The Difference Between a Cation and an Anion

A cation is an atom or molecule with a net positive charge, while an anion is an atom or molecule with a net negative electrical charge. jack0m / Getty Images

Cations and anions are both ions. The difference between a cation and an anion is the net electrical charge of the ion.

Ions are atoms or molecules which have gained or lost one or more valence electrons giving the ion a net positive or negative charge. If the chemical species has more protons than electrons, it carries a net positive charge. If there are more electrons than protons, the species has a negative charge.

The number of neutrons determines the isotope of an element, but does not affect the electrical charge.

Cation Versus Anion

Cations are ions with a net positive charge.

Cation Examples: Silver: Ag+, hydronium: H3O+, and ammonium: NH4+

Anions are ions with a net negative charge.

Anion Examples: hydroxide anion: OH-, oxide anion: O2-, and sulfate anion: SO42-

Because they have opposite electrical charges, cations and anions are attracted to each other. Cations repel other cations, while anions repel other anions.

Predicting Cations and Anions

Sometimes you can predict whether an atom will form a cation or an anion based on its position on the periodic table. Alkali metals and alkaline earths always form cations. Halogens always form anions. Most other nonmetals typically form anions (e.g., oxygen, nitrogen, sulfur), while most metals form cations (e.g., iron, gold, mercury).

Writing Chemical Formulas

When writing the formula of a compound, the cation is listed before the anion.

For example, in NaCl, the sodium atom acts as the cation, while the chlorine atom acts as the anion.

When writing cation or anion symbols, the element symbol(s) is listed first. The charge is written as a superscript following the chemical formula.

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Helmenstine, Todd. "The Difference Between a Cation and an Anion." ThoughtCo, Feb. 1, 2018, Helmenstine, Todd. (2018, February 1). The Difference Between a Cation and an Anion. Retrieved from Helmenstine, Todd. "The Difference Between a Cation and an Anion." ThoughtCo. (accessed April 23, 2018).