In this lesson we will learn about the different physical properties of metals and some of the many uses they are suitable for.
By the end of this lesson you will be able to:
- Describe physical properties common to all metals: lustre, electrical conductivity, thermal conductivity, malleability and ductility.
- Describe physical properties that vary among metals: melting point, hardness, strength and density.
- Describe the arrangement of atoms in metals and explain how it accounts for their physical properties.
- Give examples of uses of metals that take advantage of their unique properties.
- Describe alloys and give examples of how their superior properties make them more suitable in certain situations.
- Metals are a large group of elements with a common atomic arrangement, and subsequently similar physical and chemical properties.
- The unique combination of properties makes metals useful for a wide variety of purposes, ranging from the building of skyscrapers to electronics and the crafting of fine jewellery.
- Properties of metals can be improved further by combining them with other metals and non-metals to form alloys.
Arrangement of Atoms in Metals
- In metals, valence electrons are free to move between atoms, forming a ‘sea’ of delocalised electrons.
- These electrons surround a lattice of positive metal ions.
- These lattices are held together by metallic bonds, which are strong forces of attraction between positively charged metal ions and negatively charged valence electrons.
- Metallic bonding exists in all metal elements, except for mercury.
- It also exists in metal alloys, which are metallic mixtures, such as brass, which is a mixture of copper and zinc.
Properties of Metals
- Since metals have a similar arrangement of atoms, metallic structures – including pure metals and alloys – have similar properties.
- Properties common to all metals include:
- Metallic lustre.
- Electrical conductivity.
- Thermal conductivity.
- Malleability and ductility.
- Properties common to most metals include:
- High melting point.
- High strength.
- High density.
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- Lustre can be defined as a shininess due to the reflection of light.
- All metals exhibit a shiny surface when freshly cut or polished, often referred to as a metallic lustre.
- Light rays are reflected off delocalised electrons, causing the metallic surface to shine.
- Gold, silver and copper have long been prized for their lustrous appearance, making them popular in jewellery and decorative ornaments.
- Chromium plating is common in the automotive and building industries due to its mirror-like finish.
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Electrical Conductivity of Metals
- Electrical conductivity refers to the flow of electric charge through a substance.
- Electric current readily passes through metallic materials.
- Delocalised valence electrons in metals are highly mobile and therefore able to transfer electric charge.
- Gold and silver are two of the best conductors of electricity, but are only used in specialised, high-end electronic components.
- Copper and aluminium are the most common metals used in electrical wiring due to their abundance and low cost.
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Thermal Conductivity of Metals
- Thermal conductivity refers to the transfer of heat through a substance.
- Heat travels readily from one end of a metal object to another.
- Delocalised valence electrons readily acquire and transfer heat energy it to neighbouring electrons and cations.
- Copper and aluminium are two of the best thermal conductors. They are often used in heating elements, usually as alloys including other metals such as nickel and chromium.
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Malleability and Ductility of Metals
- Malleability refers to the ability of a material to be reshaped or flattened.
- Ductility refers to the ability of a material to be drawn out into wires.
- Metals are malleable and ductile (as opposed to brittle), but the amount of forces required varies.
- For example, thin pieces of lead can readily be bent into shape by hand, whereas iron requires heating and hammering.
- If sufficient force is applied, layers within the metal lattice are able to slide over each other, due to the mobility of electrons and subsequent non-directional nature of metallic bonds.
- Copper is an excellent material for electrical wiring due to its ductility.
- Gold is extremely malleable. Gilding is a decorative technique of applying gold leaf – very thin layers of gold – to materials including other metals, wood, porcelain and stone.
- While highly malleable metals are often highly ductile, this is not always the case. For example, lead has high malleability but low ductility.
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Melting Point of Metals
- Melting point refers to the temperature at which a substance changes from solid to liquid.
- Almost all metals are solids with high or very high melting points.
- Bond strength (and therefore the energy required to overcome these bonds) is generally high in metals, due to the attraction between electrons and cations.
- The strength of metallic bonds varies between metals.
- Bond strength is directly related not only to melting point, but also hardness and tensile strength.
- Therefore, metals with lower melting points are generally not as hard or strong.
- Tungsten is the metal with the highest melting point – more than 3400°C.
- Iron, titanium and platinum all have melting points above 1500°C.
- Mercury is the only metal that is not a solid at room temperature.
- Group 1 metals have relatively low melting points, decreasing down the group to caesium, which has a melting point of 28°C.
- Gallium has a melting point of 30°C.
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Hardness of Metals
- Hardness can be defined as resistance to scratching or abrasion.
- Most metals are hard, particularly the transition metals; main group metals tend to be softer.
- The hardness of metals is related to bond strength – the level of attraction between electrons and cations.
- Chromium and tungsten (group 6) are two of the hardest metals.
- Sodium and potassium (group 1) are two of the softest metals – they can easily be cut with a knife.
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Strength of Metals
- The are several ways that the strength of a metal can be defined and measured.
- Yield strength is a measure of resistance to permanent deformation.
- Tensile strength is a measure of resistance to stretching.
- Compressive strength is a measure of resistance to compression (being squashed).
- Impact strength is a measure of resistance to fracture due to impact.
- Many metals have a high strength, but it varies considerably.
- The strength of metals is related to bond strength – the level of attraction between electrons and cations.
- Iron, titanium and tungsten are high-strength metals.
- Aluminium, zinc and gold are low-strength metals.
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Density of Metals
- Density refers to the amount of mass in a given volume.
- Most metals have a high or very high density, but there is a wide range.
- Density of metals is affected by how closely atoms are packed in the lattice, as well as the density of atoms.
- Atoms are generally tightly packed in metals.
- Atomic density generally increases down periodic groups, as the neutron-proton ratio increases.
- Osmium, iridium and platinum (period 6) have the highest densities – more than 20 g/cm3.
- Lithium, sodium and potassium (group 1) have the lowest densities – less than 1 g/cm3, which means they float on water.
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- An alloy is a mixture of a metal and one or more other elements, which may be metals or non-metals.
- Alloys can be described as solid solutions, as they are usually formed by melting and combining the components, then allowing them to cool and solidify.
- Alloys are metallic structures with enhanced physical and chemical properties.
- For example, alloys are generally harder and stronger than pure metals, as the different sized atoms make it more difficult for layers within the lattice to slide over each other.
- Also, alloys often have greater resistance to corrosion, due to the incorporation of less reactive elements.
- Their enhanced properties make alloys useful for a wide variety of purposes.
Substitutional and Interstitial Alloys
- There are two types of alloys – substitutional alloys and interstitial alloys.
- Substitutional alloys are alloys where atoms of the added element substitute atoms of the main metal in the lattice.
- The atoms of the substitutional element are of a similar size to atoms of the main metal.
- Bronze and brass are examples of substitutional alloys – copper atoms are substituted by tin and zinc atoms respectively.
- Interstitial alloys are alloys where atoms of the added element occupy spaces (interstices) between atoms in the lattice, rather than displace atoms of the main metal.
- The atoms of the interstitial element are much smaller than the atoms of the main metal.
- Steel is an example of an interstitial alloy – carbon atoms occupy spaces within the iron lattice.
- Some alloys are both substitutional and interstitial.
- Stainless steel is an example of an alloy that contains substitutional and interstitial elements – iron atoms are substituted by chromium atoms while carbon atoms occupy spaces within the iron-chromium lattice.
Common Metal Alloys
- The use of metals and metal alloys dates back thousands of years. The creation of new metal alloys remains an important part of materials science.
- Some of the most common alloys and their applications are discussed below.
- Bronze was the first alloy to be made.
- It is formed by combining a small amount of tin with copper.
- Bronze is stronger and more durable than either copper or tin.
- It was a popular material for making tools and weapons during the Bronze Age.
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- Steel is the most abundantly produced metal in the world.
- It is formed by mixing small amounts of carbon (<2%) with iron.
- The result is a stronger, harder and more durable material.
- Steel is used widely in the construction and transportation industries.
- Stainless steel is steel that also contains chromium (>10%) and smaller amount of other elements, such as nickel.
- Stainless steel has a shiny, polished surface which is much more resistant to corrosion.
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- Brass is an alloy containing copper a significant proportion of zinc (usually a third or more).
- Brass is more malleable than either copper or zinc, and has a smooth, shiny and durable appearance.
- This makes it a suitable material for many applications, such as musical instruments, ornaments, gears and fittings.
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- A wide variety of aluminium alloys exist, which are formed by mixing aluminium with small amount of other elements, such as copper, magnesium, manganese, silicon, tin and zinc.
- Aluminium alloys have improved properties, such as greater strength, hardness and corrosion resistance.
- Aluminium alloys have a wide range of uses, particularly in the construction, transportation and aerospace industries.
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- Similar to aluminium, a wide range of titanium alloys are available, which are formed by mixing titanium with small amount of other elements, such as tin, aluminium, molybdenum, silicon and vanadium.
- Titanium alloys have superior strength to weight ratios, durability and corrosion resistance.
- This makes them ideal materials for many uses, such as the military, airline and aerospace industries, as well as consumer electronics and sporting goods.
- They are also non-toxic, leading to their use in medical and dental implants.
- Although an ideal material for many applications, titanium mining and production is laborious and expensive, making it a more specialised than widely used material.
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- Metals are a large group of elements with a common atomic arrangement and similar properties.
- Metallic structures consist of lattices of metal ions surrounded by delocalised electrons.
- These structures are held together by strong metallic bonds.
- Physical properties common to all metals include:
- Metallic lustre – a mirror-like shininess when freshly cut or polished.
- Malleability – the ability to be bent into different shapes.
- Ductility – the ability to be drawn into wires.
- Electrical conductivity – the ability to conduct electric charge.
- Thermal conductivity – the ability to transfer heat.
- Other properties common to most metals include:
- High melting point.
- High strength.
- High density.
- Alloys are mixtures containing a metal and one or more other elements.
- Substitutional alloys are those where atoms within the metallic lattice are replaced by other atoms.
- Interstitial alloys are those where additional atoms occupy spaces within the metallic lattice.
- Alloys are metallic structures with enhanced properties compared to their components.
(Image: TheDigitalArtist, Pixabay)
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