Ions and Ionic Bonds

Summary: Ionic bonding involves electron transfer from a metal to a non-metal, forming oppositely charged ions held in a giant ionic lattice by strong electrostatic forces. Tags: igcse chemistry bonding Created: 2026-07-14 Last Updated: 2026-07-18

An Ion is a charged particle that forms when an atom gains or loses one or more electrons to achieve a stable noble gas electron configuration. When a metal atom loses electrons, it becomes a positive ion (cation); when a non-metal atom gains electrons, it becomes a negative ion (anion). The ionic bond is the strong force of electrostatic attraction that holds these oppositely charged ions together in a fixed, repeating three-dimensional structure. This regular arrangement of alternating positive and negative ions is known as a giant ionic lattice — it is not composed of discrete molecules but of billions of ions extending in all directions. The simplest whole-number ratio of ions in the lattice is represented by the formula of the ionic compound, such as NaCl for sodium chloride. A dot-and-cross diagram is used to illustrate the transfer of electrons from metal to non-metal during ionic bond formation, with different symbols representing the outer-shell electrons of each atom.

How Ions Form

An ion is a charged particle formed when an atom gains or loses electrons to achieve a stable noble gas electron configuration (full outer shell — the octet rule).

Metals lose electrons to form positive ions (cations):

• Metals (Groups 1, 2, 13) have 1–3 outer electrons — easier to lose them than gain 5–7
• Losing electrons → more protons than electrons → overall positive charge
• e.g., Na (2.8.1) loses 1 e- → Na+ (2.8); Mg (2.8.2) loses 2 e- → Mg2+ (2.8)

Non-metals gain electrons to form negative ions (anions):

• Non-metals (Groups 15, 16, 17) have 5–7 outer electrons — easier to gain than lose
• Gaining electrons → more electrons than protons → overall negative charge
• e.g., Cl (2.8.7) gains 1 e- → Cl- (2.8.8); O (2.6) gains 2 e- → O2- (2.8)

Predicting Ion Charges

Transition metal ions (variable charge): Iron(II) = Fe2+, Iron(III) = Fe3+; Copper(I) = Cu+, Copper(II) = Cu2+ Polyatomic ions: OH- (hydroxide), NO3- (nitrate), CO32- (carbonate), SO42- (sulfate), NH4+ (ammonium), HCO3- (hydrogencarbonate), PO43- (phosphate)

The Ionic Bond

An ionic bond is the strong electrostatic attraction between oppositely charged ions.

Formation (e.g., NaCl): Na (2.8.1) loses its outer electron → Na+ (2.8); Cl (2.8.7) gains that electron → Cl- (2.8.8); Na+ and Cl- attract electrostatically.

MgO: Mg loses 2 electrons → Mg2+; O gains 2 electrons → O2-; charges +2 and -2 balance → formula MgO (1:1).

Key distinction: Ionic = electron transfer (metal↔non-metal); Covalent = electron sharing (non-metal↔non-metal); Metallic = delocalised electrons (metal↔metal).

Giant Ionic Lattice Structure

Ionic compounds do not form molecules. They form a giant ionic lattice — a regular, repeating 3D arrangement of alternating positive and negative ions extending in all directions.

Key features:

• Each cation is surrounded by anions, each anion by cations — maximising attraction, minimising repulsion
• Strong electrostatic forces of attraction in all directions throughout the lattice
• The formula (e.g., NaCl) represents the simplest whole-number ratio of ions, NOT a molecule
• In NaCl: each Na+ is surrounded by 6 Cl-; each Cl- by 6 Na+; overall ratio 1:1

Properties of Ionic Compounds

All properties derive from the giant ionic lattice and strong electrostatic forces.

Dot-and-Cross Diagrams

Use different symbols (o for one atom’s electrons, x for the other). Show outer shells, electron transfer arrows, and resulting ions in square brackets with charges.

Four key diagrams:

• NaCl: Na (2.8.1) → Na+ + e-; Cl (2.8.7) + e- → Cl- (2.8.8); 1:1 ratio
• MgO: Mg (2.8.2) → Mg2+ + 2e-; O (2.6) + 2e- → O2- (2.8); 1:1 ratio
• CaCl2: Ca → Ca2+ + 2e-; two Cl atoms each gain 1e- → 2 Cl-; 1:2 ratio
• Na2O: two Na atoms each lose 1e- → 2 Na+; O gains both → O2-; 2:1 ratio

Formula Writing from Ion Charges

Balance total positive and total negative charge so the compound is electrically neutral.

Method: Write ions with charges → find lowest common multiple of charges → determine the number of each ion → write formula with subscripts.

Examples:

• NaCl: Na+ + Cl- → charges ±1 balance → NaCl
• CaCl2: Ca2+ + 2Cl- → one +2 needs two -1 → CaCl2
• Al2O3: Al3+ + O2- → LCM of 3 and 2 = 6 → 2 Al3+ (+6) and 3 O2- (-6) → Al2O3
• Mg(NO3)2: Mg2+ + 2NO3- → one +2 needs two -1 → Mg(NO3)2
• For polyatomic ions appearing more than once, use brackets: Ca(OH)2, NOT CaOH2

Key Points

• Metals lose electrons → cations (+); Non-metals gain → anions (-)
• Group 1 → 1+; Group 2 → 2+; Group 16 → 2-; Group 17 → 1-
• Ionic bond: strong electrostatic attraction between oppositely charged ions
• Giant ionic lattice: regular alternating 3D arrangement of + and - ions
• High mp/bp: lots of energy needed to overcome strong electrostatic forces
• Conductivity: solid = no (ions fixed); molten/aqueous = yes (ions free to move)
• Brittle: layers shift → like charges align → repel → crystal shatters
• Formula: simplest whole-number ratio of ions (not a molecule)

Key Concepts from Past Papers

• Ion: a charged particle formed when an atom gains or loses one or more electrons
• Ionic bond: the strong electrostatic attraction between oppositely charged ions
• Giant ionic lattice: a regular, repeating 3D arrangement of alternating positive and negative ions held by strong electrostatic forces in all directions
• Ions are held in a giant lattice by strong electrostatic forces of attraction
• High melting point because a lot of energy is needed to overcome the strong electrostatic forces
• Conducts electricity when molten/aqueous because ions are free to move
• Does not conduct when solid because ions are in fixed positions and cannot move

Keywords from Past Papers

conduct, high, graphite, low, electricity, ions, melting, points, reacts, boiling, point, calcium, oxide, diamond, heat

Related Notes

• Atomic Structure
• Covalent Bonding
• Metallic Bonding
• Giant Structures
• Electrolysis
• Ion
• Positive Ions (Cations)
• Negative Ions (Anions)
• Ionic Bond
• Electrostatic Attraction
• Dot-and-Cross Diagram
• Ionic Compounds
• Giant Ionic Lattice

Sources

• OpenStax Chemistry 2e — Chapter 7: Chemical Bonding and Molecular Geometry, Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — Ionic Compounds, BBC (free educational resource)
• Syllabus Section 2.4 — Syllabus topic 3 (Atoms, elements and compounds — bonding), Cambridge Assessment International Education
• CK-12 Chemistry for High School — Ionic Bonding, CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/32 Feb/March 2017: Q22(b)(iii) (1m), Q22(d)(i) (1m)
• 0620/32 Feb/March 2020: Q22(b)(ii) (3m)
• 0620/32 Feb/March 2021: Q77(b)(i) (1m)
• 0620/32 Feb/March 2022: Q66(c)(i) (1m)
• 0620/32 May/June 2020: Q55(b)(ii) (1m)
• 0620/33 May/June 2016: Q55(b)(i) (1m), Q55(b)(i) (1m)
• 0620/33 May/June 2021: Q88(b)(iii) (1m)
• 0620/33 May/June 2022: Q33(b)(iv) (2m)
• 0620/33 May/June 2023: Q88(c)(ii) (2m), Q88(c)(ii) (2m)
• 0620/33 May/June 2024: Q33(a)(ii) (1m)
• 0620/33 October/November 2015: Q44(b)(i) (2m), Q33(a)(ii) (1m), Q33(a)(ii) (1m)
• 0620/33 October/November 2017: Q44(f)(i) (0m), Q55(b)(ii) (2m)

Common Misconceptions