Ionization energies and electron affinities

Ionization energies

The ionization energy of hydrogen

 H atom has only one electron, no additional ionization processes can occur. For multi-electron atoms, successive ionizations are possible.

   The first ionization energy, IE₁, of an atom is the internal energy change at 0 K, Δ U(0 K), associated with the removal of the first valence electron (equation 1.17); the energy change is defined for a gas phase process. The units are kJ mol^-1 or electron volts (eV).

   ( 1.1 )

It is often necessary to incorporate ionization energies into thermochemical calculations (e.g. Born–Haber or Hess cycles) and it is convenient to define an associated enthalpy change, ΔH(298K).

 Since the difference between ΔH(298K) and ΔU(0K) is very small , values of IE can be used in thermochemical cycles so long as extremely accurate answers are not required.

The second ionization energy, IE₂, of an atom refers to step 1.2; note that this is equivalent to the first ionization of the ion X⁺‏. Equation 1.2 describes the step corresponding to the third ionization energy, IE₃, of X, and successive ionizations are similarly defined:

       (1.2)

 Figure 1. shows the variation in the values of IE1 as a function of Z. Several repeating patterns are apparent and some features to note are:

•  the high values of IE₁ associated with the noble gases;
•  the very low values of IE₁ associated with the group 1 elements;
•  the general increase in values of IE₁ as a given period is crossed;
•  the discontinuity in values of IE₁ on going from an element in group 15 to its neighbour in group 16;
•  the decrease in values of IE₁ on going from an element in group 2 or 12 to its neighbour in group 13.
•  the rather similar values of IE₁ for a given row of d-block elements.

Each of these trends can be rationalized in terms of ground state electronic configurations. The noble gases (except for He) possess ns² np⁶ configurations which are particularly stable  and removal of an electron requires a great deal of energy. The ionization of a group 1 element involves loss of an electron from a singly occupied ns orbital with the resultant X‏ ion possessing a noble gas configuration.

The general increase in IE₁ across a given period is a consequence of an increase in Z_eff. A group 15 element has a ground state electronic configuration ns2np3 and the np level is half-occupied. A certain stability is associated with such configurations and it is more difficult to ionize a group 15 element than its group 16 neighbour.   In going from Be (group 2) to B (group 13), there is a marked decrease in IE1 and this may be attributed to the relative stability of the filled shell 2s² configuration compared with the 2s² 2p¹ arrangement; similarly, in going from Zn (group 12) to Ga (group 13), we need to consider the difference between 4s² 3d¹⁰ and 4s² 3d¹⁰ 4p¹ configurations.

The noble gases (except for He) possess ns² np⁶ configurations which are particularly stable and removal of an electron requires a great deal of energy. The ionization of a group 1 element involves loss of an electron from a singly occupied ns orbital with the resultant X‏ ion possessing a noble gas configuration.

The general increase in IE₁ across a given period is a consequence of an increase in Z_eff. A group 15 element has a ground state electronic configuration ns² np³ and the np level is half-occupied. A certain stability is associated with such configurations and it is more difficult to ionize a group 15 element than its group 16 neighbour. In  oing from Be (group 2) to B (group 13), there is a marked decrease in IE₁ and this may be attributed to the relative stability of the filled shell 2s² configuration compared with the 2s² 2p¹ arrangement; similarly, in going from Zn (group 12) to Ga (group 13), we need to consider the difference between 4s² 3d10 and 4s² 3d¹⁰ 4p¹ configurations.

Fig. 1. The values of the first ionization energies of the elements up to Rn.