A-level Applied Science/Choosing and Using Materials/Structure and properties

Relating the physical properties of materials to their structure

Materials behave as they do because of their structure; the way their atoms and molecules fit together. Materials scientists consider how materials behave at the atomic level in order to assess the suitability of a material for a specific project.

You should understand how physical properties relate to:

• the microscopic structure of crystalline, polymeric and amorphous materials;
• the structure of composite materials;
• strong bonding between particles (ionic, covalent and metallic);
• weak bonding between particles.

Strong bonding between particles

Ionic – directional

Although all cations attract all anions, each cation is surrounded by anions and vice-versa. Moving one part of the ionic lattice will cause anion-anion and cation-cation repulsions.

Covalent – directional

: Covalent bonds are between specific atoms and distorting the positions of the atoms will break the bonds.

Metallic - non-directional.

The fluid ‘sea’ of electrons can accommodate changes in the crystal structure, so the shape of the sample can change without breaking the bonds.

Compounds which are purely ionic or covalent have low ductility. Metals are much more ductile.

Weak bonding between particles.

The strong bonding of metals, ionic crystals and giant covalent molecules extends throughout the material.

Intermolecular bonding

Small covalent molecules, polymers, graphite etc. are not composed of a single molecule. What holds one molecule to another molecule? The answer is intermolecular bonding.

There are three types of intermolecular bond:

Van der Waals’ forces

Every molecule and atom will be very weakly attracted to any other molecule or atom. This is because the electrons on one particle will be attracted to the nuclei of the other, and vice-versa.

This type of bonding is very weak. Small molecules which bond in this way will be gases. Very large molecules can accumulate enough van der Waals’ force to form solids (e.g. polymers, graphite) but they are mechanically weak unless stressed along/across their covalent bonds.

Dipole-dipole interactions

Polar molecules (molecules containing atoms such as oxygen, nitrogen, fluorine, chlorine) have permanent areas of positive and negative charge. These molecules can stick together like pairs of magnets.

This bonding is stronger than van der Waals’ bonding and quite like ionic bonding. Molecules that bond in this way tend to be less volatile than pure van der Waals’ substances. Polymers with dipole-dipole bonding tend to be stiffer than hydrocarbon polymers e.g PVC.

Hydrogen bonding

If hydrogen is bound to an oxygen or nitrogen, it can bind to the oxygen or nitrogen of a second molecule. This is the strongest intermolecular bond and is akin to covalent bonding.

Molecules that hydrogen bond are the least volatile molecules. Simple molecules will be they are brittle. E.g. sugar but polymers tend to be stiff and brittle, e.g cellulose, perspex.