Ionic Bond
Summary: An ionic bond is the strong electrostatic attraction between oppositely charged ions, formed by the transfer of electrons from a metal atom to a non-metal atom. Tags: igcse chemistry Created: 2026-07-18
An ionic bond is a strong Electrostatic Attraction between oppositely charged ions — the exact definition required by the syllabus. It forms when one or more electrons are transferred from a metal atom to a non-metal atom: the metal becomes one of the Positive Ions (Cations) and the non-metal becomes one of the Negative Ions (Anions), and the resulting opposite charges attract strongly. The classic examples are compounds of Group I metals with Group VII non-metals, such as sodium chloride, but ionic bonding occurs between any metallic and non-metallic elements. Because the attraction acts equally in all directions, ionic bonds do not join ions in pairs but bind enormous numbers of ions into a regular giant ionic lattice, which explains the characteristic properties of Ionic Compounds such as high melting points and electrical conductivity when molten or aqueous. Electron transfer is represented on paper using a Dot-and-Cross Diagram, and the topic as a whole sits alongside Covalent Bonding and Metallic Bonding as one of the three types of strong chemical bonding at IGCSE.
Definition
An ionic bond is the strong electrostatic attraction between oppositely charged ions.
Every word of this definition earns credit in an exam answer:
- strong — ionic bonds require a large amount of energy to overcome, which is why ionic compounds have high melting and boiling points;
- electrostatic attraction — the force between opposite electric charges (see Electrostatic Attraction);
- oppositely charged ions — the bond is between a positive ion and a negative ion, not between atoms or molecules.
How an Ionic Bond Forms: Electron Transfer
Ionic bonds form by the complete transfer of electrons from a metal atom to a non-metal atom. This is the key contrast with Covalent Bonding, where electrons are shared:
- The metal atom loses its outer-shell electron(s), becoming a cation with a full outer shell (Electronic Configuration of a noble gas).
- The non-metal atom gains those same electron(s), becoming an anion with a full outer shell.
- The oppositely charged ions attract each other electrostatically — this attraction is the ionic bond.
At Core level, the syllabus focuses on bonds between Group I and Group VII elements (e.g. NaCl, KBr, LiF). At Supplement level, this is extended to ionic bonding between any metallic and non-metallic elements — including Group II and Group VI combinations such as MgO and CaCl₂, and compounds of transition metals.
Example: Formation of Sodium Chloride (NaCl)
Step by step, using electronic configurations:
| Step | What happens |
|---|---|
| 1 | A sodium atom (Group I, configuration 2.8.1) has one outer-shell electron |
| 2 | A chlorine atom (Group VII, configuration 2.8.7) has seven outer-shell electrons — one short of a full shell |
| 3 | The sodium atom transfers its outer electron to the chlorine atom |
| 4 | Sodium becomes Na⁺ (2.8): it now has 11 protons but only 10 electrons — see Positive Ions (Cations) |
| 5 | Chlorine becomes Cl⁻ (2.8.8): it now has 17 protons but 18 electrons — see Negative Ions (Anions) |
| 6 | Both ions have full outer shells (noble gas configurations — see Group 0 Noble Gases) |
| 7 | The Na⁺ and Cl⁻ ions are held together by strong electrostatic attraction — the ionic bond |
As electron-loss and electron-gain equations:
- Na → Na⁺ + e⁻
- Cl + e⁻ → Cl⁻
The transfer is drawn using a Dot-and-Cross Diagram: the sodium electron (shown as a dot) is drawn moving into chlorine’s outer shell (electrons shown as crosses), and the finished ions are drawn in square brackets with their charges.
Balancing Charges in Other Compounds
Where the charges are not equal and opposite in a 1 : 1 ratio, more than one ion of one type is needed so that the compound is electrically neutral overall:
| Compound | Transfer | Ions formed | Ratio |
|---|---|---|---|
| MgO | Mg gives 2 electrons to one O | Mg²⁺ and O²⁻ | 1 : 1 |
| CaCl₂ | Ca gives 1 electron to each of two Cl atoms | Ca²⁺ and 2 Cl⁻ | 1 : 2 |
| Na₂O | Two Na atoms each give 1 electron to one O | 2 Na⁺ and O²⁻ | 2 : 1 |
From Bond to Lattice
An ionic bond is non-directional: an ion attracts every oppositely charged ion around it, not just one partner. As a result, ions pack into a giant ionic lattice — a regular alternating arrangement of positive and negative ions extending in three dimensions. There are no molecules of NaCl; the formula simply gives the ratio of ions. The lattice structure and the properties it produces are covered in Ionic Compounds and Ions and Ionic Bonds, and the contrast with other giant structures is discussed in Giant Structures.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.4 (Ions and ionic bonds, Core points 2–3 and Supplement point 6), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”An ionic bond is the electron transfer itself” | Electron transfer creates the ions; the bond is the electrostatic attraction between the oppositely charged ions that results |
| ”Ionic bonds are weak because they are ‘just attraction‘“ | The electrostatic attraction is strong — overcoming it takes a lot of energy, hence high melting points |
| ”The ionic bond in NaCl joins one Na⁺ to one Cl⁻ like a pair” | The attraction acts in all directions: each ion attracts all its oppositely charged neighbours in the giant lattice |
| ”Electrons are shared in an ionic bond” | Sharing is Covalent Bonding; in ionic bonding electrons are completely transferred from metal to non-metal |
| ”Only Group I and Group VII elements form ionic bonds” | Any metal can bond ionically with any non-metal — MgO, CaCl₂ and Al₂O₃ are all ionic |
| ”The atoms are attracted because one ‘wants’ the other’s electron” | Neutral atoms do not attract ionically; the attraction only exists after the charged ions have formed |