We have already learnt that ionization energy is a measure of the tendency of the atom to form cation. In the similar way, the tendency of a gaseous atom to form anion is expressed in terms of electron affinity.
Electron affinity may be defined as the energy change taking place when an isolated gaseous atom accepts an electron to form a monovalent gaseous anion.
The values of electron affinity are expressed in kilo joules per mole of atoms. For example, electron affinity of chlorine is – 348 kJ mol-1, i.e.,
Cl(g) + e- à CI- (g) Eea = – 348 kJ mot-1
Depending on the element, the process of adding an electron can be either exothermic or endothermic.
The magnitude of electron affinity measures the tightness with which the atom can hold the additional electron. The large negative value of electron affinity reflects the greater tendency of an atom to accept the electron. For example, the elements of group-17 (the halogens) have very high negative electron affinities because they can attain stable noble gas electronic configuration by gaining an electron. Thus, halogens have great tendency to accept an electron. On the other hand, the elements of group-18 (the noble gases) have large positive electron affinities because the additional electron goes to the next principal shell resulting in a very unstable electronic configuration.
FACTORS AFFECTING ELECTRON AFFINITY
Some of the important factors which affect electron affinity are discussed below:
1. Nuclear Charge. Greater the magnitude of nuclear charge greater will be the attraction for the incoming electron and as a result, larger will be the negative value of electron affinity.
2. Atomic Size. Larger the size of an atom is, more will be the distance between the nucleus and the additional electron and smaller will be the negative value of electron affinity.
3. Electronic Configuration. Stable the electronic configuration of an atom is, lesser will be its tendency to accept the electron and larger will be the positive value of its electron affinity. For example, the elements having completely filled sub-levels of the valence shell have relatively stable configurations and consequently, possess large positive values of electron affinity.
VARIATION OF ELECTRON AFFINITY IN THE PERIODIC TABLE
Since experimental determination of electron affinity is not as easy as that of ionization energy, the sufficient data regarding electron affinities is not available. Consequently, the varying trends of electron affinities are not well-defined. The electron affinities of some elements are given in Table 6. 1 1 .
Table 6.11. Electron Affinities of Some Elements (kJ moi-1)
Variation in a Period
On moving across the period, the atomic size decreases and nuclear charge increases. Both these factors result into greater attraction for the incoming electron, therefore, electron
affinities tend to become more negative as we go from left to right across a period. However, some irregularities are observed among the elements of group 2, group 1 5 and group 18. The electronic configurations of these elements are relatively stable and hence, these elements have positive or very low negative electron gain enthalpies.
Variation Down a Group
On moving down a group, the atomic size as well as nuclear charge increases. But the effect of increase in atomic size is much more pronounced than that of nuclear charge and thus, the additional electron feels less attraction. Consequently, electron affinity becomes less negative on going down the group. Let us now examine the values of electron affinity for halogens as shown in Table 6. 1 1 .
It may be noted that the electron affinity becomes less negative as we go from chlorine to bromine to iodine. However, as we move from fluorine to chlorine the electron affinity becomes more negative whereas reverse was expected. This is because when an electron is added· to fluorine atom, it goes to the relatively compact second energy level. As a result, it experiences significant repulsion from the other electrons present in this shell. On the other hand, in chlorine atom, the added electron goes to the third energy shell which is relatively larger. Hence, it experiences Jess electron-electron repulsion. Therefore, electron affinity of fluorine is less negative as compared with chlorine. The unexpected trend is observed in case of many other elements of third period as their electron affinities are more negative than those of the elements of second period.
SUCCESSIVE ELECTRON AFFINITIES
When the first electron is added to the gaseous atom, it forms a uninegative ion and the energy change during the process is called .first electron affinity. Now, if an electron is added to the uninegative ion, it experiences a repulsive force from the anion. As a result, the energy has to be supplied to overcome the repulsive force. Thus, in order to add the second electron, the energy is required rather than released. Therefore, the value of second electron affinity is positive. Similarly, addition of third, fourth electrons, etc., also requires energy. Hence, the values of successive electron affinities are positive. For example, let us study the addition of electrons to oxygen atom
Example 6.11 . Which of the following will have the most negative electron affinity and which the least negative?
P, S, Cl, F.
Explain your answer.
Solution. Electron affinity generally becomes more negative across a period as we move from left to right. Therefore, among P, S and Cl the order of negative electron affinity is Cl > S > P. Within a group, electron affinity becomes less negative down a group. However, adding an electron to the 2p-orbital leads to greater repulsion than adding an electron to the larger 3p-orbital. Therefore, electron affinity of Cl is more negative than that of F. Hence, the element with most negative electron affinity is Cl the one with the least negative electron affinity is P.