What Is Electron Affinity?

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What is electron affinity? What does electron affinity measure? What are the electron affinity of main elements and information about electron affinity.

electron-affinityElectron Affinity

The energy change that occurs when an electron is added to a gaseous atom is called the electron affinity. The attraction of the nucleus of an atom for an additional electron brings about a release of energy (exothermic process) when a gaseous atom gains a single electron. This energy is a measure of electron affinity of an element.

\displaystyle \begin{array}{l}X\left( g \right)+{{e}^{-}}\to {{X}^{-}}\left( g \right)+energy\\H\left( g \right)+{{e}^{-}}\to {{H}^{-}}\left( g \right)+1,2x{{10}^{-22}}kJ\\O\left( g \right)+{{e}^{-}}\to {{O}^{-}}\left( g \right)+2,36x{{10}^{-22}}kJ\\Cl\left( g \right)+{{e}^{-}}\to C{{l}^{-}}\left( g \right)+6,04x{{10}^{-22}}kJ\end{array}

The trends in electron affinity as we proceed through the periodic table are not as evindent as they were for ionization energy. The halogens, which are one electron shy of a filled p subshell, have the most negative electron affinities. By gaining an electron, a halogen atom forms a stable negative ion that has a noble gas configuration. The addition of an electron to a noble gas, however, requires that the electron reşide in a higher – energy subshell that is empty in the neutral atom. Because occupying a higher – energy subshell is energetically very unfavorable, the electron affinity is highly positive. The electron affinities of Be and Mg are positive for the same reason. The electron affinity > 0 indicates that the negative ion is higher in energy than the separated atom and electron.

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