What Are Ions?

By the Periodixy Editorial Team · Last reviewed July 10, 2026

An ion is an atom (or group of atoms) that has gained or lost electrons and therefore carries an electric charge. Ions are why salt dissolves, why batteries work, why nerves fire — and they follow rules simple enough to predict from the periodic table alone.

Macro photograph of salt crystals, a familiar ionic compound — Photo by Castorly Stock on Pexels

Cations and anions

Cation: positive ion, formed by losing electrons. Na → Na⁺ + e⁻. Metals form cations.
Anion: negative ion, formed by gaining electrons. Cl + e⁻ → Cl⁻. Nonmetals form anions.

💡 Tip: Memory aid: the “t” in cation looks like a plus sign; anion = “a negative ion”.

⚠ Common mistake: Losing an electron makes an atom positive (it lost negative charge). Students often reverse this — remember electrons carry the minus.

Predicting charges from the table

Atoms form ions to reach a full outer shell, so the group number predicts the charge:

Group	Typical ion	Examples
1	+1	Li⁺, Na⁺, K⁺
2	+2	Mg²⁺, Ca²⁺
13	+3	Al³⁺
15	−3	N³⁻, P³⁻
16	−2	O²⁻, S²⁻
17	−1	F⁻, Cl⁻, Br⁻

Transition metals are flexible — iron forms Fe²⁺ and Fe³⁺ — which is why their compounds carry Roman numerals: iron(II) chloride vs iron(III) chloride.

Polyatomic ions

Some charged particles are whole groups of atoms bonded together: sulfate SO₄²⁻, nitrate NO₃⁻, ammonium NH₄⁺, hydroxide OH⁻. They act as single units in formulas — see How to Read Chemical Formulas for the full table.

Ionic compounds: charge balancing

Building a neutral formula

What is the formula of aluminium oxide?

Al forms Al³⁺; O forms O²⁻.
Find the smallest counts that cancel: 2 × (+3) = 3 × (−2) = 6.

Answer: Al₂O₃.

Ions in daily life: sodium and potassium ions fire your nerves; calcium ions contract your muscles; lithium ions shuttle back and forth inside every phone battery; dissolved ions make sports drinks “electrolytes”.

Summary

Ions are charged atoms/groups: cations (+, lost electrons), anions (−, gained electrons).
Main-group charges are predictable: metals +group number, nonmetals group number − 18.
Polyatomic ions (SO₄²⁻, NO₃⁻, NH₄⁺…) act as single units.
Ionic formulas balance total + and − charge to zero.

Frequently asked questions

What is the difference between an ion and an isotope?

Ions differ in electrons (creating charge); isotopes differ in neutrons (creating mass differences). An atom can be both at once.

Why do metals form positive ions?

Metals have few valence electrons and low ionization energy — losing one to three electrons to expose a full inner shell is energetically easy.

What are electrolytes?

Substances that release ions when dissolved, letting the solution conduct electricity — like the sodium, potassium and chloride in body fluids and sports drinks.