Ion Definition in Chemistry

An ion is defined as an atom or molecule that has gained or lost one or more of its valence electrons, giving it a net positive or negative electrical charge. In other words, there is an imbalance in the number of protons (positively charged particles) and electrons (negatively charged particles) in a chemical species.

History and Meaning

The term "ion" was introduced by English chemist and physicist Michael Faraday in 1834 to describe the chemical species that travels from one electrode to another in aqueous solution. The word ion comes from the Greek word ion or ienai, which means "to go."

Although Faraday could not identify the particles moving between electrodes, he knew that metals dissolved into a solution at one electrode and that another metal was deposited from the solution at the other electrode, so matter had to be moving under the influence of an electrical current.

Examples of ions are:

alpha particle He²⁺

hydroxide OH^-

Cations and Anions

Ions can be grouped into two broad categories: cations and anions.

Cations are ions that carry a net positive charge because the number of protons in the species is greater than the number of electrons. The formula for a cation is indicated by a superscript following the formula that indicates the number of the charge and a "+" sign. A number, if present, precedes the plus sign. If only a "+" is present, it means the charge is +1. For example, Ca²⁺ indicates a cation with a +2 charge.

Anions are ions that carry a net negative charge. In anions, there are more electrons than protons. The number of neutrons is not a factor in whether an atom, functional group, or molecule is an anion. Like cations, the charge on an anion is indicated using a superscript after a chemical formula. For example, Cl^- is the symbol for the chlorine anion, which carries a single negative charge (-1). If a number is used in the superscript, it precedes the minus sign. For example, the sulfate anion is written as:

SO₄^2-

One way to remember the definitions of cations and anions is to think of the letter "t" in the word cation as looking like a plus symbol. The letter "n" in anion is the starting letter in the word "negative" or is a letter in the word "anion."

Because they carry opposite electrical charges, cations and anions are attracted to each other. Cations repel other cations; anions repel other anions. Because of the attractions and repulsion between ions, they are reactive chemical species. Cations and anions readily form compounds with each other, particularly salts. Because ions are electrically charged, they are affected by magnetic fields.

Monatomic vs. Polyatomic Ions

If an ion consists of a single atom, it is called a monatomic ion. An example is the hydrogen ion, H⁺. By contrast, polyatomic ions, also called molecular ions, consist of two or more atoms. An example of a polyatomic ion is the dichromate anion:

Cr₂O₇^2-