Electron gain enthalpy

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In chemistry, electron gain enthalpy (related to but not equal to electron affinity) is the enthalpy change when an electron is added to a neutral isolated gaseous atom to form a gaseous anion with a -1 charge.

All elements have a negative electron gain enthalpy, but older texts mistakenly report that some elements such as inert gases have positive EGE, meaning they repel electrons. This is not recognised by modern chemists. This means they do not require energy to gain an electron; instead, they release energy. Atoms whose anions are relatively more stable than neutral atoms have a more negative EGE. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. EGE of noble gases are close to 0.

Although EGE vary in a chaotic manner across the table, some patterns emerge. Generally, nonmetals have more negative EGE than metals.

EGE trends:

The EGEs increase across a row (since the radius slightly decreases, because of the increased attraction from the nucleus, and the number of electrons in the top shell increases, helping the atom reach maximum stability) in the periodic table and decrease going down a family (because of a large increase in radius and number of electron that decrease the stability of the atom, repulsing each other).

EGEs are not limited to the elements but also apply to molecules. For instance the electron affinity for benzene is negative, that of naphthalene also negative and that of anthracene,phenanthrene and pyrene is positive. In silico experiments show that the electron affinity of hexacyanobenzene surpasses that of fullerene <ref>Remarkable electron accepting properties of the simplest benzenoid cyanocarbons: hexacyanobenzene, octacyanonaphthalene and decacyanoanthracene Xiuhui Zhang, Qianshu Li, Justin B. Ingels, Andrew C. Simmonett, Steven E. Wheeler, Yaoming Xie, R. Bruce King, Henry F. Schaefer III and F. Albert Cotton Chemical Communications, 2006, 758 - 760 Abstract</ref>.