Transcript TOPIC D: Bonding and the properties of solids

TOPIC D:
BONDING AND THE
PROPERTIES OF SOLIDS
• An alloy is a mixture of metals.
• Two types are common:
• 1. An interstitial alloy – additional, smaller atoms of a
different element fill the spaces in the metallic lattice.
• The most common example is steel where carbon atoms fill the
spaces between the iron atoms.
• Alloys of this type are less malleable and ductile than the pure
metals since the presence of the smaller atoms make the structure
more rigid and less flexible.
• Substitutional alloy – where one metals’ atoms are
replaced by another metals’ atoms.
• The metal atoms are usually of a similar radius as in
brass where copper atoms are replaced with zinc atoms.
• Substitutional alloys have similar, reduced malleability and
ductility to interstitial alloys, and have densities that
typically lay between the densities of the component
metals.
• In both cases the sea of electrons is maintained and the
alloys remain good conductors.
• Giant covalent network solids (diamond, graphite, silicon
dioxide and silicon carbide)
• The elements in group 14 can make four covalent bonds,
and as such allow then to bond together in large,
continuous networks.
• Diamond and graphite are allotropes (different forms of
carbon), made entirely of carbon atoms, and covalently
bonded together in a continuous network.
• The diamond structure is based on the 3-dimensional
tetrahedral unit.
• All the carbon atoms are bonded to one another with very
strong covalent bonds
• This makes a diamond strong and hard with a high
melting point and boiling point.
• Graphite had a 2-dimensional layered structure where
each carbon atom is bonded to three others in a plane.
• It will conduct electricity only in one plane. (There is no conduction
in the vertical plane.)
• Since each carbon atom is bonded to three others, this leaves one of
the outer electrons free. These electrons are delocalized.
• It can be used as a lubricant
• LDF’s hold the planes together
vertically, so they can slide over one another.
• Silicon and semi-conductors
• Pure semi-conductors are generally poor conductors of
electricity,
• If ‘doped’ (a deliberate introduction of an impurity) the
conductivity increases.
• When the doping is carried out with an element that has
an extra valence electron compared to silicon
• (a group 15 element like phosphorus)
• An n-type conductor is produced (n meaning ‘negative’
because of the extra electron).
• Conversely, when the doping is carried out with an
element that has one less extra valence electron
compared to silicon,
• (a group 13 element like boron)
• A p-type conductor is produced (p meaning ‘positive’
because of the one less electron).
• Disrupting the valence shells of the silicon atoms in this
manner effectively allows electrons to flow, and the
previous insulator becomes a good conductor.
• Molecular solids (iodine)
• Molecular solids are made from non-metals.
• Sometimes these might be diatomic molecules like iodine,
or much larger molecules like polymers.
• The intermolecular forces are weak, have low melting
points and do not conduct electricity.
• Iodine is held in a similar lattice structure to NaCl, only the
I2 molecules are held by weak dispersion forces.
• Solid iodine has a low melting point and a non conductor
• It typically sublimes at room temperature.