C2.2 How Structure Influences The Properties And Uses Of Substances
1 C2.2.1 Molecules
1.1 Substances consisting of simple
molecules are gases, liquids or solids with
relatively low melting and boiling points.
They 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.
1.1.1 Substances that consist of simple molecules do not
conduct electricity because the molecules do not
have an overall electric charge.
2 C2.2.2 Ionic Cmpounds
2.1 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 and
boiling points because of the large
amounts of energy needed to break
the many strong bonds.
2.1.1 When molten or in solution,
ionic compounds conduct
electricity because the ions
are free to move and carry
3 C2.2.3 Covalent Structures
3.1 Atoms that share electrons can also form giant
structures or macromolecules. Diamond and
graphite and silicon dioxide are examples of lattice
structures of atoms. All the atoms are linked to
other atoms by strong covalent bonds and so they
have very high melting points.
3.1.1 In diamond, each carbon atom forms four covalent
bonds with other carbon atoms in a giant covalent
structure, so diamond is very hard.
220.127.116.11 In graphite, each carbon atom bonds to three others, forming layers. The layers are free to slide over each other
because there are no covalent bonds between the layers and so graphite is soft and slippery.
18.104.22.168.1 In graphite, one electron from each carbon atom
is delocalised. These delocalised electrons
allow graphite to conduct heat and electricity.
22.214.171.124.1.1 Carbon can also form fullerenes with different
numbers of carbon atoms. Fullerenes can be
used for drug delivery into the body, in
lubricants, as catalysts, and in nanotubes for
reinforcing materials, eg in tennis rackets.
4 C2.2.4 Metals
4.1 Metals conduct heat and electricity because
of the delocalised electrons in their structures.
4.1.1 The layers of atoms in
metals are able to slide
over each other and so
metals can be bent and
126.96.36.199 Alloys are made from two or more
different metals. The different sized
atoms of the metals distort the
layers in the structure, making it
more difficult for them to slide over
each other and so alloys harder
than pure metals.
188.8.131.52.1 Shape memory alloys can return to their original shape
after being deformed, eg Nitinol used in dental braces.
5 C2.2.5 Polymers
5.1 The properties of polymers depend on what they are made from
and the conditions under which they are made. For example, low
density (LD) and high density (HD) poly(ethene) are produced using
different catalysts and reaction conditions. LDPE is weaker and
has a lower melting point than HDPE.
5.1.1 Thermosoftening polymers consist of individual, tangled polymer chains. Thermosetting polymers consist of
polymer chains with cross-links between them so that they do not melt when they are heated.
6 C2.2.6 Nanoscience
6.1 Nanoscience refers to structures that are 1–100nm 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, stronger and lighter
construction materials, and new cosmetics such as sun tan creams