← Unit 1: How can the diversity of materials be explained?
How can knowledge of elements explain the properties of matter?
the nature of metallic bonding and the properties of pure metals and alloys, and the nature of ionic bonding and the properties, names and formulas of binary and ternary ionic compounds
A focused VCE Chemistry Unit 1 answer on metallic and ionic bonding. Covers the metallic bonding model and how it explains malleability, conductivity and lustre; the role of alloying; the ionic bonding model and lattice structure; and the writing of names and formulas of binary and ternary ionic compounds.
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What this dot point is asking
VCAA wants you to describe the metallic bonding model and use it to explain the properties of metals and alloys, to describe the ionic bonding model and use it to explain the properties of ionic compounds, and to write correct names and formulas of binary and ternary ionic compounds.
The answer
Metallic bonding model
A metal is a regular three-dimensional lattice of positive metal cations held together by a sea of delocalised valence electrons. Each atom contributes its valence electrons to a shared pool that is free to move throughout the lattice. The non-directional electrostatic attraction between the positive cations and the negative electron sea is the metallic bond.
The model explains the standard suite of metallic properties:
| Property | Explanation |
|---|---|
| Electrical conductivity (solid and liquid) | Delocalised electrons free to move under a potential difference. |
| Thermal conductivity | Delocalised electrons transfer kinetic energy quickly through the lattice. |
| Malleability and ductility | Layers of cations slide past one another without breaking bonds; the electron sea adapts. Non-directional bonding. |
| High melting and boiling points | Strong electrostatic attraction between cations and the electron sea; takes a lot of energy to overcome. |
| Lustre | Delocalised electrons absorb and re-emit light at all visible wavelengths. |
| High density | Cations pack closely in the lattice. |
Alloys
An alloy is a mixture of a metal with one or more other elements (often other metals). Examples: bronze (Cu + Sn), brass (Cu + Zn), stainless steel (Fe + Cr + Ni + C), solder (Sn + Pb).
Alloying typically makes a metal harder and stronger but less malleable. The model explanation: the foreign atoms are a different size and disrupt the regular planes of cations. Layers cannot slide past one another as easily, so the alloy resists deformation. Most pure metals are too soft for structural use; alloying is the standard fix.
Ionic bonding model
An ionic compound is a 3D lattice of alternating positive and negative ions held together by the strong electrostatic attraction between opposite charges (the ionic bond). It is non-directional: every cation is attracted to every nearby anion.
Ionic compounds form when a metal (low ionisation energy) transfers electrons to a non-metal (high electronegativity), producing cations and anions in whole-number ratios so the lattice is electrically neutral.
Properties:
| Property | Explanation |
|---|---|
| High melting and boiling points | Strong electrostatic forces between many ions in the lattice; lots of energy to disrupt. |
| Brittle | A shift of one layer brings like-charged ions next to each other; repulsion shatters the crystal. |
| Hard but brittle | Strong bonds resist scratching, but the crystal cracks along planes. |
| Electrical conductivity: only when molten or in solution | Ions are fixed in the solid lattice; molten or dissolved, they are mobile and can carry charge. |
| Often soluble in water | Polar water molecules surround and stabilise the separated ions. |
| Form crystals with regular geometry | Reflects the underlying lattice structure (e.g. NaCl forms cubic crystals). |
Writing names and formulas
Binary ionic compound = a metal cation + a non-metal anion. The anion takes the -ide suffix (chloride, oxide, sulfide, nitride).
Ternary ionic compound = uses a polyatomic ion (carbonate CO3^2-, sulfate SO4^2-, nitrate NO3^-, phosphate PO4^3-, hydroxide OH^-, ammonium NH4^+, hydrogencarbonate HCO3^-).
Procedure for the formula:
- Identify the charges of the cation and anion.
- Cross over the charges (without the signs) to get the subscripts.
- Reduce to the lowest whole-number ratio.
- Use brackets around a polyatomic ion if you need more than one of it.
Examples:
| Cation | Anion | Formula | Name |
|---|---|---|---|
| Na^+ | Cl^- | NaCl | sodium chloride |
| Mg^2+ | O^2- | MgO | magnesium oxide |
| Al^3+ | O^2- | Al2O3 | aluminium oxide |
| Ca^2+ | NO3^- | Ca(NO3)2 | calcium nitrate |
| Ammonium NH4^+ | SO4^2- | (NH4)2SO4 | ammonium sulfate |
| Fe^3+ | OH^- | Fe(OH)3 | iron(III) hydroxide |
| Cu^+ | S^2- | Cu2S | copper(I) sulfide |
For transition-metal cations with more than one common charge (Fe^2+/Fe^3+, Cu^+/Cu^2+), the charge is written as a Roman numeral in brackets.
Worked example
Sodium phosphate. Cation: Na^+. Anion: PO4^3-. Cross over: 3 Na for 1 PO4. Formula Na3PO4. Name: sodium phosphate.
Iron(III) sulfate. Cation: Fe^3+. Anion: SO4^2-. Cross over: 2 Fe for 3 SO4. Brackets needed around the polyatomic. Formula Fe2(SO4)3. Name: iron(III) sulfate.
Bronze. Mostly copper with a smaller percentage of tin. The lattice of Cu cations is disrupted by Sn atoms of a different size, so the layers cannot slide as easily. Bronze is harder than pure copper but less malleable.
Common traps
Drawing electrons as static dots in a metallic lattice. They are delocalised, not paired between specific atoms. The sea description is essential.
Saying ionic compounds conduct in the solid state. They do not. Ions are fixed in the lattice. Only molten or dissolved ionic compounds conduct.
Forgetting brackets around polyatomic ions. Calcium nitrate is Ca(NO3)2, not CaNO32 or CaN2O6.
Missing Roman numerals for transition metals. Iron forms Fe^2+ and Fe^3+. The compound name must specify which (iron(II) chloride vs iron(III) chloride).
Not reducing to lowest ratio. Mg^2+ and O^2- cross-over gives Mg2O2; reduce to MgO.
Using crossing-over without checking ratios. For Al^3+ and N^3- crossing over gives Al3N3, which reduces to AlN. The check (positive total + negative total = 0) avoids miscounting.
In one sentence
A metal is a lattice of positive cations in a sea of delocalised valence electrons, explaining conductivity, malleability and lustre (and why alloys are harder than pure metals), while an ionic compound is a 3D lattice of cations and anions held by strong electrostatic forces, with the formula built by balancing charges and the name using -ide or polyatomic-ion conventions plus Roman numerals where the cation charge varies.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
2024 VCE4 marksUse the metallic bonding model to explain why copper is (a) a good electrical conductor and (b) malleable. (c) Explain why bronze (a copper-tin alloy) is harder than pure copper.Show worked answer →
A 4-mark answer needs the model named, then each property explained in terms of it.
The model. A metal is a lattice of positive metal cations (Cu^2+ in this case is loose; for the purposes of the model the kernels are positive) held together by a sea of delocalised valence electrons free to move throughout the lattice.
(a) Electrical conductivity: the delocalised electrons are free to move through the lattice when a potential difference is applied. Moving charge is an electric current. Copper conducts well because each Cu atom contributes its valence electrons to the sea.
(b) Malleability: when a force is applied, layers of cations can slide past one another without breaking the bonding, because the bonding is non-directional. The sea of electrons re-adjusts around the new arrangement. The metal deforms without shattering.
(c) Bronze (Cu/Sn) is harder: tin atoms are a different size from copper atoms. They disrupt the regular lattice planes, making it harder for layers of cations to slide past one another. The result is less malleability, but more hardness and tensile strength.
2025 VCE3 marksWrite the name and formula of the ionic compound formed from: (a) magnesium and chlorine, (b) aluminium and oxygen, (c) calcium and the phosphate ion.Show worked answer →
A 3-mark answer needs the right cation/anion charges and a neutral formula in lowest whole-number ratio.
(a) Mg^2+ and Cl^-: the formula must be electrically neutral. One Mg^2+ balances two Cl^-, giving MgCl2 (magnesium chloride).
(b) Al^3+ and O^2-: cross over the charges. Two Al^3+ (total +6) balance three O^2- (total -6), giving Al2O3 (aluminium oxide).
(c) Ca^2+ and PO4^3-: cross over the charges. Three Ca^2+ (total +6) balance two PO4^3- (total -6), giving Ca3(PO4)2 (calcium phosphate). Note the brackets around the polyatomic ion when more than one is needed.
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