por What is electron affinity?
The Electronic affinity It is defined as the amount of energy released when one mole of atoms in the gaseous state combines with one mole of electrons to form one mole of anions, also in the gaseous state. In other words, it refers to the negative of the enthalpy change of the following process:
As its name implies, electron affinity (EA) 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.
Electron affinity interpretation
Due to the way it is defined, a high electron affinity implies that the enthalpy change 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 the electron affinity is an indirect measure of the stability of an anion.
The greater the electron affinity of an atom, the greater tendency it will have to form the anion. This is why atoms such as chlorine, whose electron affinity is 349 kJ/mol, tend to easily form anions (in this case the chloride anion), while other atoms such as magnesium, whose electron affinity is negative, do not form anions. .
Clarification about electron affinity and ionization energy
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 so. Consider, for example, an atom X.
Its electron affinity refers to the energy change of the process shown above in the first equation. However, its ionization energy refers to the change in energy when the atom loses an electron:
Although this reaction appears to be the opposite reaction to the previous one, it is not (note that the electric charges are not the same in either case).
What determines electron affinity?
To know which characteristics of an atom influence the value of its electron 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:
Electronic configuration. Some electronic configurations are more stable than others. In general, the shell-filled configuration (like that of the noble gases) is the most stable of all. This is then followed by the half-full shell configuration, in which all valence shell orbitals have half as many electrons as they could (for example, 4s14p3).
electronic repulsion. If an anion of charge -1 is compared 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
Electron 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 in the periodic table. Generally speaking, electron affinity increases as the size of the atom decreases.
In this sense, the size of the atoms varies depending on the period and the group in which it is found, so its electronic affinity also varies as explained below:
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 electron 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 electron 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 /mole).
exceptions
The previous rule is not always fulfilled.
First, when going from the alkali metals to the alkaline earth metals, the electron affinity decreases. This is because for alkali metals (electronic configuration ns1) it is favorable to capture an electron, since that way they would finish filling their s orbital.
In the case of alkaline earths (electronic configuration ns2) capturing an electron is unfavorable because they already have their s orbital filled. The same happens when going from the halogens (which have the highest electron affinities of all the elements) to the noble gases.
Variation of electron affinity across a group
In the case of groups, the behavior is even less predictable. The general rule is that AE increases from bottom to top, in the same direction as atomic radius decreases. For alkali metals and halogens, this rule holds quite well. However, this is not the case with most other groups.
Examples of electron affinity of some representative elements
The following table shows the electron affinity values in (kJ/mol) of the representative elements sorted by group:
Here are some examples of electron affinity along with the reaction they refer to:
1. Electron affinity of hydrogen
2. Electron affinity of oxygen
3. Electron 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 electron affinity is strongly negative, despite the fact that the O2- 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 the negative charges in O2- destabilizes said ion in the gaseous state, but in the solid state the charge is stabilized by the cations that surround it.