What Is an Ion? Chemistry Definition

What Is an Ion - Definition in Chemistry
An ion is an atom or molecule with a net electrical charge. In chemistry, this involves the gain or loss of electrons.

In chemistry, the definition of an ion is an electrically charged atom or molecule. This means the atom or molecule has an unequal number of protons and electrons. Neutrons carry no net electrical charge, so they don’t affect ions directly. In chemical reactions, ions form when atoms or molecules gain or lose valence electrons. Only nuclear reactions change the number of protons in a chemical species.

Anions and Cations

The two types of ions are anions and cations.

  • An anion has a net negative electrical charge. It contains more electrons than protons. For example, a chlorine atom with 17 protons and 18 electrons (Cl) is an anion. An anion may be larger than its original atom if it gains an additional electron shell.
  • A cation has a net positive electrical charge. It contains more protons than electrons. For example, a hydrogen ion with one proton and zero electrons (H+) is a cation. A cation is smaller than its original atom because the greater nuclear charge draws the electrons in closer.

One way to remember the cation and anion definitions is thinking of the “t” in cation like a “+” sign. Another option is remembering “anion” as sounding like “a negative ion.”

Because they carry opposite electrical charges, anions and cations attract each other. Anions repel other anions, while cations repel other cations. Both electrical and magnetic fields affect anions and cations.

Examples of Ions

Chemical notation indicates ions by following an element symbol or chemical formula with a superscript that shows whether a charge is positive (+) or negative (-) and the amount of charge. If an electrical charge is 1, only list the “+” or “-” symbol. Here are some ion examples:

  • H+
  • Ca2+
  • Fe2+
  • Fe3+
  • Cl
  • P3-
  • NH4+
  • H3O+
  • HCO3
  • SO42-

Monatomic and Polyatomic Ions

An ion containing only one atom is a monatomic ion. Examples of monatomic ions include H+, O2-, and Cl. An ion containing multiple ions is a polyatomic ion or molecular ion. Examples of polyatomic ions are ammonium (NH4+), hydronium (H3O+), chlorate (CO3), and hydroxide (OH).


The word ion comes from the Greek word ion or ienai, meaning “to go.” English physicist and chemist Michael Faraday coined the term in 1834 as a way of describing the way a chemical species travels from one electrode to another in an aqueous solution.

While Faraday did not identify the nature of the particles moving between electrodes, he saw metal dissolve at one electrode and deposit at the other electrode. So, somehow electrical current influenced the movement of matter.


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