Chapter 5: Metals

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Chapter 5 Metals
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Uses of Metals
Week 6, Lesson 1

• 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.

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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.

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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.

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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
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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.

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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.

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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.

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Explaining the Properties of Metals
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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.

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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.

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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.

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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.

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Modifying Metals
Week 6, Lesson 2

• 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.

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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.

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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.

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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.

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Substitutional Alloys cont
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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.

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Interstital Alloys cont
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Varying Compositions

• By varying the composition of alloys, materials
  with specific properties can be produced.

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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.

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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.

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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.

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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.

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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.

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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.

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Heat Treatment cont