Metallic Bonding and Comparing Bond Types
In 1986, researchers at IBM Zurich discovered a ceramic compound that conducted electricity at −238 °C, shattering the assumption that only metals with free electrons could carry current.
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Q1 · Think about properties of metals you've observed, they conduct electricity, can be bent without shattering, and feel solid, how do you think the atoms inside a metal are arranged to allow all of this?
Q2 · Why do you think the type of bonding in a material (ionic, covalent, or metallic) would affect what that material can and cannot be used for?
● Know
- How metallic bonds form in a lattice of cations and delocalised electrons
- The key properties of metals (malleable, ductile, conductive, high melting point)
- How to compare ionic, covalent, and metallic bonding
● Understand
- Why metals conduct electricity (free electrons can move)
- Why metals are malleable and ductile (layers can slide without breaking bonds)
- Why ionic compounds are brittle but metals are not
● Can do
- Describe the metallic bonding model
- Explain metal conductivity and malleability using the sea-of-electrons model
- Compare and contrast ionic, covalent, and metallic bonding in a table
Place an ice cube, a copper coin, a rock-salt crystal, and a diamond side by side on a bench: all four are solid at room temperature, yet if you tried to melt them, one would melt in your hand, one at 801 °C, one at 1085 °C, and one would still be solid at 3500 °C. The type of bonding in a material determines its physical properties in a predictable way. Ionic compounds (e.g. NaCl, MgO): high melting points (801 °C and 2852 °C respectively), brittle, conduct only when dissolved or molten, often soluble in water. Simple covalent molecules (e.g. H₂O, CO₂): low melting points (0 °C and −78.5 °C), soft, don't conduct electricity. Giant covalent structures (e.g. diamond, SiO₂): extremely high melting points (>3500 °C), very hard, don't conduct electricity. Metallic (e.g. Cu, Fe): variable melting points, malleable, ductile, conduct electricity and heat.
These patterns arise because properties are determined by the forces that must be overcome when the material is used or processed. A high melting point means strong forces between particles, either multiple ionic bonds or actual covalent bonds must break. A low melting point means weak forces, only gentle intermolecular attractions between neutral molecules. Conductivity depends on whether charged particles (electrons or ions) can move freely. Once you know the bond type, you know the entire property profile before ever measuring it.
Predicting unknown compound X: melting point 780 °C, brittle, conducts electricity when molten but not as solid, soluble in water. Diagnosis: ionic. Confirmed, compound X is potassium bromide (KBr), an ionic compound matching every predicted property from bond-type reasoning alone.
CSIRO materials scientists routinely predict properties of new ceramic and polymer materials computationally before synthesising them in the lab, saving years of experimental work. This bond-type reasoning, taught in Year 9, is the same foundational logic used in Australia's advanced manufacturing sector to screen millions of candidate materials computationally.
You can predict the type of bonding from a compound's formula using a three-step rule. (1) If the formula is a single element symbol (e.g. Cu, Fe, Al): metallic. (2) If the formula contains a metal symbol plus a non-metal symbol (e.g. NaCl, MgO, CaF₂): ionic. (3) If the formula contains only non-metal symbols (e.g. H₂O, CO₂, HCl, CH₄, SiO₂): covalent. For covalent, distinguish simple molecular (small molecule, generally low melting point) from giant covalent (usually contains Si, or is carbon like diamond/graphite, with very high melting point).
Worked examples: MgO, Mg is a metal, O is a non-metal → ionic. HCl, both H and Cl are non-metals → covalent (simple molecular, gas at room temperature). Cu, single metal element → metallic. SiO₂, both Si and O are non-metals, but Si forms giant covalent structures → giant covalent. The formula-to-bond-type prediction is a skill that becomes automatic with practice and immediately unlocks the full property profile of any compound.
Predict and explain: (a) KBr, K is metal, Br is non-metal → ionic → high melting point, brittle, conducts when dissolved. (b) CCl₄, C and Cl both non-metals, small molecule → simple covalent → low melting point (−23 °C), doesn't conduct, doesn't dissolve in water.
Australian mining companies test for bond type during ore analysis: a compound that conducts electricity when dissolved (ionic) is treated differently from one that doesn't (covalent). The distinction determines which extraction process, electrolysis for ionic, solvent extraction for covalent, is applied at mines across WA and QLD.
Bond type is the first filter in any material selection process. An engineer designing a component for a jet engine combustion chamber (operating at 1600 °C) immediately eliminates all simple covalent materials (too low melting point) and most metals (melt below 1600 °C). The shortlist becomes: ionic ceramics (zirconia, alumina) and giant covalent ceramics (silicon carbide). This eliminates thousands of candidates in seconds using only bond-type knowledge.
For electrical applications: circuit boards need insulators (covalent: fibreglass, epoxy resin) to prevent unwanted current flow, but also conductors (metallic: copper tracks) to route current to components. Connecting them requires solder, a metallic alloy. Every layer of a circuit board is engineered using bond-type knowledge. The insulator is covalent; the conductor is metallic; the whole assembly is designed so bond types never mix in the wrong location.
Alumina (Al₂O₃, ionic ceramic): melting point 2072 °C, electrical insulator, hard (Mohs 9). Used as spark plug insulators in every car engine in Australia, the ionic bonding gives both the thermal resistance to survive thousands of ignition events and the electrical insulation to prevent short circuits.
NGK Insulators (Japan, with Australian distributors) manufactures spark plugs with alumina ceramic insulators for the entire Australian automotive market. The engineers who specify these parts use exactly the bond-type property prediction you've learned, ionic ceramics for insulation and heat resistance, metallic alloys for the electrode that must conduct and resist spark erosion.
Bond type is the first in any material selection process. For a 1600 °C jet engine chamber, an engineer eliminates simple covalent materials because their melting points are too . The shortlist becomes ceramics and giant covalent ceramics, which have the highest melting points. Circuit boards need such as fibreglass to prevent unwanted current flow. They also need metallic such as copper to route current to components.
At the start of this lesson, you heard that sodium chloride melts at 801 °C, water melts at 0 °C, and diamond melts at over 3500 °C, three solids with wildly different melting points, and the difference is entirely due to bonding type. This single idea lets you predict properties before ever testing a material in a lab.
Now that you've worked through the lesson, how has comparing the three bond types changed your thinking? Could you now predict whether a substance has a high or low melting point, and whether it conducts electricity, just from knowing what type of bonding it has?
Q1. Describe the structure of a metallic bond using the 'sea of electrons' model. Explain why this model accounts for electrical conductivity.
Q2. Using your knowledge of metallic and ionic bonding, explain why a copper wire can be bent easily but a salt crystal shatters when struck.
Q3. Compare ionic, covalent, and metallic bonding. For each type, describe: how the bond forms, one property it causes, and one application of a material with that bond type.