Ionic bonding occurs when a metal transfers electrons to a non-metal, forming oppositely charged ions that attract each other strongly. The result is a giant three-dimensional lattice — not a discrete molecule — held together by electrostatic forces, which is why table salt is a hard crystal with a melting point of 801 °C.
How does ionic bonding form?
Ionic bonds form between metals (which lose electrons to form positive ions, cations) and non-metals (which gain electrons to form negative ions, anions). The driving force is that both elements achieve a full outer electron shell — the same electronic configuration as a noble gas — which is a lower-energy, more stable arrangement.
Worked example — sodium chloride (NaCl)
- Sodium (Na): electronic configuration 2,8,1 — has 1 electron in its outer shell.
- Chlorine (Cl): electronic configuration 2,8,7 — needs 1 electron to fill its outer shell.
Sodium transfers its outer electron to chlorine:
- Na → Na⁺ + e⁻ (sodium loses 1 electron; electronic configuration becomes 2,8 — same as neon)
- Cl + e⁻ → Cl⁻ (chlorine gains 1 electron; electronic configuration becomes 2,8,8 — same as argon)
The Na⁺ and Cl⁻ ions have opposite charges and attract each other by electrostatic attraction, forming the ionic bond.
How do you work out the charges on common ions?
The charge on a metal ion is usually the same as its group number. Non-metals form ions with a charge of (8 − group number):
| Element | Group | Ion formed | Charge |
|---|---|---|---|
| Sodium (Na) | 1 | Na⁺ | +1 |
| Magnesium (Mg) | 2 | Mg²⁺ | +2 |
| Aluminium (Al) | 3 | Al³⁺ | +3 |
| Oxide (O) | 6 | O²⁻ | −2 |
| Chloride (Cl) | 7 | Cl⁻ | −1 |
For ionic compounds to be electrically neutral, the total positive charge must equal the total negative charge. To write the formula of magnesium oxide: Mg²⁺ and O²⁻ → MgO (charges balance 1:1). For magnesium chloride: Mg²⁺ and Cl⁻ → MgCl₂ (one Mg²⁺ balances two Cl⁻).
What is the structure of an ionic compound?
Ions in an ionic compound do not exist as separate molecules. They arrange themselves in a giant ionic lattice — a regular, repeating three-dimensional pattern in which each positive ion is surrounded by negative ions, and each negative ion is surrounded by positive ions.
In sodium chloride:
- Each Na⁺ ion is surrounded by 6 Cl⁻ ions (and vice versa).
- The ratio of Na⁺ to Cl⁻ is 1:1, hence the formula NaCl.
- The lattice extends in three dimensions, creating a macroscopic crystal.
The strong electrostatic forces act in all directions throughout the lattice — this is why so much energy is required to break the structure apart.
What are the properties of ionic compounds and why?
| Property | Observation | Explanation |
|---|---|---|
| High melting and boiling points | NaCl melts at 801 °C; MgO at 2852 °C | Many strong electrostatic bonds throughout the lattice must be broken |
| Hard and brittle | Crystals scratch other surfaces but shatter when struck | Rigid lattice; a shift in layers brings like charges together → repulsion shatters the crystal |
| Conduct electricity when dissolved in water | Sodium chloride solution conducts; the solid does not | Ions are mobile in solution; in the solid, ions are fixed in the lattice |
| Conduct electricity when molten | Liquid sodium chloride conducts | Ions become mobile when the lattice melts |
| Often soluble in water | Many ionic compounds dissolve readily | Water molecules surround and separate ions from the lattice (hydration) |
The critical distinction is that ionic solids do not conduct electricity because the ions are locked in fixed positions. Only when the lattice is broken — by melting or dissolving — do ions become free to move and carry charge.
How do you draw dot-and-cross diagrams for ionic compounds?
A dot-and-cross diagram shows the electron transfers during ionic bond formation. By convention, dots represent electrons from one atom and crosses from the other.
For NaCl:
- Draw Na with 1 outer electron (dot).
- Draw Cl with 7 outer electrons (crosses).
- Show the electron moving from Na to Cl (an arrow is sometimes added).
- Draw Na⁺ with an empty outer shell in square brackets with a + charge.
- Draw Cl⁻ with 8 electrons in its outer shell in square brackets with a − charge.
For MgO:
- Mg has 2 outer electrons; O has 6 outer electrons.
- Mg transfers both electrons to O.
- Mg²⁺ (empty outer shell) and O²⁻ (8 electrons in outer shell).
Frequently asked questions
Why do ionic compounds have high melting points?
An ionic compound consists of many millions of oppositely charged ions held together by strong electrostatic forces throughout the entire lattice. To melt the compound you must supply enough thermal energy to overcome all these attractions simultaneously and separate the ions from their fixed positions. Because the forces act in all directions and are strong, a very large amount of energy — reflected in a high melting point — is required. Compounds with ions of higher charge (e.g. Mg²⁺ and O²⁻ in MgO) have even stronger forces and even higher melting points.
Why does solid sodium chloride not conduct electricity?
Electrical conduction requires charged particles that are free to move. In solid sodium chloride the ions (Na⁺ and Cl⁻) are locked in fixed positions in the giant lattice by strong electrostatic attraction. They cannot move in response to an applied electric field. When the solid is dissolved in water or melted, the lattice breaks down and the ions become free to migrate — allowing the substance to conduct electricity.
What is the difference between an ionic bond and a covalent bond?
In an ionic bond, electrons are transferred from a metal to a non-metal, forming oppositely charged ions that are held together by electrostatic attraction. In a covalent bond, electrons are shared between two non-metal atoms, with both nuclei attracted to the shared pair. Ionic bonding produces giant lattice structures with high melting points that conduct when molten/dissolved; simple covalent compounds form discrete small molecules with low melting points that generally do not conduct electricity.
How do you predict the formula of an ionic compound?
First, identify the charges on the ions involved (using the periodic group or a supplied data table for transition metals). Then adjust the numbers of each ion so the total positive charge equals the total negative charge. For aluminium oxide: Al³⁺ and O²⁻. To balance charges, take 2 × Al³⁺ (total +6) and 3 × O²⁻ (total −6) → formula is Al₂O₃. Always write the metal (positive ion) first, then the non-metal (negative ion), with no charge signs in the final formula.
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