Ionization Energy: Potential, Methods For Determination

The ionization energy refers to the minimum amount of energy, usually expressed in units of kilojoules per mole (kJ / mol), which is required to produce the release of an electron located in an atom in the gas phase that is in its state fundamental.

The gaseous state refers to the state in which it is free from the influence that other atoms can exert on themselves, as well as any intermolecular interaction is ruled out. The magnitude of ionization energy is a parameter to describe the force with which an electron binds to the atom of which it is a part.

Ionization energy

In other words, the greater the amount of ionization energy required, the more difficult it will be to detach the electron in question.

Ionization potential

The ionization potential of an atom or molecule is defined as the minimum amount of energy that must be applied to cause the detachment of an electron from the outermost shell of the atom in its ground state and with a neutral charge; that is, the ionization energy.

It should be noted that when speaking of ionization potential, a term that has fallen into disuse is being used. This is because previously the determination of this property was based on the use of an electrostatic potential to the sample of interest.

By using this electrostatic potential two things happened: the ionization of the chemical species and the acceleration of the process of shedding the electron that it was desired to remove.

So when starting to use spectroscopic techniques for its determination, the term “ionization potential” has been replaced by “ionization energy.”

Likewise, it is known that the chemical properties of atoms are determined by the configuration of the electrons present in the outermost energy level in these atoms. So, the ionization energy of these species is directly related to the stability of their valence electrons.

Methods for determining ionization energy

As previously mentioned, the methods for determining ionization energy are mainly given by photoemission processes, which are based on the determination of the energy emitted by electrons as a consequence of the application of the photoelectric effect.

Although it could be said that atomic spectroscopy is the most immediate method for determining the ionization energy of a sample, there is also photoelectron spectroscopy, in which the energies with which electrons are bound to atoms are measured.

In this sense, ultraviolet photoelectron spectroscopy – also known as UPS for its acronym in English – is a technique that uses the excitation of atoms or molecules through the application of ultraviolet radiation.

This is done in order to analyze the energetic transitions of the outermost electrons in the chemical species studied and the characteristics of the bonds they form.

X-ray photoelectron spectroscopy and extreme ultraviolet radiation are also known, which use the same principle described above with differences in the type of radiation that is impinged on the sample, the speed with which the electrons are expelled and the resolution obtained.

First ionization energy

In the case of atoms that have more than one electron in their outermost level – that is, the so-called polyelectronic atoms – the value of the energy necessary to remove the first electron from the atom that is in its ground state is given by the following equation:

Energy + A (g) → A + (g) + e

“A” symbolizes an atom of any element and the detached electron is represented as “e “. Thus the first ionization energy is obtained, referred to as “I 1 “.

As can be seen, an endothermic reaction is taking place, since energy is being supplied to the atom to obtain an electron added to the cation of that element.

Likewise, the value of the first ionization energy of the elements present in the same period increases proportionally to the increase in their atomic number.

This means that it decreases from right to left in a period, and from top to bottom in the same group of the periodic table.

In this sense, noble gases have high magnitudes in their ionization energies, while the elements belonging to the alkali and alkaline earth metals have low values ​​of this energy.

Second ionization energy

In the same way, by removing a second electron from the same atom, the second ionization energy is obtained, symbolized as “I 2 “.

Energy + A + (g) → A 2+ (g) + e

The same scheme is followed for the other ionization energies when starting the following electrons, knowing that, followed by the detachment of the electron from an atom in its ground state, the repulsive effect between the remaining electrons decreases.

As the property called “nuclear charge” remains constant, a greater amount of energy is required to tear off another electron of the ionic species that has the positive charge. So the ionization energies increase, as seen below:

I 1 <I 2 <I 3 <… <I n

Finally, in addition to the effect of the nuclear charge, the ionization energies are affected by the electronic configuration (number of electrons in the valence shell, type of orbital occupied, etc.) and the effective nuclear charge of the electron to be shed.

Due to this phenomenon, most of the molecules of an organic nature have high values ​​of ionization energy.


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  5. Carey, FA (2012). Advanced Organic Chemistry: Part A: Structure and Mechanisms. Obtained from

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