ELECTRON AFAFINITY

    The chemical properties of the elements are also determined by the tendency of their atoms to accept an additional electron. This tendency is measured by the electron affinity, i.e., the energy released when a gaseous atom accepts an electron forming a free gaseous ion.

X(g)+e = X-(g) + energy released

    The energy released is measured in electron volt per atom (e. v. /atom) or kilo joule per mole (k. j./mole). Thus, larger is the energy released, the greater is the electron affinity. in other words, it is a measure of the force with which the added electron is bound by the nucleus of the atom.

There may be first, second or higher electron affinity just like ionization potential. 

         O(g)+e = O-(g) + 1.47 e. v.

         O- (g)+e = O2- (g) - 8.75 e. v.

    The second and higher electron affinities are always negative because there occurs a repulsion between negatively charged ion and electron being added and energy has to be given for this addition of electron.

MEASUREMENT OF ELECTRON AFFINITY

The electron affinity of an elements can be determined by Born Haber cycle. With the help of this cycle, the following equation has been deduced for the determination of lattice energy of ionic crystal, MX:     

      △H = S+I+1/2D +E+U

where S = Sublimation energy of metal M.

           I = Dissociation energy of X2 molecule.

          E = Electron affinity of an element X.

          U = Lattice energy of ionic crystal, MX.

         Δ H = Heat of formation of ionic crystal, MX.

The electron affinity of an element X can be calculated if the values of different thermochemical quantities are known. 


FACTORS AFFECTING THE VALUE OF ELECTRON: AFFINITY:

In general, electron affinity depends upon the following factors :

  1. Size of the atom :  The larger the size of the atom, lesser will be the force of attraction between the nucleus and additional electron. Consequently, electron affinity will be low. Thus, bigger is the size of the atom, lesser will be the electron affinity.
  2. Charge on the nucleus: The higher is the charge on the nucleus, the greater will be the force of attraction between the nucleus and additional electron, consequently electron affinity will be high.
  3. Electronic configuration: The atoms having stable electronic configuration, e.g., completed shells, completed of half completed subshells do not have a tendency to accept additional electrons. Such atoms have either zero or very low electron affinity.
     The electron affinities of some of the elements have been determined by absorption spectroscopy and are given :


Applications in explaining the chemical behavior: 

  1. The elements having higher value of electron affinity can easily accept additional electron. These elements are electronegative. They can easily from anions and ionic compounds.
  2. The elements having higher value of electron affinity act as strong oxidizing agents, e. g. Halogens.
  3. The relative value of ionization potential and electron affinity give an idea about the nature of bond formed between two elements.







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