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

Electron 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.