Definition of electron affinity and its trend in a period
Answers
Electron affinity is defined as the amount of energy released or spent when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion.
Periodic Trends in Electron Affinity
Although Eea varies greatly across the periodic table, some patterns emerge. Generally, nonmetals have more positive Eea than metals. Atoms, such as Group 7 elements, whose anions are more stable than neutral atoms have a higher Eea. 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 Eea while mercury has the lowest.
Eea generally increases across a period (row) in the periodic table, due to the filling of the valence shell of the atom. For instance, within the same period, a Group-17 atom releases more energy than a Group-1 atom upon gaining an electron because the added electron creates a filled valence shell and therefore is more stable.
A trend of decreasing Eea 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.
These trends are very similar to those in ionization energy and change for similar (though opposing) reason.
