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What is electronic affinity?

What is electronic affinity?

The electron affinity is defined as the amount of energy released when one mole of atoms in a gaseous state is combined with one mole of electrons to form one mole of anion, also in gaseous state. In other words, it refers to the negative of the enthalpy change of the following process:

As its name implies, electron affinity (AE) is a measure of the tendency of an atom to bond with an electron. That is, it measures the affinity of an atom for electrons.

Interpretation of electronic affinity

Due to the way it is defined, a high electron affinity implies that the enthalpy variation is very negative. This in turn indicates that the process is energetically favorable and that the products are more stable than the reactants. For this reason, we could also say that electron affinity is an indirect measure of the stability of an anion.

The greater the electron affinity of an atom, the greater its tendency to form the anion. This is why atoms such as chlorine, whose electronic affinity is 349 kJ / mol, tend to easily form anions (in this case the chloride anion), while other atoms such as magnesium, whose electronic affinity is negative, do not form anions. .

Electron affinity is often thought of as the opposite of ionization energy (the tendency of a gaseous atom to lose an electron), but this is not the case. Consider, for example, an X atom.

Its electron affinity refers to the energy change of the process shown earlier in the first equation. However, its ionization energy refers to the energy change when the atom loses an electron:

Although this reaction seems to be the opposite reaction to the previous one, it is not like that (note that the electric charges are not the same in either case).

What determines electron affinity?

To know what characteristics of an atom influence the value of its electronic affinity, the stability of the original atom must be considered, as well as that of the anion that is formed. If the anion is more stable than the atom, then the electron affinity will be high, otherwise it will be low or even negative.

But how do you know which of the two species is more stable? For that, we rely on two factors:

  1. Electronic configuration. There are electronic configurations more stable than others. In general, the full shell configuration (such as noble gases) is the most stable of all. This is followed by the semi-full shell configuration, in which all the orbitals in the valence shell have half as many electrons as they could have (for example, 4s 1 4p 3 ).
  2. Electronic repulsion. If you compare an anion of charge -1, with an anion of charge -2, in the second case there will be much more repulsion between the electrons, which destabilizes the anion.

Periodic trend of electron affinity

Electronic affinity is one of the periodic properties of elements. That is, it is a property that varies predictably from one element to another based on its position on the periodic table. Generally speaking, electron affinity increases as the size of the atom decreases.

Variation of electron affinity over a period

At least for the representative elements (those belonging to the s and p blocks of the periodic table), it can be observed that the electronic affinity has a general tendency to increase from left to right, due to the increase in the effective nuclear charge that is capable of attract electrons more strongly.

For example, if we take the 3rd period of the periodic table, we can see that the electronic affinity of Li (60 kJ / mol) is less than that of oxygen (141 kJ / mol) and this is less than that of fluorine (328 kJ / mol).

Exceptions

The above rule is not always true.

First, when moving from alkali metals to alkaline earth metals, the electron affinity decreases. This is because for alkali metals (electronic configuration ns 1 ) it is favorable to capture an electron, since this way they would finish filling its s orbital.

In the case of alkaline earths (electronic configuration ns 2 ), capturing an electron is unfavorable because they already have their s orbital full. The same happens when going from halogens (which have the highest electronic affinities of all elements) to noble gases.

Variation of electron affinity throughout a group

In the case of groups, the behavior is even less predictable. The general rule of thumb is that AE increases from the bottom up, in the same direction that the atomic radius decreases. For alkali metals and halogens, this rule works quite well. However, this is not the case with most other groups.

Examples of electronic affinity of some representative elements

The following table shows the electron affinity values ​​in (kJ / mol) of the representative elements ordered by group:

Below are some examples of electron affinity along with the reaction they refer to:

1. Electron Affinity for Hydrogen

2. Electronic affinity of oxygen

3. Electronic affinity of an anion

Another common example is the case of the electron affinity of an anion such as O  . The AE in this case is given by the energy associated with the following process:

As can be seen, this electronic affinity is strongly negative, despite the fact that the O 2- ion has the electronic configuration of neon (a noble gas) and is a very common ion in many ionic solids.

The reason is that the repulsion of negative charges in O 2- destabilizes said ion in the gaseous state, but in the solid state the charge is stabilized by the cations that surround it.

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