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Topic 2: Properties and structure of materials

Describe ionic bonding as the electrostatic attraction between oppositely charged ions in a regular three-dimensional lattice, predict the formula of binary ionic compounds, and relate physical properties (melting point, electrical conductivity, brittleness, solubility) to lattice structure

A focused answer to the QCE Chemistry Unit 1 dot point on ionic bonding. Explains how electron transfer forms cations and anions held in a 3D lattice by electrostatic attraction, predicts formulae for binary ionic compounds, and links lattice structure to the high melting point, brittleness, conductivity only when molten or dissolved, and variable solubility of ionic substances.

Generated by Claude OpusReviewed by Better Tuition Academy8 min answerUpdated 2026-05-18

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What this dot point is asking

QCAA wants you to describe ionic bonding as electrostatic attraction between cations and anions arranged in a 3D lattice, predict the formula of a binary ionic compound from the charges of its ions, and explain the characteristic properties of ionic compounds (high melting points, conductivity only when molten or dissolved, brittleness, variable water solubility) in terms of the lattice structure.

The answer

An ionic bond is the electrostatic attraction between oppositely charged ions, typically formed by complete transfer of one or more electrons from a metal atom to a non-metal atom. In the solid state, the ions are arranged in a regular three-dimensional lattice in which each ion is surrounded by ions of opposite charge. The properties of ionic compounds follow from the strength and geometry of this lattice.

Forming an ionic compound

Metals lose electrons to form cations (positive ions); non-metals gain electrons to form anions (negative ions). Both species typically reach a noble-gas electron configuration (closed-shell stability).

Worked examples:

Na: $...3s^1 \rightarrow Na^+: ...2p^6$ (Ne configuration).
Mg: $...3s^2 \rightarrow Mg^{2+}: ...2p^6$ (Ne configuration).
Al: $...3s^2 3p^1 \rightarrow Al^{3+}: ...2p^6$ (Ne configuration).
Cl: $...3s^2 3p^5 \rightarrow Cl^-: ...3p^6$ (Ar configuration).
O: $...2s^2 2p^4 \rightarrow O^{2-}: ...2p^6$ (Ne configuration).

The electrostatic attraction between cation and anion is the ionic bond. It is not directional in the way a covalent bond is; an ion attracts every oppositely charged ion in its vicinity.

Predicting formulae

The compound must be electrically neutral overall. Use the smallest whole-number ratio that balances charge.

Cation	Anion	Ratio	Formula
Na+	Cl-	1:1	NaCl
Ca^2+	Cl-	1:2	CaCl_2
Al^3+	O^2-	2:3	Al_2O_3
Mg^2+	O^2-	1:1	MgO
Fe^3+	SO_4^2-	2:3	Fe_2(SO_4)_3
NH_4^+	NO_3^-	1:1	NH_4NO_3

The cross-over method (swap charge magnitudes as subscripts, then simplify) is reliable for binary ionic compounds. For polyatomic ions (NH_4^+, NO_3^-, SO_4^2-, CO_3^2-, OH-, PO_4^3-), use brackets when there is more than one of the polyatomic unit.

The ionic lattice

In the solid state, ions are arranged in a regular repeating 3D pattern (a crystal lattice). The geometry depends on the relative sizes of the cation and anion (the radius ratio). Common QCE-relevant lattices:

Rock-salt structure (NaCl). Each Na+ is surrounded by 6 Cl-; each Cl- is surrounded by 6 Na+. Cubic.
Caesium chloride structure (CsCl). Each ion has 8 nearest neighbours of opposite charge. Suited to similar-sized cations and anions.
Fluorite structure (CaF_2). Each Ca^2+ has 8 F-; each F- has 4 Ca^2+.

The lattice energy is the energy released when gaseous ions combine to form the solid lattice. It governs melting point and solubility.

\text{Lattice energy} \propto \frac{q_1 q_2}{r}

where q_1 and q_2 are the charges and r is the inter-ionic distance. Higher charges and smaller ions give a larger lattice energy.

Physical properties from lattice structure

High melting and boiling points. Strong electrostatic attractions throughout the lattice; large lattice energy. Comparing within ionic compounds, higher charges and smaller ions give higher melting points. MgO (charges 2+/2-) melts above 2800 degrees C; NaCl (1+/1-) melts at 801 degrees C.

Hard but brittle. The lattice resists deformation, so the solid is hard. But under stress, one layer of ions can shift over another so that like charges align; the resulting repulsion shatters the crystal along that plane. This is why ionic crystals cleave cleanly under impact.

Electrical conductivity only when molten or dissolved. Conduction requires mobile charge carriers. In the solid, ions are fixed in the lattice. In the molten state, the lattice has broken down and ions can migrate. In aqueous solution, ions are dissociated and surrounded by water (solvation shells) and free to move. Solid ionic conductors are an exception covered in materials chemistry, not in QCE Unit 1.

Variable solubility in water. Water is a polar solvent that solvates ions effectively. Solubility depends on the balance between lattice energy (must be overcome to dissociate the solid) and hydration energy (released when ions are solvated). Compounds with comparable lattice and hydration energies tend to dissolve; compounds with very high lattice energy relative to hydration energy do not. AgCl is famously insoluble (very high lattice energy, modest hydration energy); NaCl is very soluble. QCE Chemistry provides solubility rules to memorise; Unit 2 builds on these.

Naming ionic compounds

Cation name first, anion name second.
Monatomic anion: use the element root plus -ide (chloride, oxide, nitride).
Polyatomic anion: use the standard name (sulfate, nitrate, carbonate, hydroxide, phosphate).
For transition metals with variable oxidation state, indicate the cation charge with a roman numeral: iron(II) chloride is FeCl_2; iron(III) chloride is FeCl_3.

Worked example: predicting properties

Question. Predict whether sodium oxide or magnesium oxide has the higher melting point, and explain your reasoning.

Na_2O has charges +1 / -2; MgO has charges +2 / -2. The product of the charges (proportional to lattice energy) is 1 x 2 = 2 for Na_2O versus 2 x 2 = 4 for MgO. Also Mg^2+ is smaller than Na+ (smaller r), increasing the lattice energy further. Therefore MgO has the higher melting point. Actual values: Na_2O 1132 degrees C; MgO 2852 degrees C. Consistent with the prediction.

This kind of comparative-strength reasoning is a routine QCAA EA short response.

Common traps

Writing formulae with non-simplified subscripts. Mg^2+ with O^2- gives 1:1, so MgO. Never Mg_2O_2.

Calling ionic compounds "molecules". They are formula units, not molecules. NaCl in the solid is an extended lattice, not discrete pairs.

Claiming solid ionic compounds conduct because they contain ions. The ions must be free to move. In the solid lattice they are not.

Forgetting the brittleness mechanism. Brittleness is not just "ionic compounds are weak"; it is specifically the like-charge repulsion when layers shift.

Using "ionic bond strength" loosely. The relevant property is lattice energy, set by charge magnitudes and ionic radii. Quote both factors.

In one sentence

Ionic bonding is the electrostatic attraction between oppositely charged ions arranged in a regular three-dimensional lattice, with formulae predicted by charge balance, and the characteristic properties (high melting point, brittleness, conductivity only when molten or dissolved, variable solubility) all follow from the strength and geometry of that lattice.

Past exam questions, worked

Real questions from past QCAA papers on this dot point, with our answer explainer.

2024 QCAA-style4 marksMagnesium chloride is an ionic compound used as a road de-icer. (a) Write balanced equations showing the electron transfer between a Mg atom and Cl atoms. (b) Write the formula of magnesium chloride and explain why the ratio is 1:2. (c) Explain why magnesium chloride conducts electricity when molten or dissolved in water but not as a solid.

Show worked answer →

A 4-mark answer needs the electron transfer, the ratio reasoning, and the conductivity explanation.

(a) Electron transfer.

Mg \rightarrow Mg^{2+} + 2 e^-

Cl + e^- \rightarrow Cl^-

Mg has two valence electrons in 3s; loses both to reach the [Ne] configuration. Each Cl has seven valence electrons in 3s/3p; gains one to reach the [Ar] configuration.

(b) Formula MgCl_2. The compound must be electrically neutral overall. One Mg^2+ contributes +2; two Cl- contribute -2 in total. Hence the 1:2 ratio and formula MgCl_2.

(c) Conductivity. Conduction requires charge carriers that are free to move. In solid MgCl_2 the ions are locked in fixed positions in the lattice and cannot migrate. When the solid is melted (above approximately 714 degrees C) or dissolved in water, the lattice breaks down and the Mg^2+ and Cl- ions are free to move, so the molten or aqueous form conducts.

Markers reward correct half-equations with electrons balanced, the 1:2 ratio justified by charge neutrality, and explicit reference to free-moving ions in the molten or aqueous state.

2023 QCAA-style3 marksCompare the melting points of NaCl (801 degrees C) and MgO (2852 degrees C). Explain the difference in terms of lattice structure and electrostatic attraction.

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A 3-mark answer needs reference to charge magnitude, ionic radii and lattice energy.

Both NaCl and MgO have the same rock-salt lattice geometry. The energy required to melt the lattice is set by the strength of the electrostatic attraction between the ions, which depends on (i) the magnitudes of the charges and (ii) the inter-ionic distance.

In NaCl: charges are +1 and -1.
In MgO: charges are +2 and -2.

Coulomb's law: the force of attraction scales with the product of the charges. The MgO attraction involves a product of 2 x 2 = 4 versus 1 x 1 = 1 for NaCl, roughly four times stronger per ion pair. Additionally, Mg^2+ and O^2- are smaller than Na+ and Cl-, so the inter-ionic distance is smaller, further increasing the attraction.

Result: MgO has a much higher lattice energy and therefore a much higher melting point.

Markers reward the charge-product reasoning, the radius reasoning, and the explicit link to electrostatic attraction strength.