity of hydrogen is ΔH in the reaction: H(g) e- → H-(g); ΔH -73 kJ/mol, so the electron affinity of hydrogen is 73 kJ/mol. The plus sign isnt cited, though, so the Eea  is simply written as 73 kJ/mol. Sources Anslyn, Eric V.; Dougherty, Dennis A.  (2006). Modern Physical Organic Chemistry. University Science Books. ISBN 978-1-891389-31-3.Atkins, Peter; Jones, Loretta (2010). Chemical Principles the Quest for Insight. Freeman, New York. ISBN 978-1-4292-1955-6.Himpsel, F.; Knapp, J.; Vanvechten, J.; Eastman, D. (1979). Quantum photoyield of diamond(111)—A stable negative-affinity emitter. Physical Review B. 20 (2): 624. doi:10.1103/PhysRevB.20.624Tro, Nivaldo J. (2008). Chemistry: A Molecular Approach (2nd Ed.). New Jersey: Pearson Prentice Hall. ISBN 0-13-100065-9.IUPAC (1997). Compendium of Chemical Terminology (2nd Ed.) (the Gold Book). doi:10.1351/goldbook.E01977