An ion is an atom or molecule that has a number of electrons different from the number of protons, which causes the atom or molecule to acquire a net non-neutral electrical charge. That is, they are electrically charged atoms or moleculeseither a positive charge or a negative charge.

The term ion, from the Greek ἰών [ion] (“going” or “going”), was introduced by the chemist Michael Faraday in 1834 to designate chemical species that traveled from one electrode to another and that were unknown until then.

General types and characteristics

Electrons have a negative charge and protons have a positive charge. If an ion has more electrons than protons, the net charge on the ion will be negative. This type of ion is known as anion. If the ion has fewer electrons than protons, the net charge will be positive and the ion is known as cation:

  • Anion: ion with more electrons than protons. Its electrical charge is negative and equal to the number of electrons minus the number of protons. It is denoted with the symbol of the atom or molecule and a superscript with the charge number (number of electrons in excess) followed by the negative sign (−). If there is only one extra electron, the number can be dropped. For example, the anion Cl is the anion formed by the chlorine atom and an extra electron; the anion S2− is the anion formed by a sulfur atom with two extra electrons.
  • Cation: ion with more protons than electrons. Its electric charge is positive and equal to the number of protons minus the number of electrons. It is denoted with the symbol of the atom or molecule and a superscript with the number of excess protons followed by the plus (+) sign. For example, the Hg2+ It is the cation formed by a mercury atom with two less electrons, or two more protons, with respect to the neutral state.

All ions have the following characteristics, regardless of the sign of their electrical charge:

  • are attracted to an opposite electrical charge (negative for positive and positive for negative).
  • are repelled by an electric charge of the same sign.
  • when they move, their trajectory can be deflected by the action of magnetic fields.

The size of an ion is measured by the ionic radius. Compared to the neutral state, the anions are usually larger, since the electron cloud has a great influence on this radius and with more electrons repelling each other, the radius of this cloud increases. In contrast, cations are usually smaller.


The formation of ions, or ionizationcan be understood as the process of loss or gain of electrons or protons from an electrically neutral state.

Generally, the formation of monatomic ions is due to the loss or gain of electrons from the valence shell of an atom, which is the outermost electronic shell. The innermost shell electrons do not participate in ionization, since they interact more closely with the positive charge of the nucleus.

An atom can be ionized with multiple mechanisms; For example, a cation can be formed by stripping electrons from an atom by applying energy through heat, radiation, particle bombardment, or any other process in which a valence shell electron can absorb enough energy to escape the atom.

But the most common ionization mechanism in nature is transfer of electrons between two atoms. This transfer occurs in favor of stable electronic configurations and each atom that participates in the transfer will give or accept electrons depending on the most energetically favorable action.

For example, the sodium atom, symbol Na, has a single electron in its valence shell surrounding two complete and stable shells. The chlorine atom, symbol Cl, has 7 electrons in its valence shell and only one is missing to complete it. When both atoms meet, sodium tends to give up the electron and chlorine to accept it, so that both are left with complete electronic orbitals and ionization occurs. Sodium forms a cation (Na+) and chlorine an anion (Cl); The electrical interaction between the two forms a ionic bond and together they form sodium chloride or common salt, NaCl.

Na → Na+ + e
Cl + e → Cl
no+ +Cl → NaCl

Substances formed by ionic bonding, such as sodium chloride, tend to have good solubility in polar solvents, since the polarity of the solvent molecules can solvate the ions in the molecule and keep them stably separated.

In fact, the ions on Earth are mostly present in the solid state and in solution, being very little abundant in gaseous state, state in which they are very reactive. An ionic substance in the gaseous state is called plasma and it needs a lot of energy to form and remain stable, for example, plasmas are formed in the hottest areas of a flame or with electric discharges.

Unlike monatomic ions, molecular ions tend to form by the gain of other elemental ionsAs the hydron or hydrogen cation H+which is very common to also refer to as a proton, especially in chemistry.

Hydron is very unstable in solution and is always bound to other substances. In aqueous solution it can bind to the water molecule and form oxonium (H3EITHER+); it can also bind to other substances, for example it can bind to ammonia (NH3) to form the ammonium cation (NH4+).

There may also be proton exchange as an ionization process. For example, sulfuric acid ionizes and forms an anion after donating a hydron to the water molecule:

HtwoSW4 + HtwoOR ⇌ H3EITHER+ + HSO4

ionization potential

The ionization potential, or ionization energy, is a measure of the energy required to transform a given atom into an ion. It is defined as the energy required to remove an electron in its lowest energy stable state from an atom or molecule in the gaseous state.

The energy to release a second electron would be called the second ionization energy, and thus the n ionization energy to release n electron when all the n – 1 electrons have already been given off.

Each ionization energy is greater than the previous one, with increasing difference, because when an orbital remains empty, the following orbitals are complete, a state in which they tend to be stable. For example, the Na ion+ it has all its orbitals complete, whereas the neutral sodium atom has one unpaired electron in its valence shell; the ionization energy of sodium for the first electron is much lower than the second ionization energy.