What is an ion?

What Does Ion Mean

We explain what an ion is and how it is constituted and some examples. Also, what is an anion and a cation.

An ion is an atom or a molecule that has gained or lost electrons.

What is an ion?

In chemistry , an electrically charged particle is known as an ion and is made up of an atom or molecule that is not electrically neutral, that is, that in its constitution has gained or lost electrons . The process by which ions are produced is called "ionization."


The ions can be made up of two or more atoms (polyatomic) of different nature, or of a single atom (monatomic). In any of the cases, we will speak of a cation (or cations) when it is a positively charged ion (that is, that the initial neutral atom or molecule gave electrons), and we will speak of an anion (or anions) when it comes to a negatively charged ion (that is, the initial neutral atom or molecule accepted electrons).

Other types of ions are also known, based on their electrical charge , known as dianions (when they present two negative charges), zwitterions (when they present a positive and a negative charge that are isolated, but in the same compound, due to what is neutral), or ionic radicals (ions of enormous reactivity and instability because they have free electrons). Generally speaking, ions are very reactive and tend to combine with other ions, atoms, or molecules through electrostatic interactions.

Ions play an indispensable role in life, especially those of calcium, potassium and sodium , the importance of which in the transit of cell membranes and neurotransmitters has been widely studied. Moreover, understanding ions has enabled us to develop the technology of the plasma and even measure the quality of the water based on ionic salts dissolved in it.

See also: Chemical bond


Sulfite is a polyatomic anion.

It is known as anion (or anions) to ions that have a negative electrical charge , that is, they have gained electrons in a chemical reaction that gave rise to them. They can be made up of one or more atoms, but even in the latter case the overall charge of the molecule (its oxidation state ) is always negative.

There are three types of anions:

  • Monoatomic. Those made up of a single atom that has gained electrons. For example: Chloride (Cl - ).
  • Polyatomic. They come from a molecule that has gained electrons in a chemical reaction, or from an acid that has lost protons . For example: Sulfite (SO 3 2- ).
  • Acids . They come from a polyprotic acid (which have multiple ionizable hydrogens) from which protons have been extracted. For example: Diacid phosphate (H 2 PO 4 - ).


The cations have lost one or more electrons.

Cations are ions that have a positive electric charge , that is, they have lost one or more electrons. Thus, like anions, cations can also be composed of one or more atoms, provided that the total charge of the compound, in this case, is positive.

One of the most important functions of cations is their participation in biological processes . For example, the Na + and K + cations play a fundamental role in the transmission of nerve impulses.

Examples of ion

Azide is a simple anion.

The best known ions are:

  • Simple cations. Composed of a single positively charged atom:
    • Aluminum (Al 3+ )
    • Cesium (Cs + )
    • Chromium (III) or chromic ion (Cr 3+ )
    • Chromium (VI) or perchromic ion (Cr 6+ )
    • Hydrogen or proton (H + )
    • Helium or alpha particle (He 2+ )
    • Lithium (Li + )
    • Iron (II) or ferrous ion (Fe 2+ )
    • Iron (III) or ferric ion (Fe 3+ )
    • Nickel (III) or nickel ion (Ni 3+ )
    • Tin (II) or stannous ion (Sn 2+ )
    • Tin (IV) or stanic ion (Sn 4+ )
  • Polyatomic cations. Composed of two or more positively charged atoms:
    • Ammonium (NH 4 + )
    • Oxonium (H 3 O + )
    • Nitronium (NO 2 + )
    • Mercury (I) or mercury ion (Hg 2 2+ )
  • Simple anions. Composed of a single negatively charged atom:
    • Azide (N 3 - )
    • Bromide (Br - )
    • Carbide (C 4- )
    • Chloride (Cl - )
    • Fluoride (F - )
    • Phosphide (P 3- )
    • Nitride (N 3- )
    • Sulfide (S 2- )
  • Oxoanions. Composed of oxygen and other elements, they have a negative charge:
    • Arsenate (AsO 4 3- )
    • Borate (BO 3 3- )
    • Hypobromite (BrO - )
    • Bicarbonate (HCO 3 - )
    • Chlorate (ClO 3 - )
    • Chlorite (ClO 2 - )
    • Hypochlorite (ClO - )
    • Dichromate (Cr 2 O 7 2- )
    • Hydrogen sulfate or bisulfate (HSO 4 - )
    • Hydrogensulfite or bisulfite (HSO 3 - )
    • Silicate (SiO 4 4- )
  • Anions of organic acids. Coming from organic molecules, they have a negative charge:
    • Acetate (C 2 H 3 O 2 - )
    • Oxalate (C 2 O 4 2- )
    • Bioxalate (HC 2 O 4 - )
  • Other anions. Negatively charged and more than one atom:
    • Bisulfide (HS - )
    • Amide (NH 2 - )
    • Cyanate (OCN - )
    • Thiocyanate (SCN - )
    • Cyanide (CN - )
    • Hydroxide (OH - )

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