Chapter 5 Metals

Uses of Metals Metals are common in everyday life because they have a range of useful properties. Most metals are found combined with non-metals in ‘ores’. We extract metals from ores because metals have a combination of useful properties that are not found in other materials.

Useful properties of Metals The elements we classify as metals have all or most of the following properties. They: Are good conductors of electricity Are good conductors of heat Are malleable (they can be shaped by beating/rolling) Are ductile (can be drawn into a wire) Exhibit a range of melting temps and relatively high boiling point Generally have high densities Are lustrous (reflective/shiny) Are often hard with high tensile strength

Properties of Metals Not all metals have these properties. Group 1 elements (alkali metals) are soft and react vigorously with water Mercury is liquid at room temp However, these elements exhibit most other properties so are classified as metals

Properties and Structure The metallic model must be one in which: Some of the particles are charged and free to move There are strong forces of attraction between particles throughout the metal structure

The metallic bonding model The chemists’ model of an atom: Positive ions (called cations) are arranged in a closely packed structure. This structure is described a a regular, three dimensional lattice. The much smaller negatively charged electrons are released from the outer shell of metal atoms are free to move around the lattice. They are called delocalised electrons. Electrons in the inner shell are not free to move so they are known as localised electrons. The ions are held together in the lattice by the electrostatic force of attraction between them and the delocalised electrons. This is the metallic bonding.

Metals have high boiling/melting points For most metals, large amounts of heat energy are required to overcome the strong forces of attraction between the positive ions and the delocalised electrons in the metal lattice.

Metals are good conductors of heat When the delocalised electrons bump into each other and into the ions, they transfer energy. When you heat a metal you give the particles more energy, particles vibrate more rapidly and thus the heat is transferred rapidly throughout the lattice.

Metals are good conductors of electricity The delocalised electrons in a metallic lattice are free to move. If a source of electric current is applied, electrons are forced in at one end and an equal number flow out the other. This produces a current flow.

Metals are lustrous Because of the presence of free electrons in their lattice, metals reflect light and appear shiny.

Metals are malleable and ductile When beaten into sheets or drawn into a wire, layers of the positive ions are forced across each other. The delocalised electrons move so that they still surround the positive ions (remember that opposites attract!)

Metals are generally dense The ions in a metal lattice are closely packed. The density of the metal atoms depends upon the mass of the metal ions, their radius and the way in which they are packed together.

Reactivity of metals Metals tend to react by losing electrons. The chemical reactivity of a metal depends upon the ease of which the metal is able to remove an electron (ionisation energy!!)

Limitations of the Metallic Bonding model This model explains many properties of metals. However, it cannot explain: The range of melting temperatures and densities of different models The differences in electrical conductivity Magnetic nature All you need to know in Unit 1/2 is the metallic bonding model.

Modifying Metals Only a few metals are used in their pure form. Eg. Copper, Aluminium. However, most metals need to be modified in order to produce the desired properties for particular uses. The properties of a metal can be significantly altered by adding small amounts of another substance (usually carbon or another metal). The substances are melted together, mixed and allowed to cool. The final product is called an alloy.

Making Alloys There are two main types of alloys: Substitutional alloys: Are made from elements that have fairly similar chemical properties and atoms of a similar size.

Making Alloys There are two main types of alloys: Interstitial alloys: A small proportion of an element with significantly smaller atoms is added to a metal. Eg. Carbon is added to Iron to increase its hardness. This is steel.

Alloys By varying the composition of alloys, materials with specific properties can be obtained:

Altering the structure of a metal “a crystal is a region in a solid in which the particles are arranged in a regular way” Metal solids are made up of large numbers of small crystals. Each individual crystal is a lattice of ions surrounded by delocalised electrons, but the arrangement of individual crystals is random. At the point where one crystal meets another, the regular lattice is disrupted. The way a metal behaves (malleability/brittleness) will depend to some extent on the size of the these crystals and the way they are arranged. The crystal structure can be altered in two ways:

Work Hardening Heat Treatment Hammering or working cold metals causes rearrangement of crystals and a hardening of the metal (also more brittle). Annealing involves heating the metal to a moderate temp and leaving it to cool slowly. Quenching involves heating to a moderate temp. The metal is cooled quickly to form tiny crystals, making the metal hard but brittle Tempering is when quenched metals are warmed again to a much lower temp which reduce the brittleness whilst retaining the hardness. Intermediate size crystals are formed. Heat Treatment

Metal Fatigue Sometimes metal structures fail. A metal structure may fail because: There is a flaw in the metal itself (fault in the production process) There is a design flaw or weak point in the structure (a faulty joint or weld) Constant use and stress have caused the metal to fail at some point (this is true metal fatigue)

Questions Chapter 5 Review Questions: 2, 3, 5, 6, 8, 13, 14, 15, 18, 19, 20, 21, 28