Ionic compounds have giant structures in which many strong electrostatic forces hold the ions tightly together. This means they are solids at room temperature, they have high melting points and boiling points.
When they have been melted, the ions are free to move to carry electrical charge. Some ionic solids dissolve in water because water molecules can split up the lattice.
Atoms in molecules are held together by strong covalent bonds which act only between the atoms within the molecules. Simple molecules have little attraction for each other with relatively low melting points and boiling points.
The forces of attraction between molecules, 'intermolecular forces', are weak. They overcome when molecular substances melt or boil. Those with the smallest molecules, H2, Cl2 and CH4, have the weakest intermolecular forces and are gases at room temperature. Larger molecules have stronger attractions and so may be liquids at room temperature, like Br2 and C6H14, or solids with low melting points.
Molecules have no overall charge and cannot carry an electric current.
Covalent bonds are strong and difficult to break; intermolecular forces are much weaker and more easily overcome.
Giant Covalent Substances
Atoms of some elements can form several covalent bonds, they join together in giant covalent structures (sometimes called macromolecules'). Every atom in the structure is joined to several other atoms by strong covalent bonds. It takes an enormous amount of energy to break down the lattice and so these substances have very high melting points.
Diamond (form of carbon) and silica (silicon dioxide) have regular three-dimensional giant structures and so they are very hard and transparent.
Graphite is a form of carbon in which the atoms join in flat two-dimensional layers. There are only weak forces between the layers and so they slide over each other, making graphite slippery and grey. There are free electrons (delocalized electrons) in the structure.
Another type of a giant structure is a fullerene, which is found when carbon behaves in a way as to form large "cages" of carbon atoms between the bonds.
Giant Metallic Structures
Metal atoms are arranged in layers. When a force is applied the layers of atoms can slide over each other. They can move into a new position without breaking apart, so the metal bends or stretches into a new shape. This means that metals are useful for making wires, rods and sheet materials.
Delocalised electrons hold the atoms in place, they are free to move throughout the metal structure, this means that they can flow as an electric current without changing the metal. They can also carry heat energy.
Nanoscience and Nanotechnology
'Nano' means 'one-thousand-millionth'
When atoms are arranged into very small particles they behave different to ordinary materials made of the same atoms. Nanoparticles contain a few hundred atoms arranged in a particular way. Their structures and very small sizes give them new properties that can make them very useful materials.
Nanoparticles have very large surface areas exposing many more atoms at their surface than normal materials. Electrons can move through them more easily than ordinary materials. They can be very sensitive to light, heat, pH, electricity and magnetism. Nanotechnology uses nonparticles as very selective sensors, highly efficient catalysts, new coatings and construction materials with special properties, and to make drugs more effective.