Chapter 5: Metals
Week 6, Lesson 1
Uses of Metals
• Metals in different forms are very noticeable all around us.
• We can see that:
– The strength of metals is useful when building robust
structures, such as towers and bridges.
– And, the lightness and strength of some metals are useful in
sailing boats, aircrafts, vehicles and frames of houses and
buildings.
– The ability to form wire from metals is applied in many different
objects, from braces to wire fencing to jewellery.
– Electricity is transmitted by metals in industry, domestic
appliances, in our computers, DVD players and mobile phones.
– Metals can be used to create diverse objects which can be
moulded and shaped, such as door handles and locks and
shopping trolleys.
Useful Properties of Metals
• The elements that are classified as metals, have all or
most of the following properties:
–
–
–
–
–
Good conductors of electricity
Good conductors of heat
Are malleable; they can be shaped by beating or rolling
Are ductile; can be drawn into a wire
Exhibit a range of melting temperature and relatively high
boiling temperatures
– Have high densities
– A lustrous or reflective, when freshly cut or polished
– Are often hard, with high tensile strength.
Useful properties of metal cont…
• However, not all metals have these properties.
• For example, mercury is a liquid at room
temperature which means it has a very low
melting temperature.
• Chromium is brittle rather than malleable.
• Group 1 metals have some properties that make
them different to all other metals.
• For example, they are all soft and can be cut with
a knife and react vigorously with water to give
hydrogen gas.
Properties and Structure
• The physical properties and inferred structural
features of metals.
PROPERTY
WHAT THIS TELLS US ABOUT THE
STRUCTURE
Metals conduct electricity in a solid state
Metals have charged particles that are
free to move
Metals are malleable and ductile
The forces between the particles must be
able to adjust when the particles are
moved
Metals generally have high densities
The particles are closely packed together
Metals tend to have high boiling
temperatures
The forces between particles must be
strong
Metals are lustrous or reflective
Metals can reflect light
Properties and Structure cont…
• Using the table on the previous slide, chemists
have developed a model to explain the properties
of metals.
• This model needs to describe what the charged
particles in metals are like and how they are
arranged.
• The model must be one in which:
– Some of the particles are free to move
– There are strong forces of attraction between particles
throughout the metal structure.
The Metallic Bonding Model
• The only particles that are small enough to
move through a solid lattice are electrons.
• If a metal atom loses one or more electrons
from its outer shell it forms a positive ion, or
cation.
Metallic Bonding Model cont…
• It is now believed that in a solid sample of metal:
– Positive ions are arranged in a closely packed structure. This
structure is described as a regular, 3D lattice of positive ions.
The ions occupy fixed positions in the lattice.
– The much smaller negatively charged electrons that have been
released from the outer shell of the metal atoms free to move
throughout the lattice. These electrons are delocalised electrons
because they belong to the lattice as a whole. The delocalised
electrons come from the valence shell. Electrons that are not
free to move throughout the lattice are said to be localised.
Electrons in the inner shells are localised.
– The ions are held in the lattice by the electrostatic force of
attraction between them and the delocalised electrons. This
attraction extends throughout the lattice and is called metallic
bonding.
Explaining the Properties of Metals
Other Properties of Metals
• Metals are good conductors of heat.
– When the delocalised electrons bump into each
other and the metal ions, they transfer energy.
– Heating a metal gives the particles more energy
and they vibrate more rapidly.
– The electrons transmit the energy rapidly
throughout the lattice.
Other Properties of Metals cont…
• Metals are lustrous.
– Because there are electrons that are free to move
in the lattice, metals reflect light and are shiny.
• Metals are generally dense.
- The ions in the metal lattice are closely packed.
The density of the metal depends on the mass of
the metal ions, their radius and the way in which
they are packed into the lattice.
Other Properties of Metals cont…
• Metals tend to react by losing electrons.
– The chemical reactivity of a metal therefore
depends on the ease with which electrons can be
removed from its atoms.
Limitations of the Metallic Bonding
Model
• Although this model explains many properties
of metals, there are some that cannot be
explained as simply:
– The range of melting temperature and densities of
different metals
– The differences in electrical conductivity between
metals
– The magnetic nature of metals such as cobalt, iron
and nickel.
Week 6, Lesson 2
Modifying Metals
• A few metals are used in their pure form.
• Aluminium and copper are two examples of this.
• Aluminium does not appear to corrode, has a low
density and conducts heat well.
• It is used in cookware and food wraps.
• Copper is used in household electrical wiring
because of its electrical conductivity.
• The presence of impurities affects the ease if
current flow, so copper must be pure.
Modifying Metals cont…
• Most other metals need to be modified in order
to produce the desired properties for use.
• Iron for example, is usually converted to steel by
adding 2% of carbon.
• There are very few pieces of jewellery that are
purely gold because it is soft and easily
deformed.
• Most jewellery is is produced as 18- or 9-carat
gold. These materials contain a certain amount of
silver and copper which makes them harder.
Modifying Metals cont…
• The properties of a metal can be significantly
altered by adding small amounts of another
substance, usually a metal or carbon.
• The substances are melted together, mixed and
then allowed to cool.
• The resultant solid is called an alloy.
• Alloying is a common modification of metals to
change their properties and make them more
useful.
• Another modification is changing the crystal
structure of the metal by heat treatment of
working the metal.
Making Alloys – Substitutional Alloys
• Substitutional alloys are made from elements that have fairly
similar chemical properties and atoms of similar size.
• Australian silver coins are made from an alloy that is 75%
copper and 25% nickel.
• The nickel atoms take the place of some of the copper atoms
in the lattice.
• Both the nickel and copper atoms are attracted to the sea of
electrons so the lattice is still strongly bonded.
• Because of the slight difference in the size of the two atoms
there is a restriction when the layers within the lattice move
relative to each other.
• This makes the alloy harder and less malleable than pure
copper.
Substitutional Alloys cont…
Interstitial Alloys
• A small proportion of an element with significantly
smaller atoms is added to a metal.
• For example, Carbon is added to iron to increase its
hardness.
• The resulting product is steel.
• In steel, the smaller carbon atoms fit randomly in the
hollows between the packed metal ions.
• In interstitial alloys the presence of the different
atoms in the lattice will make it more difficult for
layers to slide past each other.
• So, this is also harder and less malleable than pure
iron.
Interstital Alloys cont…
Varying Compositions
• By varying the composition of alloys, materials
with specific properties can be produced.
Alloys cont…
• In general alloys are harder and less malleable
than the metals from which they are made.
They are also poorer electrical conductors
than the pure metals.
Work Hardening and Heat
Treatment
• The way a metal is prepared will also affect
how it behaves.
• Many metals are prepared in liquid state, then
cooled.
• The rate at which a metal is cooled can have a
significant impact on the properties of the
solid.
• The model we used to describe the structure
of metals referred to the arrangement of
particles within a single metal crystal.
Work Hardening and Heat
Treatment cont…
• A crystal is a region in a solid in which the
particles are arranged in a regular way.
• A sample of a solid metal consists of a large
number of small crystals.
• Each individual crystal is a lattice of ions
surrounded by a sea of delocalised electrons
but the arrangement of individual crystals
with respect to one another is random.
• At the point where one crystal meets another,
the regular lattice is disrupted.
Work Hardening and Heat
Treatment cont…
• The way a metal behaves will depend on the size of these
crystals and the way that they are arranged.
• Generally, the smaller the crystals, the harder the metal
because there will be less free movement of layers of ions
over each other.
• Smaller crystals, however, also means more areas of
disruption between them and this usually means that a lattice
will be more brittle.
• Two ways of altering the crystal structure of metals are work
hardening and heat treatment.
Work Hardening
• Hammering or working cold metals causes a
rearrangement of crystal grains and a hardening of
the metal.
• This effect can be seen by bending a paper clip.
• If it is bent once it remains fairly pliable.
• But if it was bent backwards and forwards several
times it snaps.
• Bending causes the crystal grains to be rearranged
making the metal harder but more brittle.
Heat Treatment
• There are three possible outcomes of the heat
treatment of metals.
– Annealing: involves heating the metal to a moderate
temperature then leaving it to cool slowly. This allows
larger crystals to form and the metal produced is softer
and more ductile.
– Quenching: also involves heating the metal to a moderate
temperature, however it is cooled quickly (quenched) to
form tiny crystals. This hardens the metal but also makes it
brittle
– Tempering: when quenched metals are warmed again but
to a lower temperature. This reduces the brittleness of the
material, but also retains the hardness.
Heat Treatment cont…