What is an ion?

An ion is a molecule or atom that has a positive or negative electrical charge. That is, an ion is an atom whose electrical charge is not neutral.

The word ion comes from the Greek ἰών [ion], which means ‘what goes’. This word was first used in the English language in 1834 thanks to the scientist Michael Faraday, who in 1830 had proposed the existence of ions. Then, in 1884, the scientist Arrhenius developed the theory that led to its verification.

The ions are formed by ionization. The term refers to the phenomenon of gain or loss of electrons from the atom when it is subjected to certain processes. The loss or gain of electrons allows the atom to acquire an electrical charge, transforming itself into an ion. If the electrical charge is positive, it is called a cation; if it is negative, it is called an anion. It is concluded that non-ionized atoms are electrically neutral.

To represent ions, the following form is used:

  • Write the symbol for the atom (for example, To the for aluminum);
  • A plus (+) or minus (-) symbol is added to indicate whether the charge is negative or positive;
  • If the number of electrons gained or lost is greater than 1, the amount is indicated.

For example,

  • A simple hydrogen cation is represented as: H+
  • A simple aluminum cation is represented as follows: Al3+

The ions can be monatomic (consisting of a single atom) or polyatomic (consisting of two or more atoms).

For example,

  • Monatomic: Be2+ (Beryllium); cs+ (Cesium); Li+ (Lithium).
  • Polyatomic: NH4+ (Ammonium); H3EITHER+ (Hydronium or Oxonium); NOtwo+ (Nitronium).
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When two ions with different charges approach each other, a union called an ionic bond is established between them. Various chemical compounds are formed from these bonds.

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  • What is ionization?
  • Atom
  • ionic bond

types of ions

There are two types of ions, defined according to the electrical charge they contain. Their names are cations and anions.


Anions are negatively charged ions. They are formed as a result of the gain of electrons. Therefore, anions are characterized by having more electrons than protons in their composition. The word anion means ‘the one that goes down’.

examples of anions

  • arsenide as3
  • Azide N3−
  • Bromide Br
  • C-carbide4−
  • Fluoride F
  • Phosphide P3−
  • Oxide OR2−
  • Peroxide ORtwotwo
  • Sulfide S2−


Cations are positively charged ions. They are formed as a result of the loss of electrons. This means that cations always have more protons than electrons. The word cation means ‘the one that goes up’.

examples of cations

  • Calcium Catwo+
  • Chromium(II)Crtwo+
  • Copper(I)Cu+
  • Iron(II) Fetwo+
  • Mercury(II)Hgtwo+
  • Nickel (III) Ni3+
  • Silver Ag+
  • Lead(IV)Pb4+
  • Potassium K+
  • Sodium Na+
  • zinc-zntwo+