Chapter 5 Metals Week 6 Lesson 1 Uses

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, 3 D 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…