# Ionic bonding **Ionic bonding** is the type of [[Chemical_bond|chemical bond]] that arises from the electrostatic attraction between oppositely charged [[Ion|ions]], and it is the primary interaction holding together the ionic compounds, or [[Salt_(chemistry)|salts]]. It forms when one [[Atom|atom]], usually a metal, gives up one or more [[Electron|electrons]] to another, usually a nonmetal, so that the first becomes a cation and the second an anion; the two then attract by [[Coulomb's_law|Coulomb's law]], and in a solid each ion attracts every oppositely charged ion around it, so the bond is the property of a whole lattice rather than of a single pair.[^os-af-ch4] The strength of the bonding is measured by the [[Lattice_energy|lattice energy]], the energy released when the separated gaseous ions come together into the crystal, which for [[Sodium_chloride|sodium chloride]] is 787 kJ per mole.[^blackstock-bh] On the Chemistry flagship the article serves the section *Ionic bonding and the lattice* (Part IV — Bonding), and its microsim is the sibling of the electronegativity dial on the Chemical bond page: the same two atoms, now taken to the ionic corner of the triangle, become an ion pair. In the microsim below the reader sets the charges of the two ions, from 1 to 3, and their radii, and watches the [[Potential_energy|potential-energy]] well `U(r) = −k·z₊·z₋·e²/r + B/rⁿ` deepen and narrow: the first term is the Coulomb attraction, the second the short-range Born repulsion that keeps the electron shells from overlapping, and the minimum of the curve at `r₀` is the equilibrium separation.[^sanny-91] The equation the sim answers is how the depth of that well, and with it the lattice enthalpy, grows with the product of the charges and falls with the sum of the radii, with the Portal Book's Born–Haber values for NaCl (−787 kJ/mol) and MgCl₂ (−2526 kJ/mol) drawn as anchors.[^blackstock-lattice] ## Overview An ionic bond is what remains when the electronegativity difference between two atoms is so large that sharing gives way to transfer. In the language of the van Arkel–Ketelaar triangle it is the top corner, reached when `ΔEN` is large and one partner is a metal; the Portal Book puts Na–Cl there at `ΔEN = 2.1`.[^blackstock-en] Once the electron has moved, the [[Electron_configuration|electron configurations]] of both ions are usually those of a [[Noble_gas|noble gas]]: Na⁺ has the ten electrons of neon and Cl⁻ the eighteen of argon, which is the [[Octet_rule|octet rule]] read as a statement about ions. What holds them together is nothing more specific than electrostatics. A cation and an anion attract with a force that falls off as the inverse square of their separation and does not depend on direction, so an ion has no reason to prefer one neighbour and every reason to surround itself with as many oppositely charged ions as its size allows.[^os-af-ch4] That is why an ionic compound has no molecules. The formula NaCl is a ratio, one sodium for every chlorine, and the "bond" is the sum of all the attractions and repulsions in a crystal of some 10²² ions. The sim's pair potential is the first term of that sum, one cation and one anion alone, and the lattice enthalpy it reports is the pair well scaled by the geometry of the crystal. ### Properties of ionic bonds The properties of ionic solids follow from a lattice of charges held by non-directional forces. They are [[Hardness|hard]] and have high melting points, because moving an ion means pulling it away from several oppositely charged neighbours at once; sodium chloride melts at about 800 °C.[^os-af-ch4] They are brittle rather than malleable: a blow that shifts one plane of ions by half a spacing brings like charges face to face, and the crystal cleaves along that plane instead of bending, which is the opposite of what a [[Metallic_bonding|metal]] does.[^sanny-91] They do not conduct electricity as solids, because the ions are fixed, but conduct well when molten or dissolved, because the charge carriers are then free; that difference is how Arrhenius recognised ions in solution and how [[Electrolysis|electrolysis]] extracts sodium and chlorine from molten salt.[^os-af-ch4] They [[Solubility|dissolve]] in polar solvents, above all [[Water|water]], whose molecules surround each ion with their oppositely charged ends and repay much of the lattice energy, and they are insoluble in nonpolar ones, which cannot. Each of these properties is a reading of the depth of the well the sim draws: deepen it, by raising the charges or shrinking the ions, and the melting point, hardness and resistance to dissolution all rise together.[^blackstock-lattice] ## Formation Transferring an electron from a metal atom to a nonmetal atom costs energy, and an ionic bond forms only because the lattice pays it back with interest. For sodium chloride the Portal Book's Born–Haber cycle lays out the account in kilojoules per mole of NaCl: 107 to turn solid [[Sodium|sodium]] into gaseous atoms, 496 of [[Ionization_energy|ionisation energy]] to ionise those atoms, 122 to split half a mole of [[Chlorine|chlorine]] into atoms, −349 recovered when each chlorine atom accepts the electron, and then the lattice enthalpy, which must be −787 for the sum to equal the measured enthalpy of formation of −411.[^blackstock-bh] Read from the middle, the electron transfer alone is unfavourable, `496 − 349 = +147 kJ/mol`; a sodium atom and a chlorine atom in the gas phase would rather stay neutral. The ions form because, once formed, they can fall into a well 787 kJ/mol deep. The microsim shows the same account for a single pair in electronvolts, the unit the Physics Portal Book uses. Ionising sodium costs 5.14 eV and chlorine's electron affinity returns 3.62 eV, so the transfer costs 1.52 eV per pair.[^sanny-91] Bringing the two ions to the separation of the gas-phase NaCl molecule, 0.236 nm, releases Coulomb energy `k·e²/r₀ = 1.44 eV·nm / 0.236 nm = 6.1 eV` (derived), which more than repays the transfer.[^sanny-91] The repulsive term reduces the gain: with `U(r) = −k·e²/r + B/rⁿ`, requiring the slope to vanish at `r₀` fixes `B = k·e²·r₀ⁿ⁻¹/n`, and the depth of the well becomes `U(r₀) = −(k·e²/r₀)·(1 − 1/n)`. The exponent `n` is a display parameter in the sim (ILLUSTRATIVE; the Born exponent of a real pair is fitted to compressibility data): at `n = 8` the well is 5.34 eV deep and the pair is bound by `5.34 − 1.52 = 3.82 eV` relative to the neutral atoms (derived). Dragging the charge slider to `z₊ = z₋ = 2` quadruples the Coulomb term at the same `r₀`, which is the first thing the reader sees and the reason [[Magnesium|magnesium]] oxide is far harder to melt than NaCl. ## Structures Because the attraction is not directional, the structure of an ionic solid is a packing problem: each ion takes as many oppositely charged neighbours as can touch it without the like-charged neighbours touching one another. The number of nearest neighbours is the [[Coordination_number|coordination number]], and for two ions of radii `r₊` and `r₋` simple geometry sets the limits. Six anions can surround a cation octahedrally when the radius ratio `r₊/r₋` exceeds `√2 − 1 = 0.414`, and eight can surround it cubically when the ratio exceeds `√3 − 1 = 0.732` (derived from the geometry of the octahedron and the cube). Sodium chloride adopts the first arrangement, the rock-salt structure, in which every Na⁺ has six Cl⁻ neighbours and every Cl⁻ six Na⁺, on a face-centred [[Cubic_crystal_system|cubic]] lattice whose [[Unit_cell|unit cell]] contains four of each; caesium chloride adopts the second, with eight-fold coordination on a simple cubic lattice.[^os-af-ch4] The crystal itself, with its planes and its cleavage, is the Materials science flagship's rock-salt cell and is treated in the article on sodium chloride. The sim draws the pair, not the crystal, but it labels the nearest-neighbour distance it uses. In the NaCl crystal that distance is 0.282 nm, larger than the 0.236 nm of the isolated molecule, because each ion is now pulled by six neighbours rather than one and the repulsions between second neighbours push the lattice apart.[^sanny-91] The [[Crystal_structure|crystal structure]] is determined experimentally by [[X-ray_crystallography|X-ray diffraction]], which measures the lattice spacing directly, and the radius of an individual ion is then inferred by apportioning the measured spacings between cation and anion, so that ionic radii are consistent within a table rather than absolute. ## Strength of the bonding The strength of ionic bonding is the lattice enthalpy, and the Portal Book states its two controlling variables plainly: it grows with the charges of the ions and falls with their radii.[^blackstock-lattice] The sim's well makes both visible: raising `z₊·z₋` from 1 to 4, as in MgO against NaCl, quadruples the Coulomb term at fixed `r₀`, and increasing the radii, which pushes `r₀` outward, shallows the well as `1/r₀`. The book's anchors are the two chlorides: NaCl at −787 kJ/mol and MgCl₂ at −2526 kJ/mol, a ratio of 3.2, more than the factor of two the charge alone supplies, because Mg²⁺ is also smaller than Na⁺ and pulls its chlorides closer.[^blackstock-lattice] The lattice enthalpy cannot be measured directly; it is inferred from a Born–Haber cycle, and the book quotes both 769 and 787 kJ/mol for NaCl in different places, a reminder that the number depends on the auxiliary data used.[^blackstock-769] The lattice can also be computed; the calculation is the sim's pair well with the crystal's geometry folded in (ILLUSTRATIVE). Summing the Coulomb energy over every ion of the rock-salt lattice multiplies the pair term by the Madelung constant, 1.7476 for that structure, and the repulsion contributes the same `(1 − 1/n)` factor, so `U_lattice = −M·k·z₊·z₋·e²/r₀·(1 − 1/n)`. With `r₀ = 0.282 nm` and `n = 8` this gives −7.81 eV per ion pair, which is −753 kJ/mol, about 4 % below the Born–Haber value of −787 (derived); the Coulomb sum alone would give −861 kJ/mol, so the repulsion costs an eighth of the attraction.[^sanny-91][^blackstock-bh] The agreement is the evidence that the ionic model is right for NaCl; where computed and Born–Haber values disagree, as for silver chloride, the book takes the gap as the signature of a bond that is not purely ionic.[^blackstock-agcl] The book's magnesium chlorides show why the strength matters. MgCl would form with an enthalpy of only −94 kJ/mol; MgCl₂, despite paying a second ionisation energy of 1451 kJ/mol, forms with −643, because its lattice is more than three times deeper; MgCl₃ would need a third ionisation of 7733 kJ/mol, reaching into the neon core, which no lattice can repay, so its enthalpy of formation would be +3949 kJ/mol and it does not exist.[^blackstock-mgcl] The book cautions that its lattice values for MgCl and MgCl₃ are estimates found on the web rather than tabulated data; the argument survives because the numbers are so far apart.[^blackstock-agcl] ## Polarization power effects The pair well treats each ion as a hard sphere of charge, and real ions are not. A cation's [[Electric_field|electric field]] pulls the electron cloud of a neighbouring anion toward itself, distorting it; the more the cloud is distorted, the more electron density sits between the nuclei and the more the bond acquires covalent character. Fajans' rules, set out in 1923, summarise when the distortion is large: a small, highly charged cation has the strongest polarising power, a large anion with a loosely held outer shell is the most polarisable, and cations without a noble-gas configuration, such as Ag⁺ and the ions of the [[Transition_metal|transition metals]], polarise more strongly than alkali-metal cations of the same size and charge.[^fajans1923] The sim's radius slider shows the geometric half of this: shrinking the cation at fixed charge concentrates its field and deepens the well, which is the polarising power rising. The chemical consequence is that the Born–Haber lattice enthalpy, which is what nature actually pays, exceeds the value computed for point charges by an amount that measures the covalent contribution. The Portal Book's example is [[Silver|silver]] chloride, for which the theoretical and Born–Haber lattice energies differ, while for the alkali halides they agree.[^blackstock-agcl] The same rules predict the trends a chemist sees on the bench: the anhydrous chlorides of [[Aluminium|aluminium]] and the heavier transition metals sublime and dissolve in organic solvents like covalent compounds, iodides are more covalent than fluorides of the same metal, and the [[Electronegativity|electronegativity]] difference alone, which the sibling sim uses, under-predicts the covalency of any bond involving a small, highly charged or d-block cation. ## Comparison with covalent bonding No bond is purely ionic. The electronegativity difference of Na–Cl, 2.1, gives an estimated ionic character of 67 % by Pauling's relation, and H–Cl at 0.9 gives 18 % (derived from the book's values); the two lie on one continuous scale, and "ionic" and "covalent" name its ends.[^blackstock-en][^pauling1960] What distinguishes the two ends is where the electrons are and, in consequence, what the solid is like. A [[Covalent_bond|covalent bond]] is a pair of electrons localised between two nuclei, directional, and the property of a molecule; an ionic bond is a transferred pair, non-directional, and the property of a lattice. Covalent solids of discrete molecules melt low because only weak [[Intermolecular_force|intermolecular forces]] must be overcome, while network covalent solids such as [[Diamond|diamond]] melt very high because every bond must be broken; ionic solids sit high because every ion is bound to several neighbours.[^os-af-ch4] Ionic solids conduct when molten and covalent liquids do not, and ionic solids shatter where metals bend. The two sims are built to be compared. On the Chemical bond page the electron cloud between two nuclei skews with `ΔEN` until, at the ionic corner, it has essentially left one atom for the other; this page picks up at that point and asks what the two resulting ions do. The bond energy of a covalent pair, such as the 436 kJ/mol of H–H, and the lattice enthalpy of an ionic solid, such as the 787 kJ/mol of NaCl, are both measured in kilojoules per mole, but the first is the cost of breaking one bond and the second the cost of dispersing a whole crystal into ions, so the numbers are not directly comparable and the sim reports them on separate readouts.[^blackstock-h2][^blackstock-bh] In practice most bonds between a metal and a nonmetal are treated as ionic with a covalent correction, and most bonds between nonmetals as covalent with a polar correction, which is what the two readouts, `% ionic` on one page and the well depth on the other, are for. ## See also - [[Lattice_energy]] - [[Salt_(chemistry)]] - [[Sodium_chloride]] - [[Chemical_bond]] - [[Born–Haber_cycle]] - [[Covalent_bond]] - [[Electronegativity]] - [[Coulomb's_law]] ## References [^os-af-ch4]: Flowers, Paul; Neth, Edward; Robinson, William, et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 4, "Chemical Bonding and Molecular Geometry", pp. 185–244 (§4.1 ionic bonding: formation of ions, properties of ionic compounds, the rock-salt and caesium chloride structures; page to pin). Portal Book 051. https://openstax.org/details/books/chemistry-atoms-first-2e [^blackstock-bh]: Blackstock, Lindsay; Brewer, Sharon; Cinel, Bruno (2022). *Chemical Bonding and Organic Chemistry*. Chapter "Chemical Bonding I — Basic Concepts", pp. 224–225 (Born–Haber cycle for NaCl: 107 + 496 + 122 − 349 + LE = −411 kJ/mol, so LE = −787 kJ/mol; lattice enthalpies cannot be measured directly, p. 224). Portal Book 054. https://open.umn.edu/opentextbooks/textbooks/chemical-bonding-and-organic-chemistry [^sanny-91]: Sanny, Jeff; Ling, Samuel (2016). *University Physics Volume 3*. OpenStax. Chapter 9, "Condensed Matter Physics", §9.1 "Types of Molecular Bonds", pp. 393–440 (ionic bond of NaCl: ionization energy of Na 5.14 eV, electron affinity of Cl 3.62 eV, transfer cost 1.52 eV; Coulomb energy at the equilibrium separation; the Coulomb-plus-repulsion form `U = −k·e²/r + A/rⁿ`; the crystal separation 0.282 nm; brittleness of ionic crystals; page to pin). Portal Book 079. https://openstax.org/details/books/university-physics-volume-3 [^blackstock-lattice]: Blackstock, Lindsay; Brewer, Sharon; Cinel, Bruno (2022). *Chemical Bonding and Organic Chemistry*. Chapter "Chemical Bonding I — Basic Concepts", pp. 222–223 (lattice formation and dissociation enthalpies have the same magnitude and opposite sign; lattice enthalpy grows with ionic charge and falls with ionic radius) and pp. 226–228 (MgCl₂ cycle with lattice enthalpy −2526 kJ/mol). Portal Book 054. [^blackstock-en]: Blackstock, Lindsay; Brewer, Sharon; Cinel, Bruno (2022). *Chemical Bonding and Organic Chemistry*. Chapter "Chemical Bonding I — Basic Concepts", p. 234 (ΔEN examples: H–Cl 0.9; Na–Cl 2.1, ionic). Portal Book 054. [^blackstock-769]: Blackstock, Lindsay; Brewer, Sharon; Cinel, Bruno (2022). *Chemical Bonding and Organic Chemistry*. Chapter "Chemical Bonding I — Basic Concepts", p. 219 (NaCl lattice dissociation quoted as 769 kJ/mol) against pp. 222–225 (787 kJ/mol). Portal Book 054. [^blackstock-agcl]: Blackstock, Lindsay; Brewer, Sharon; Cinel, Bruno (2022). *Chemical Bonding and Organic Chemistry*. Chapter "Chemical Bonding I — Basic Concepts", p. 226 (theoretical and Born–Haber lattice energies differ for AgCl; the lattice values used for MgCl and MgCl₃ are "found on the web"). Portal Book 054. [^blackstock-mgcl]: Blackstock, Lindsay; Brewer, Sharon; Cinel, Bruno (2022). *Chemical Bonding and Organic Chemistry*. Chapter "Chemical Bonding I — Basic Concepts", pp. 226–228 (MgCl: 148 + 738 + 122 − 349 − 753 = −94 kJ/mol; MgCl₂: 148 + 738 + 1451 + 244 − 698 − 2526 = −643 kJ/mol; MgCl₃: 148 + 738 + 1451 + 7733 + 366 − 1047 − 5440 = +3949 kJ/mol). Portal Book 054. [^fajans1923]: Fajans, Kasimir (1923). "Struktur und Deformation der Elektronenhüllen in ihrer Bedeutung für die chemischen und optischen Eigenschaften anorganischer Verbindungen." *Die Naturwissenschaften* 11 (10): 165–172. [^pauling1960]: Pauling, Linus (1960). *The Nature of the Chemical Bond*, 3rd ed. Ithaca: Cornell University Press. Chapter 3, "The partial ionic character of covalent bonds and the relative electronegativity of atoms" (the relation `fraction ionic = 1 − exp(−ΔEN²/4)`; page to pin). [^blackstock-h2]: Blackstock, Lindsay; Brewer, Sharon; Cinel, Bruno (2022). *Chemical Bonding and Organic Chemistry*. Chapter "Chemical Bonding I — Basic Concepts", p. 231 (H₂ → 2H, +436 kJ/mol). Portal Book 054. ## External links - [Chemistry: Atoms First 2e](https://openstax.org/details/books/chemistry-atoms-first-2e), OpenStax — §4.1, the open text behind Portal Book 051 - [University Physics Volume 3](https://openstax.org/details/books/university-physics-volume-3), OpenStax — §9.1, the open text behind Portal Book 079 - [Chemical Bonding and Organic Chemistry](https://open.umn.edu/opentextbooks/textbooks/chemical-bonding-and-organic-chemistry), Open Textbook Library record for Portal Book 054 - For the pair's other external links, see the Wikipedia article's *External links* section. <!-- MATTERSIM:BEGIN g33 — Matter & Energy Cluster microsim (framework build, specs/sims/Ionic_bonding.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Ionic bonding* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Ionic_bonding.html" data-title="Ionic bonding"></div> *Built from `MICROSIM_GUIDE/specs/sims/Ionic_bonding.json`; part of the [[PORTAL_Matter|Matter portal]] spine (section sims and See-also variants).* <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Ionic_bonding) : [Wikitube](https://en.wikitube.io/wiki/Ionic_bonding) · pinned revision [1359556665](https://en.wikipedia.org/w/index.php?oldid=1359556665) · 2026-09-11 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Chemistry]]. --- *Matter & Energy Cluster child articles, wave 1 · 2026-09-11 · drafted · Chemistry row K19 · sim pending (matter/Ionic_bonding).*