What is electron affinity

Electro affinity

The term electron affinity is understood to mean the energy that is released when an atom accepts an additional electron. The electron affinity is thus the counterpart to the ionization energy. The electron affinity can be understood as a measure of how strongly a neutral atom (or a neutral molecule) can bind another (additional) electron[1].

An alternative definition is the following: electron affinity is the energy that one has to use to remove an electron from an anion[2]. The electron affinity could therefore also be described as the ionization energy of an anion.

The RÖMPP is even more precise[1]. Here the electron affinity is defined as the energy difference between the ground state of the neutral atom and the ground state of the corresponding anion.

The atoms, of course, have the highest electron affinities, because they only have one or two electrons to complete the outer shell, i.e. the halogens and the elements of the 6th main group (oxygen, sulfur, etc.). Chlorine has the highest electron affinity with a value of -3.617 eV, followed by fluorine (-3.399), bromine (-3.365) and iodine (-3.059)[1]. The negative sign stems from the fact that the atoms release energy when they absorb an electron; the process is exothermic.

Conversely, the alkali metals and the alkaline earth metals have the lowest electron affinity. These metals do not "want" to absorb additional electrons, but on the contrary give off their outer electrons (octet rule). Sodium, for example, has an electron affinity of only -0.548 eV[1].

The alkaline earth metals even have positive values: magnesium and beryllium +0.20 eV, calcium +0.10 eV, strontium +0.05 eV and barium +0.15 eV[3].

For experts:

Actually, the alkali metals should have a lower electron affinity than the alkaline earth metals, because the alkali metals only have to give up a single electron to get into the noble gas state. With the shell model or the spherical cloud model one can no longer explain this fact, for this one needs the orbital model. According to the orbital model, the alkali metals have only one electron in the s orbital of the outer shell. The aim is to have two electrons in this s orbital. This is exactly what is true of the alkaline earth metals. Therefore, the "urge" to take up more electrons is not as great with the alkaline earth metals as with the alkali metals, which "want" to occupy their s-orbital with two electrons.

Lithium, for example, has an EA of -60 kJ / mol[4]; when an additional electron is picked up, energy is released. The Li atom has a singly occupied 2s orbital, which is energetically unfavorable. The Li--Ion, on the other hand, has a doubly occupied 2s orbital, and that is energetically more favorable.

Beryllium, on the other hand, already has a doubly occupied 2s orbital, the p orbitals are empty. By taking up another electron, an energetically less favorable configuration with a singly occupied p-orbital is achieved. Therefore a Be atom is not so "eager" to take up another electron. Correspondingly, the EA has a positive value, namely +19 kJ / mol[4]. That is, the uptake of an electron is an endothermic process.

Fluorine has with -328 kJ / mol[4] a very large electron affinity. This is no wonder, since the electron configuration of the fluorine atom is very unfavorable. Only one electron is missing to complete the outer shell.

Swell:

  1. Römpp Chemie-Lexikon, 9th edition 1992
  2. Spectrum Lexicon of Physics, 1998
  3. Wikipedia
  4. www.chemie.de