Electron Affinity

Electron affinity is defined as the amount of energy released when an electron is added to the valence shell of the isolated gaseous species (which may be an atom or an ion or a molecule). The positive value of the electron affinity indicates that the process, i.e. X_(g) +e → X^-, is exothermic and the negative value indicates the process to be an endothermic one. Thus, the convention accepted in defining the electron affinity apparently contradicts the established convention in thermodynamics. It happens so because in the given definition the term, 'energy release' has been coined but in all electron affinity processes energy release may not occur and in some cases energy absorption may occur.

Thus,

F + e → F^-, ΔH = -328 KJmol^-1 and EA = 328 KJmol^-1

N + e → N^-, ΔH = 20.3 KJmol^-1 and EA = -20.3 KJmol^-1

From the definition, it appears that the electron affinity is the reverse process of the ionization process, i.e.

X^- → X + e, IE(X^-) = ΔH = EA(X).

In fact, the ionization process (which accumulates positive charge) and electron affinity (which accumulates negative charge) are related in the same way in the relation involving the enthalpy change (ΔH) of the process and the amount of charge accumulation (q) on the species. By neglecting the higher terms, we get the following quadratic equation:

ΔH = αq + βq²

Where, ΔH (thermodynamic convention) = ∑IE or ∑EA, and q is the charge on the species. ΔH relates with EA when q is –ve, and ΔH relates with IE when q is +ve. The magnitude of β is large for the non-polarisable and small species while it is small for the large and polarizable species. For example,

β(F) > β(Cl) > β(Br)

As in the case of ionization energies, the successor electron affinity is such as EA₁, EA₂, EA₃.... can also be defined. EA₂ leads to the di-negative species while EA₃ produces the tri-negative species. It is always more difficult to place an additional electron to a species which is already bearing a negative charge compared to the process involving a neutral species. This is why; higher electron affinity is always thermodynamically less favourable. In fact, EA₂ for all species are negative, i.e. the processes are endothermic or the endothermic nature outweighs the exothermic nature in the formation of O^2-, S^2- etc.

O + e → O^- (ΔH = -141 KJmol^-1)

O- + e → O^2- (ΔH = +780 KJmol^-1)

Here, EA₁(O) → 141 KJmol^-1 while the EA₂(O) = -780 KJmol^-1

S + e → S^- (ΔH = -200 KJmol^-1)

S^- + e → S^2- (ΔH = +492 KJmol^-1)

Here, EA₁(S) = 200 KJmol^-1 while EA₂(S) = -492 KJmol^-1

First and second electron affinity of elements

The factors which control the ionization process come under consideration for the process measuring the electron affinity. In general, the factors favouring the ionization process disfavour the electron affinity process, i.e. higher ionization energy leads to higher electron affinity. But the species having half-filled or full-filled level possess high ionisation energy and lower electron affinities. The important factors are:

Size of the species: If Z* for electrons at the periphery is more or less the same for different species (e.g. in a group of the representative elements), the electrostatic attraction towards the nucleus experience by the electrons at the periphery is less for the largest species. In fact, for the representative elements, in a group the effect of slight increasing trend of Z* is less important than the effect of increasing trained of n (principle quantum number) when we move from top to bottom. For such systems, the accommodation of an additional electron which is to be bound at the periphery by the attractive force of the nucleus is disfavoured more for the larger species. Thus in general, the smaller atoms in a group possess higher electron affinities.

Effective nuclear charge: The species having highest Z* the periphery show higher electron affinities.

Nature of the orbital into which the new electron gets accommodation: The orbitals which can better penetrate into the electron clouds are more suitable to house the incoming electron. Of the incoming electron follows the sequence: ns > np > nd > nf.

Nature of the electronic configuration: If the already bare and extra stability due to either the half-filled or full-filled level, then such species are very much reluctant in accepting the incoming electron. On the other hand, if the newly added electron creates the half-filled or full-filled level, the process is favoured.

This aspect disfavours the Gr IIA (2) elements (ns²), Gr II B (12) elements [(n-1)d¹⁰ns²], Gr VA (15) elements [ns²np³], noble gases [ns²np⁶] but favours the Gr VIIA (17) elements [ns²np⁵]

Variation of electron affinity in the periodic table

In a period: The effective nuclear charge increases and size decreases with the increase of atomic number in a period. This is why, the electron affinity, in general increases in a period. In fact it reaches the climax for the Gr VIIA (17) elements.

Because of the special effect of the electronic configuration, the general trend is violated in some cases. These are:

Gr IIA (2) metals: Be, Mg, Ca, etc. are having the ns² orbital structure. Hence accommodation of the incoming electron brings the configuration ns²np¹. This process is disfavoured in two ways: The addition of the new electron destroys the full-filled level structure and accommodation of the new electron occurs in the p level which is less penetrating. Here it is worth noting that for alkali metals, the accommodation occurs in the ns level giving rise to ns² configuration. Thus the process is favoured more in the Gr IA(1) elements compared to the Gr IIA (2) elements.

Transition series: In the transition series, the electron affinity values are given below in KJmol^-1

Sc = 0     Ti = 20    V = 50    Cr = 64    Mn = 0    Fe = 24     Co = 70    Cu = 118    Zn = 0

The drop in Mn arises as the starting species is stabilized due to its half-filed d level. The incoming electron breaks down this extra stability. The drop in Zn arises due to its full filled structure.

Post-lanthanides: Due to the lanthanide contraction, among the post-lanthanides, the electron affinity is unusually high.

In a group: for the representative elements moving down in a group the size gradually increases due to the opening of new principal quantum number. In fact, the effect of slight increasing trend of Z* is less important compared to the effect of increasing principle quantum number (n) moving down in a group. This is why, the electron affinity falls down with the increase of atomic number.

There are some exceptions and these are:

F < Cl, O < S, N < P, B < Al: Here it is interesting to note that the electronegativity sequence is in the opposite order and it is expected from the size sequence. To explain the observed sequence of electron affinity we are to consider the other factors. Though the electrostatic attractive pull towards the nucleus favours the second period elements more compared to the third period elements The added electron creates and unfavorable effect, i.e. electron-electron repulsion, which is more for the second period elements because of their smaller sizes. The repulsion force is not so large in the third period elements because of their larger size. In addition to this, possibility of delocalization of the increased electron density in the vacant 3d orbital reduces the repulsions for the third period elements.  This possibility to reduce the electron-electron repulsion through the participation of d orbitals is significant for the heavier species, e.g. S, Cl, in the third period. On the other hand, for the second period elements, this mechanism does not operate because of the absence of any suitable d orbital. Thus the reduced electron electron repulsions in the third period elements outweigh the favour due to the larger electrostatic attraction experienced in their corresponding second period elements.

Reference

1) Concise inorganic chemistry by J. D. Lee.

2) Inorganic Chemistry by James E. Huheey, Ellen A Keither, Richard L. Keither, Okhil K. Medhi.

3) Shriver and Atkins Inorganic Chemistry.