2.1 Giant Ionic Structures
Ionic compounds have giant structures in which many strong electrostatic forces hold the ions together.
It takes a lot of energy to break the bonds which hold a giant ionic lattice together, so all the ionic compounds have high melting and boiling points, and are solids at room temperature.
However, when an ionic compound has been melted the ions are free to move. This allows them to carry electrical charge, so the liquids conducts electricity.
Some ionic solids dissolve in water because water molecules can split up the lattice. The ions are free to move in the solutions and so they also conduct electricity.
Ionic compounds cannot conduct electricity when solid because the ions can only vibrate about fixed positions; they cannot move around.
2.2 Simple Molecules
Substances made up of simple molecules have low melting and boiling points.
Although the covalent bonds in simple molecules are strong, the forces between them are weak.
These WEAK INTERMOLECULAR FORCES explain why substances made of simple molecules have low melting and boiling points.
Simple molecules have no overall charge, so they cannot carry electrical charge. Therefore substances made of simple molecules do not conduct electricity.
The smaller the molecules, the weaker the intermolecular forces.
2.3 Giant Covalent Structures
Some covalently bonded substances have giant structures. These substances have high melting and boiling points.
Graphite contains giant layers of covalently bonded carbon atoms. However, there are no covalent bonds between the layers. This means they can slide over each other, making graphite soft and slippery.
The atoms in diamond have a different structure and cannot slide like this. So diamond is a very hard substance.
Graphite can conduct electricity because of the delocalised electrons along its layers.
As well as diamond and graphite, carbon also exists as fullerenes which can form large cage-like structures based on hexagonal rings of carbon atoms.
2.4 Giant Metallic Structures
When we bend and shape metals the layers of atoms in the giant metallic structure slide over each other.
Alloys are mixtures of metals and are harder than pure metals because the layers in the structure are distorted.
If a shape-memory alloy is deformed it can return to its original shape on heating.
Delocalised electrons in metals enable electricity and heat to pass through the metal easily.
2.5 The Properties of Polymers
Monomers affect the properties of the polymers they produce. Changing reaction conditions can also change the properties of the polymer that is produced.
- High pressure + a trace of oxygen ---> low density poly(ethene)
- Catalyst at 50C + slightly raised pressure ---> high density poly(ethene)
HD poly(ethene) has a higher softening temperature and is stronger than LD.
Thermosoftening polymers will soften or melt easily when heated due to their weak intermolecular forces. They are made up of individual, tangled together polymer chains.
Thermosetting polymers will not soften but will eventually char if heated very strongly. They have strong covalent bonds forming cross links between their polymer chains.
Nanoscience is the study of small particles that are between 1 and 100 nanometres in size.
Nanoparticles behave differently from the bulk materials they are made from.
Their very small sizes give them a large surface area and new properties that can make them very useful materials.