4.2 Bonding and Structure
🎯 Learning outcomes
After completing this lesson, you should be able to:
- describe, in simple terms, the lattice structure of a crystalline solid which is:
- giant ionic, including sodium chloride and magnesium oxide
- simple molecular, including iodine, buckminsterfullerene (C₆₀) and ice
- giant molecular, including silicon(IV) oxide, graphite and diamond
- giant metallic, including copper
- describe, interpret and predict the effect of different types of structure and bonding on the physical properties of substances
- deduce the type of structure and bonding present in a substance from given information
1🔷Introduction: crystal lattices
A crystal lattice is a regularly repeating three-dimensional arrangement of atoms, ions, or molecules in a solid.
Many ionic, covalent and metallic solids are crystalline. The physical properties of a crystal — its melting point, hardness, electrical conductivity and solubility — depend heavily on which of four broad lattice types it forms:
The rest of this lesson looks at each of these four lattice types in turn, then pulls the properties together into one summary table.
2🧂Giant ionic lattices
In an ionic lattice, oppositely charged ions are held together by strong electrostatic forces of attraction, extending in all directions throughout the crystal.
Sodium chloride and magnesium oxide share this same lattice arrangement — only the ions themselves differ (Mg²⁺ and O²⁻ in place of Na⁺ and Cl⁻).
- high melting and boiling points — a lot of energy is needed to overcome the strong electrostatic forces throughout the giant lattice
- hard but brittle — a blow can shift a layer of ions so that like charges align, and the resulting repulsion shatters the crystal
- many dissolve in water, since polar water molecules can surround and separate the ions
- conduct electricity only when molten or dissolved — the ions must be free to move; in the solid lattice, they are fixed in place
3🧊Simple molecular lattices
In a simple molecular lattice, individual molecules occupy fixed positions in a regular pattern. The atoms within each molecule are held by strong covalent bonds, but between molecules there are only weak intermolecular forces.
Because only weak intermolecular forces need to be overcome to separate the molecules — the strong covalent bonds inside each molecule stay intact — simple molecular lattices like iodine have low melting points and are often volatile (iodine visibly sublimes on gentle heating).
Ice is also a simple molecular lattice: water molecules are held in a fixed, open arrangement by hydrogen bonds — a relatively strong intermolecular force, but still far weaker than a covalent bond, so ice still melts at a modest temperature.
Buckminsterfullerene (C₆₀) is a third example: a discrete, spherical molecule made of 60 carbon atoms, held together by strong covalent bonds within the “ball” — but only weak intermolecular forces hold separate C₆₀ molecules to each other in the solid, so it behaves as a simple molecular lattice too.
4💎Giant molecular (covalent) structures
A giant molecular (or giant covalent) structure is a single, continuous network of atoms joined by strong covalent bonds extending throughout the whole crystal — there are no individual small molecules.
Diamond
Each carbon atom forms four strong covalent bonds to four neighbouring carbon atoms, arranged tetrahedrally, building up a rigid three-dimensional network.
This makes diamond extremely hard, with a very high melting point — every atom is locked into the network by strong covalent bonds, and a huge number of bonds must be broken to melt or scratch it. With no free ions or delocalised electrons, diamond does not conduct electricity.
Graphite
Carbon can also form flat, hexagonal layers: each carbon atom is covalently bonded to only three others within its layer. The fourth outer-shell electron of each carbon is delocalised, free to move within the plane of the layer.
Because the layers are only held together by weak forces, they can slide over one another — this is why graphite is soft and slippery, and useful as a lubricant and in pencil “lead”. The delocalised electrons within each layer allow graphite to conduct electricity — unusually for a giant covalent structure.
Graphite is the standard exception to “giant covalent structures don’t conduct electricity” — make sure you can explain why (delocalised electrons within the layers) rather than just stating that it does.
Silicon(IV) oxide (SiO₂)
Silicon dioxide has a diamond-like structure: each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom bridges two silicon atoms, extending into a rigid giant covalent network.
Like diamond, silicon dioxide is a hard, high-melting-point solid with no free ions or delocalised electrons, so it does not conduct electricity.
Diamond, graphite and buckminsterfullerene are all made of carbon atoms only — different structural forms of the same element like this are called allotropes.
5⚡Giant metallic lattices
In a metal such as copper, positively charged metal ions are arranged in a regular, close-packed lattice. Each atom loses its outer-shell electrons into a shared “pool” that surrounds every ion in the structure.
Metallic bonding is the strong electrostatic attraction between the lattice of positive metal ions and the delocalised “sea” of electrons surrounding them.
This model explains several characteristic metallic properties:
- good electrical conductivity (solid and molten) — the delocalised electrons are free to flow throughout the lattice
- good thermal conductivity — the mobile electrons can also transfer heat energy quickly
- high melting and boiling points — strong electrostatic attraction acts throughout the giant lattice
- malleable and ductile — when a force is applied, layers of ions can slide over one another and still be held in place by the mobile “sea” of electrons, which simply flows to re-surround the ions in their new positions
Don’t say metallic bonds “break and re-form” when a metal bends — the delocalised electrons are non-directional, so the bonding is maintained continuously as the ion layers slide, rather than breaking and reforming from scratch. This is exactly why metals are malleable and ductile, while ionic lattices (with fixed, directional attraction between specific ions) are brittle and shatter instead.
6📊Structure, bonding & physical properties
Bringing all four lattice types together lets you compare — and predict — their physical properties directly:
| Structure | Particles | Forces holding lattice together | Melting / boiling point | Electrical conductivity | Example |
|---|---|---|---|---|---|
| Giant ionic | ions | strong electrostatic attraction (all directions) | high | only molten or dissolved | NaCl, MgO |
| Simple molecular | molecules | weak intermolecular forces (strong covalent bonds within each molecule) | low | never conducts | I₂, C₆₀, ice |
| Giant molecular | atoms | strong covalent bonds (all directions) | very high | no (except graphite) | diamond, SiO₂, graphite |
| Giant metallic | cations + delocalised e⁻ | strong electrostatic attraction (ions ↔ electron sea) | high | always conducts (solid & molten) | copper |
Given unlabelled data on melting point and conductivity, work through these clues:
- high melting point + conducts only when molten/dissolved → giant ionic
- high melting point + never conducts → giant molecular (diamond, SiO₂)
- high melting point + does conduct as a solid → giant metallic, or graphite
- low melting point + never conducts → simple molecular
📌 Exam tips & quick summary
✅ Do remember
- giant ionic: strong electrostatic forces, high mp, conducts only molten/dissolved, hard but brittle
- simple molecular: weak IMFs between molecules, strong covalent bonds within — low mp, never conducts
- giant molecular: one continuous covalent network — very high mp, hard, doesn’t conduct (except graphite)
- giant metallic: cations + delocalised electron sea — conducts as solid and liquid, malleable, ductile
- diamond, graphite and C₆₀ are all allotropes of carbon
🚫 Do not confuse
- “giant molecular” (one huge covalently bonded network) ≠ “simple molecular” (many small, separate molecules)
- ionic lattices are brittle (directional attraction breaks on impact); metals are malleable (non-directional electron sea re-forms around shifted ions)
- graphite conducts because of delocalised electrons in its layers — this is an exception, not the rule, for giant covalent structures
🔁 Quick recap
- Giant ionic: NaCl, MgO — high mp, conducts molten/dissolved only
- Simple molecular: I₂, C₆₀, ice — low mp, never conducts
- Giant molecular: diamond, graphite, SiO₂ — very high mp, doesn’t conduct (except graphite)
- Giant metallic: copper — conducts solid & molten, malleable, ductile