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. Ordinary chemical reactions change only the arrangement of electrons. Only nuclear reactions change the number of protons in an atom.
Ions play essential roles in chemistry, biology, electricity, batteries, and many everyday processes, from nerve signaling to the operation of rechargeable batteries.
Key Takeaways: Ion Definition and Examples
- An ion is an atom or molecule with a net electrical charge.
- Ions form when atoms or molecules gain or lose electrons.
- Losing electrons produces a positively charged cation.
- Gaining electrons produces a negatively charged anion.
- Ions conduct electricity when they move through solutions or molten salts.
- Common examples include Na⁺, Cl⁻, Ca²⁺, NH₄⁺, and SO₄²⁻.
- Ions are essential for living organisms, electrochemistry, and many industrial processes.
Why Ions Are Important
Ions play essential roles in chemistry, biology, and technology.
Examples include:
- Sodium (Na⁺), potassium (K⁺), and calcium (Ca²⁺) ions help transmit nerve impulses and regulate muscle contractions.
- Hydrogen ions (H⁺) determine the acidity of solutions.
- Electrolytes in sports drinks contain ions that help maintain fluid balance.
- Batteries generate electrical energy through the movement of ions.
- Many minerals dissolve in water as ions and are transported through living organisms and ecosystems.
Ions allow electric current to flow through electrolyte solutions, form ionic compounds, regulate biological processes, and drive electrochemical reactions. Living cells depend on sodium, potassium, calcium, and chloride ions to transmit nerve impulses and control muscle contractions, while industries rely on ions in batteries, electroplating, water treatment, and mineral processing.
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 is usually larger than its original atom because the added electron increases electron-electron repulsion and reduces the effective pull of the nucleus on each electron.
- 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 losing electrons reduces electron-electron repulsion and allows the nucleus to pull the remaining electrons 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.
Why Do Opposite Charges Attract?
Opposite electrical charges attract because of the electrostatic force. Positive ions attract negative ions, often forming ionic compounds such as sodium chloride (NaCl). Likewise, ions with the same charge repel one another. These attractions and repulsions strongly influence the physical properties of ionic compounds and electrolyte solutions.
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-
Common Ions and Their Charges
| Ion | Charge |
|---|---|
| H⁺ | +1 |
| Li⁺ | +1 |
| Na⁺ | +1 |
| K⁺ | +1 |
| Mg²⁺ | +2 |
| Ca²⁺ | +2 |
| Ba²⁺ | +2 |
| Al³⁺ | +3 |
| Cl⁻ | −1 |
| Br⁻ | −1 |
| I⁻ | −1 |
| O²⁻ | −2 |
| S²⁻ | −2 |
| N³⁻ | −3 |
| PO₄³⁻ | −3 |
| SO₄²⁻ | −2 |
| NO₃⁻ | −1 |
| OH⁻ | −1 |
| NH₄⁺ | +1 |
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+), carbonate (CO3–), and hydroxide (OH–).
How Ions Form
Ions form when atoms or molecules gain or lose electrons. Metals usually lose electrons to form cations because doing so produces a stable electron configuration. Nonmetals generally gain electrons to form anions for the same reason. This tendency explains why ionic compounds commonly form between metals and nonmetals.
For example:
- Sodium loses one electron to form Na⁺.
- Magnesium loses two electrons to form Mg²⁺.
- Chlorine gains one electron to form Cl⁻.
- Oxygen gains two electrons to form O²⁻.
Many atoms form ions to achieve a more stable electron configuration, often resembling the electron arrangement of a noble gas.
Where Are Ions Found?
Ions occur naturally in many environments:
- Seawater contains dissolved sodium, chloride, magnesium, sulfate, and other ions.
- Blood and body fluids contain electrolyte ions such as Na⁺, K⁺, Ca²⁺, and Cl⁻.
- Earth’s atmosphere contains ions produced by cosmic rays and lightning.
- Plasmas, including stars and neon signs, contain large numbers of ions.
- Ionic compounds such as table salt contain ions arranged in crystal lattices.
How to Tell Whether an Ion Is Positive or Negative
The easiest way to determine whether an ion is positive or negative is to compare the numbers of protons and electrons.
- More protons than electrons → positive ion (cation)
- More electrons than protons → negative ion (anion)
For example:
- Na⁺ has 11 protons and 10 electrons, so it has a +1 charge.
- Cl⁻ has 17 protons and 18 electrons, so it has a −1 charge.
- Ca²⁺ has 20 protons and 18 electrons, giving it a +2 charge.
Do Molecules Form Ions?
While many ions consist of single atoms, entire molecules can also carry a charge. These are called polyatomic ions. Examples include ammonium (NH₄⁺), sulfate (SO₄²⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), and hydroxide (OH⁻). In these ions, covalent bonds hold the atoms together while the entire group has an overall positive or negative charge.
What Is an Electrolyte?
An electrolyte is a substance that produces ions when dissolved in water or melted. Because ions carry electrical charge, electrolyte solutions conduct electricity.
Examples of electrolytes include:
- Sodium chloride (NaCl)
- Potassium chloride (KCl)
- Hydrochloric acid (HCl)
- Sodium hydroxide (NaOH)
Pure water conducts electricity poorly because it contains very few ions, while salt water conducts electricity much more effectively because it contains many dissolved ions.
History
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. Later discoveries by J. J. Thomson and others explained that ions form through the gain or loss of electrons rather than changes in the atomic nucleus.
Ions vs. Atoms
| Atom | Ion |
|---|---|
| Electrically neutral | Has a net charge |
| Equal numbers of protons and electrons | Unequal numbers of protons and electrons |
| May gain or lose electrons | Already has gained or lost electrons |
| Does not move in an electric field | Moves toward an oppositely charged electrode |
Ions vs. Isotopes
| Ion | Isotope |
|---|---|
| Different number of electrons | Different number of neutrons |
| Charge changes | Charge usually stays neutral |
| Chemical behavior changes | Chemical behavior is nearly identical |
| Examples: Na⁺, Cl⁻ | Examples: Carbon-12, Carbon-13, Carbon-14 |
Common Misconceptions
Ions are always atoms.
False. Many ions are molecules, such as sulfate (SO₄²⁻) and ammonium (NH₄⁺).
Only metals form ions.
False. Metals usually form cations, but nonmetals commonly form anions.
Electrons and protons change during ordinary chemical reactions.
False. Ordinary chemical reactions involve the transfer or sharing of electrons. Only nuclear reactions change the number of protons.
An ion is unstable.
Not necessarily. Many ions are extremely stable and persist indefinitely in solutions, minerals, and biological systems.
Frequently Asked Questions
Why do ions conduct electricity?
Because charged particles move through molten compounds or solutions and carry electric current.
Can an ion be neutral?
No. A neutral atom has no net charge, so it is not an ion.
Is an electron an ion?
No. An electron is a subatomic particle. An ion is an atom or molecule with a net charge.
Why are ions important in the human body?
Ions regulate nerve impulses, muscle contractions, heartbeat, fluid balance, and many enzyme reactions.
Are all salts made of ions?
Yes. Ionic salts consist of positively charged cations and negatively charged anions held together by electrostatic attraction.
References
- Cillispie, Charles (ed.) (1970). Dictionary of Scientific Biography (1st ed.). New York City: Charles Scribner’s Sons. ISBN 978-0-684-10112-5.
- James, Frank A. J. L. (ed.) (1991). The Correspondence of Michael Faraday. Vol. 2: 1832-1840. ISBN 9780863412493.
- Knoll, Glenn F. (1999). Radiation Detection and Measurement (3rd ed.). New York: Wiley. ISBN 978-0-471-07338-3.
- Masterton, William; Hurley, Cecile (2008). Chemistry: Principles and Reactions. Cengage Learning. ISBN 0-495-12671-3.