01 · Formation
How ions form: electron gain and loss
Atoms are neutral because they contain equal numbers of protons (in the nucleus) and electrons (in shells). An ion forms when an atom gains or loses electrons — the proton count stays the same, but the electron count changes, creating an imbalance of charge.
CATION (+)
Na → Na⁺ + e⁻
Sodium loses one outer-shell electron to achieve the electron configuration of neon (full outer shell). The resulting Na⁺ ion has 11 protons but only 10 electrons: net charge +1.
Metals typically form cations.
ANION (−)
Cl + e⁻ → Cl⁻
Chlorine gains one electron to achieve the electron configuration of argon (full outer shell). The Cl⁻ ion has 17 protons and 18 electrons: net charge −1.
Non-metals typically form anions.
The charge an ion carries corresponds to how many electrons were transferred. Sodium always forms Na⁺ (loses 1); magnesium Mg²⁺ (loses 2); aluminium Al³⁺ (loses 3). Transition metals can form multiple ions: Fe²⁺ (ferrous) and Fe³⁺ (ferric). This variability arises from the closely spaced 3d orbitals — explained in Atomic structure.
Polyatomic ions
Some ions contain more than one atom covalently bonded together but carrying an overall charge. Examples:
- NH₄⁺ — ammonium (1 N, 4 H; +1 charge)
- SO₄²⁻ — sulfate (1 S, 4 O; −2 charge)
- NO₃⁻ — nitrate (1 N, 3 O; −1 charge)
- HCO₃⁻ — bicarbonate (1 H, 1 C, 3 O; −1 charge)
- OH⁻ — hydroxide (1 O, 1 H; −1 charge)
02 · Bonding
Ionic bonding and lattice structures
When a metal transfers electrons to a non-metal, the resulting cations and anions attract each other by electrostatic force — an ionic bond. In the solid state, ions do not exist as isolated pairs; they pack into a regular three-dimensional lattice that minimises repulsion and maximises attraction.
Sodium chloride (NaCl) is the classic example. Each Na⁺ is surrounded by 6 Cl⁻ ions, and each Cl⁻ by 6 Na⁺. The strength of the lattice (lattice enthalpy) determines the physical properties:
- High melting point — the lattice requires considerable energy to break.
- Brittle — shifting a layer of ions lines up like charges, which then repel catastrophically.
- Conducts when molten or dissolved — ions become free to move.
- Does not conduct in solid state — ions are locked in place.
See Chemical bonding for how ionic bonding compares to covalent and metallic bonding, and how to predict which type will form from the electronegativity difference between elements.
03 · Aqueous chemistry
Ions in solution: acids, pH, and electrolytes
When an ionic compound dissolves in water, its lattice breaks apart as water molecules surround each ion (hydration). The ions are then free to move and carry charge — this is why ionic solutions conduct electricity.
The hydrogen ion (H⁺) — or more accurately the hydronium ion (H₃O⁺) — determines acidity. The pH scale measures its concentration:
pH = −log₁₀[H⁺]
Strong acids (HCl, H₂SO₄, HNO₃) fully dissociate into ions in water; weak acids (CH₃COOH) partially dissociate. The pH glossary entry and the Acids and bases guide develop this in detail.
In electrochemistry, the movement of ions through solution completes the circuit in both galvanic and electrolytic cells. The salt bridge in a Daniell cell maintains electrical neutrality by allowing ions to migrate between the two half-cells. See Electrochemistry.
In nuclear physics, highly energetic ions (plasma) are the fuel state in fusion reactors — at the temperatures required for fusion (>100 million K), electrons are stripped from nuclei to produce a plasma of free ions and electrons. See Nuclear fusion 2026.
04 · Biology
Why biology depends on ions
| Ion | Name | Charge | Origin | Key biological role |
|---|---|---|---|---|
| Na⁺ | Sodium | +1 | Na loses 1e⁻ | Nerve impulses, osmosis |
| K⁺ | Potassium | +1 | K loses 1e⁻ | Na⁺/K⁺ pump, resting potential |
| Ca²⁺ | Calcium | +2 | Ca loses 2e⁻ | Muscle contraction, bones |
| Mg²⁺ | Magnesium | +2 | Mg loses 2e⁻ | Chlorophyll, ATP hydrolysis |
| Fe²⁺/Fe³⁺ | Iron | +2/+3 | Redox cycling | Haemoglobin, electron transport chain |
| Cl⁻ | Chloride | −1 | Cl gains 1e⁻ | Salt balance, CFTR channel |
| HCO₃⁻ | Bicarbonate | −1 | CO₂ + H₂O | Blood pH buffering |
| H⁺ | Hydrogen / proton | +1 | Loss of e⁻ from H | Acids, pH, oxidative phosphorylation |
| OH⁻ | Hydroxide | −1 | Water dissociation | Bases, pH |
The resting membrane potential of a neuron (about −70 mV) is maintained by the Na⁺/K⁺-ATPase pump, which moves 3 Na⁺ out for every 2 K⁺ in, consuming ATP. An action potential is a rapid reversal of this gradient as voltage-gated Na⁺ channels open. See Homeostasis for how ion balance is regulated across the whole organism.
Ion — common questions
- A cation (positive) has lost electrons; an anion (negative) has gained electrons. In electrolysis and galvanic cells, cations migrate to the cathode (negative electrode) and anions to the anode (positive electrode). See the Electrochemistry guide.
- An atom has equal protons and electrons (neutral). An ion results when one or more electrons are lost or gained. Some ions are polyatomic — such as ammonium (NH₄⁺), sulfate (SO₄²⁻), or hydroxide (OH⁻). These are held together by covalent bonds but carry an overall charge. See Covalent bond.
- Atoms form ions to achieve a full outer shell. Metals on the left of the periodic table lose electrons; non-metals on the right gain them. Noble gases already have full outer shells and so rarely form ions. See Atomic structure and Chemical bonding.
- An ionic bond is the electrostatic attraction between a cation and an anion formed by electron transfer. Ionic compounds form giant lattices — sodium chloride is a classic example (Na⁺ and Cl⁻ in a 1:1 ratio). Contrast with a covalent bond, where electrons are shared rather than transferred.
- Free ions carry charge through solution. This is the basis of electrochemistry: galvanic cells and electrolytic cells both rely on ionic conduction through solution and electronic conduction through the external circuit. See the Electrochemistry guide and the Acids and bases guide.