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Electron Affinity

Module by: Andrew R. Barron. E-mail the author

The electron affinity (EA) of an element is defined as the energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion.

graphics1.jpg
(1)

Electron affinities are more difficult to measure than ionization energies and are usually less accurately known. Electron affinities are large and negative for elements such as fluorine and oxygen, and small and positive for metals.

Electron affinities generally become smaller as you go down a Group of the periodic table (Table 1). This is because the electron being added to the atom is placed in a larger orbital, where it spends less time near the nucleus of the atom, and also the number of electrons on an atom increases as we go down a column, so the force of repulsion between the electron being added and the electrons already present on a neutral atom becomes larger. Electron affinities are further complicated since the repulsion between the electron being added to the atom and the electrons already present on the atom depends on the volume of the atom. Thus, for the nonmetals in Groups 6 (VIA) and 7 (VIIA), this force of repulsion is largest for the very smallest atoms in these columns: oxygen and fluorine. As a result, these elements have a smaller electron affinity than the elements below them in these columns as shown in Table 1.

Table 1: The electron affinity for the non-metallic halogens.
Element Electron affinity (kJ/mol)
F -322
Cl -349
Br -325
I -295

Although there is a general trend that for Group 1 (IA) to Group 17 (VIIA) elements the electron affinity increases across the Periodic table from left to right, the details of the trend are more complex. As may be seen from Figure 1, there is a cyclic trend. The explanation of this is a consequence of the unusually stable electron configurations exhibited by atoms with filled or half filled shells, i.e., helium, beryllium, nitrogen and neon (see Table 2). These configurations are so stable that it actually takes energy to force one of these elements to pick up an extra electron to form a negative ion.

Figure 1: Plot of the electron affinity for the elements hydrogen to fluorine. N.B. Values for helium, beryllium, nitrogen, and neon are not known with any accuracy but are all positive.
Figure 1 (graphics2.jpg)
Table 2: Electron affinities of the elements hydrogen to neon. N.B. Values for helium, beryllium, nitrogen, and neon are not known with any accuracy but are all positive.
Element Electron affinity (kJ/mol) Electron configuration
H -72.8 1s1
He +ve 1s2
Li -59.8 [He] 2s1
Be +ve [He] 2s2
B -27 [He] 2s2 2p1
C -122.3 [He] 2s2 2p2
N +ve [He] 2s2 2p3
O -141.1 [He] 2s2 2p4
F -328.0 [He] 2s2 2p5
Ne +ve [He] 2s2 2p6

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Definition of a lens

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A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

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