• Substances that consist of simple molecules are gases, liquids or
solids that have relatively low melting points and boiling points.
Substances that consist of simple molecules have only weak forces
between the molecules (intermolecular forces). It is these
intermolecular forces that are overcome, not the covalent bonds,
when the substance melts or boils.
• Substances that consist of simple molecules do not conduct
electricity because the molecules do not have an overall
• Ionic compounds have regular structures (giant ionic lattices) in
which there are strong electrostatic forces in all directions
between oppositely charged ions. These compounds have high
melting points and high boiling points.
• When melted or dissolved in water, ionic compounds conduct
electricity because the ions are free to move and carry the current.
• Atoms that share electrons can also form giant structures or
macromolecules. Diamond and graphite (forms of carbon) and
silicon dioxide (silica) are examples of giant covalent structures
(lattices) of atoms. All the atoms in these structures are linked to
other atoms by strong covalent bonds and so they have very high
• In diamond, each carbon atom forms four covalent bonds with
other carbon atoms in a giant covalent structure, so diamond is
• In graphite, each carbon atom bonds to three others, forming
layers. The layers are free to slide over each other and so graphite
is soft and slippery.
In graphite, one electron from each carbon atom is delocalised.
These delocalised electrons allow graphite to conduct heat and
Metals conduct heat and electricity because of the delocalised
electrons in their structures.
• The layers of atoms in metals are able to slide over each other and
so metals can be bent and shaped.
• Nanoscience refers to structures that are 1-100 nm in size, of the
order of a few hundred atoms. Nanoparticles show different
properties to the same materials in bulk and have a high surface
area to volume ratio, which may lead to the development of new
computers, new catalysts, new coatings, highly selective sensors
and stronger and…