Ionization in physics and chemistry: concept, process and examples

Ionization is a fundamental process for endless physical, chemical or natural phenomena, the latter often being a mixture of physicochemical changes. For example, in physical ionizations, in principle, no chemical reactions occur; that is, the production of the ions does not imply the breaking or the formation of new bonds.

Many natural phenomena, such as the Northern Lights, are based on the ionization of molecules in the atmosphere . Source: Pixabay.

However, in natural phenomena, said ionization is usually accompanied by chemical reactions, such as the formation of ozone at low altitudes. Also, during electrical storms, nitrogen oxides are generated by lightning and their intense temperatures, which oxidize nitrogen in the air.

Ionization in chemistry


During the operation of the batteries of a toy ions and electrons are continuously produced until they are discharged. Source: DJ Shin / CC BY-SA (

In chemical ionization ions are produced by breaking or forming bonds. Two substances gain or lose electrons irreversibly: the one that loses the electrons acquires a positive charge, while the one that gains them retains a negative charge.

In batteries, one substance loses electrons while another gains them. These electrons travel and activate the electrical circuits of a computer, to finally return to a second compartment of the battery, where the other substance waits for them to trap them and become negatively charged. This is an example of a chemical ionization carried out by an oxide-reduction reaction.

Heterolytic breakdown

Another type of chemical ionization is due to heterolytic breakdown. When this break occurs, the AB bond breaks to form the A + and B  ions , as the electrons are directed towards the atom for which they “feel” the greatest affinity. Heterolytic breakdown is usually the first step that governs the mechanisms of many organic reactions.


Electric charges can not only appear by the movement of electrons itself, but by the bonds that are broken or formed in an exchange of atoms. This is the case of autoionization, which occurs between two molecules of the same compound.

Of all chemical ionizations, autoionization is the simplest and most imperceptible, since it does not have the ability to conduct electrical current by itself.

Static electricity

This girl’s hair is positively charged and repels each other due to static electricity. Source: Biswarup Ganguly / CC BY (

Ionization also has a lot to do with physical phenomena. In general, ionization in physics does not involve chemical reactions. The atoms of matter become electrically charged without losing their original identity.

Surfaces can exchange electrons when they rub if one of them is more efficient at storing them, leading to static electricity. We no longer speak only of atoms, but of a whole material that acquires a negative or positive charge, and that when equilibrium is reestablished, an electric discharge can literally be generated between two materials or bodies.

Gas ionization

Scattered gaseous atoms can lose electrons if they are subjected to an electric field. In doing so, the atoms become excited and release electrons. Then, as they cool down and return to their lower energy state, they give off photons. And the process repeats over and over again. The result: a source of lighting or electricity.

Ionization by radiation or collisions

Similarly, matter can gain energy contributed by different types of radiation (gamma, X-rays, ultraviolet, etc.) or by colliding with high-energy particles (alphas, betas and neutrons).

The energy supplied is such that an electron is instantly detached from the atom. This type of ionization is related to multiple radioactive and cosmic phenomena.

Water ionization

Water dissociation

Electric charges can appear in the water due to its ionization. This is of the chemical type, since two water molecules suddenly react with each other to dissociate and generate the ions H 3 O + and OH  :

2H 2 O (l) ⇌ H 3 O + (aq) + OH  (aq)

This equilibrium has been very displaced towards the water, so that only a negligible quantity of these ions is produced.

Air ionization

Molecules in the air do not participate in any autoionization equilibrium. Oxygen, nitrogen, argon, water vapor and the other gases that make up the air, do not go around exchanging atoms or electrons. Therefore, air is not a good conductor of electricity under normal conditions.

However, if it is subjected to an electric field, radiation, or high temperatures, it can ionize and become a good conductor. This is what happens, for example, when electrical rays fall from clouds to the ground, electrons traveling through the air with a sudden burst of light.

Examples of ionization

Throughout the previous sections, some examples of ionization have been named. Finally, some others will be mentioned.

Metal oxidation

When metals are oxidized, either by the action of terrestrial oxygen or by the attack of acidic substances, they acquire positive charges to form oxides or salts, respectively.

For example, iron is ionized by oxygen to transform into Fe 3+ ions , located in the crystals of iron oxides. Magnesium, on the other hand, dissolves in sulfuric acid to lose electrons and remain as Mg 2+ ions in the magnesium sulfate salt.

Dissolution of gases in water

Some gases, such as ammonia and hydrogen chloride, dissolve in water to lose their neutrality and lead to the formation of ions. For example, when ammonia dissolves partially liberates NH + and OH  ions . Meanwhile, hydrogen chloride, when dissolved, will produce H 3 O + and Cl  ions completely.

A similar case is that of the dissolution of salts in water, such as magnesium sulfate, MgSO 4 . However, the Mg 2+ and SO 2- ions are already present in the salt crystals before they dissolve in the water.

Lamps or lighting tubes

In steam lamps or lighting tubes, gaseous atoms are excited with electric discharges to produce a certain amount of light and colors. For example, this ionization takes place in sodium or mercury lamps, as well as in the famous neon lights with their bright colors at the entrances of restaurants or shops.

Ionization energies

Ionization energy, a periodic property, is studied to characterize chemical elements according to the ease with which their gaseous atoms donate their electrons to become positively charged. For example, metals tend to have the lowest ionization energies, while non-metallic elements and noble gases have the highest.

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