Bulk Properties of Chemical Structures

Why arrangement matters

A material’s properties depend not just on which bonds it contains, but on where those bonds are. Bonds may hold an enormous, repeating structure together, join atoms within separate small molecules, or run along long polymer chains. This arrangement helps determine whether particles can move, whether the material bends or breaks, and whether it conducts electricity.

Ionic compounds: a rigid lattice

An ionic compound contains positive and negative ions in a repeating three-dimensional lattice. Attractions act in many directions, holding the ions in place. If a force shifts one layer of ions, ions with the same charge can come alongside one another. Their repulsion makes the crystal split rather than change shape: ionic solids are brittle.

The fixed ions cannot carry charge through the solid, so a solid ionic compound does not conduct electricity. When it melts or dissolves in water, its ions can move and it can conduct. Heating an ionic lattice until it melts requires substantial energy because attractions throughout the structure must be overcome.

Simple molecules: separate particles

In a simple molecular substance, strong covalent bonds join atoms within each molecule, but the molecules are separate from one another. Melting or boiling moves molecules apart by overcoming forces between them; it does not normally break the covalent bonds inside them. These intermolecular forces are generally much weaker than the bonds in a giant structure, so simple molecular substances often have comparatively low melting and boiling points. They generally do not conduct electricity because they have no mobile charged particles.

Giant covalent structures: bonds throughout

A giant covalent structure is a continuous network of atoms joined by covalent bonds. There are no separate small molecules to pull apart: melting the structure requires many strong covalent bonds to be broken, so melting points are usually very high. The connected, fixed network also explains why many such solids are hard but brittle rather than malleable. Electrical behaviour depends on the particular arrangement; graphite conducts, whereas diamond does not.

Polymers: long bonded chains

A polymer contains long chains of atoms joined by strong covalent bonds. Forces between neighbouring chains hold the chains together. Because chains are much longer than simple molecules, there can be more attraction between them; many polymers are solids at room temperature. Chains may be able to move past one another without breaking the covalent bonds along them, so a polymer can bend or change shape. Its exact behaviour depends on how its chains are arranged and held together.

Metals: layers that can slide

A metal has a giant, regular arrangement of positive ions surrounded by mobile, delocalised electrons. The electrons carry charge, making metals good electrical conductors. When a force shifts layers of metal ions, attraction between the ions and delocalised electrons continues to hold the structure together. The layers can therefore slide without the metal splitting: metals are malleable. This contrasts with the repulsion produced when an ionic lattice shifts.

Arrangement → property

• Ionic lattice: fixed ions make the solid non-conducting; mobile ions in a melt or solution conduct. Shifting the lattice brings like charges together, so it fractures.
• Simple molecules: separate molecules are held together by weaker intermolecular forces. They often melt or boil at relatively low temperatures and do not conduct.
• Giant covalent structure: covalent bonds extend through the network, giving a high melting point; graphite conducts, but diamond does not.
• Polymers: strong bonds run along long chains, with forces between chains. Chain movement helps explain flexibility; properties vary with chain arrangement.
• Metals: mobile electrons conduct, while layers of ions can slide without the bonding being lost, making metals malleable.