Lonization Energies

Ionization energy. - The ionization energy is indicated as the energy needed to remove an electron
from the isolated atom in its fundamental state, taking it to an infinite distance from the nucleus.

We speak of first ionization energy when the less strongly bound electron is removed, transforming the atom into an ion with a positive charge. For example in the case of sodium

  Na → Na ++ e-

The second ionization energy is that necessary to tear a second electron from the Na ion, third ionization energy that is necessary to tear off a third from the Na + ion, and so on.

The ionization energies are commonly expressed in joules for each mole of electrons, meaning the value of the energy needed to tear an electron to each of the atoms existing in a mass of the element considered. Another widely used unit of measurement is the electronvolt for each individual atom.

It should be noted that the first ionization energies have particularly high values   for noble gases, in relation to the high stability of the electronic configuration of these elements. The opposite happens for alkaline metals, for which it is a matter of removing an isolated electron from the nucleus. For alkaline metals, the second ionization energy becomes high, which corresponds to the removal of electrons forming part of saturated orbitals and moreover starting from an already positively charged ion.

With the same electronic configuration of the outermost layer the ionization energy decreases with the increase of the atomic number of the element considered, in relation to the fact that the ejected electron is less strongly attracted to the nucleus.
Electronic affinity

Electronic affinity. - The term electronic affinity indicates the energy that is developed when an electron joins a neutral atom, supposed to be isolated and in the ground state. For example in the case of chlorine

Cl + e- → Cl -

   Electronic affinity is a size that is difficult to measure, and for some elements it is negative. As a general trend it can be noted that the electronic affinity is maximum for the elements that, with the purchase of an electron, assume the electronic configuration of the noble gases. It is instead negative for noble gases, which obviously have no tendency to buy new electrons. The same happens for other elements (Be, Mg) with the most external (orbital ns) saturated electronic substrate.
Electronic affinity

Electronic affinity. - The term electronic affinity indicates the energy that is developed when an electron joins a neutral atom, supposed to be isolated and in the ground state. For example in the case of chlorine

Cl + e- → Cl -

  Electronic affinity is a size that is difficult to measure, and for some elements it is negative. As a general trend it can be noted that the electronic affinity is maximum for the elements that, with the purchase of an electron, assume the electronic configuration of the noble gases. It is instead negative for noble gases, which obviously have no tendency to buy new electrons. The same happens for other elements (Be, Mg) with the most external (orbital ns) saturated electronic substrate.
Rule of the octet

The so-called rare gases or noble gases, ie the elements helium, neon, argon, crypto, xenon and radon are poorly reactive. Of the first three we do not know to date any real compound, the others react with great difficulty, creating poorly stable compounds. Noble gases are also the only gaseous elements at room temperature that present a monoatomic molecule.

Since the formation of compounds, or in any case of pluriatomic molecules, starting from single atoms is always accompanied by a variation of the electronic configuration of the atoms themselves, the poor reactivity of the noble gases must correspond to a particular stability of their electronic configuration. This is evidently related to the fact that in these elements the higher energy orbitals are saturated. With the sole exception of helium, in which there are only two electrons that saturate the orbital 1s, all the other noble gases have the s2 p6 configuration in the outermost electronic layer.

In view of this particular stability it is logical that also the other elements tend to reach this same electronic configuration through appropriate displacements or groupings of their outermost electrons, which happens precisely during the formation of chemical bonds.

The above concepts are summarized in the so-called Lewis rule or octet rule: in the formation of bonds, each atom tends, through the transfer, acquisition or pooling of electrons, to reach the electronic configuration of the corresponding noble gases in the presence of eight electrons in the sep orbitals of the outermost layer.

The octet rule is of validity far from general, presenting a high number of exceptions. However, it is very useful as a first indicative criterion in the study of chemical bonds.