Properties of Metals

Learning Objective

In this lesson we will learn about the different physical properties of metals and some of the many uses they are suitable for.

Learning Outcomes

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.

 

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Introduction

  • 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.

 

uses of metals construction    uses of metals electronics    uses of metals jewellery

The unique properties of metals make them suitable for a wide variety of uses.

(Images: Engin_Akyurt, Pixabay; Magnascan, Pixabay; nuzree, Pixabay)

 

Metallic Structures

  • 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.
  • Since they have a similar arrangement of atoms, metallic structures – including pure metals and alloys – have similar properties.
    Properties common to all metals include lustre, electrical conductivity, thermal conductivity, malleability and ductility.
    Properties common to most metals include solid state, hardness, high strength and high density.

 

Metallic lattice

 

Physical Properties Common to all Metals

Lustre

Description

  • Lustre can be defined as a mirror-like shininess.
  • All metals exhibit a shiny surface when freshly cut or polished, often referred to as a metallic lustre.

Explanation

  • Light rays are reflected off delocalised electrons, causing the metallic surface to shine.

 
metallic shiny lustre

Metals have a characteristic lustre.

(Image: BarbeeAnne, Pixabay)

 

Electrical Conductivity

Description

  • Electrical conductivity refers to the flow of electric charge through a substance.
  • Electric current readily passes through metallic materials, due to the free movement of electrons.

Explanation

  • Valence electrons in metals are highly mobile and therefore able to transfer electric charge.

 
electrical conductivity copper wire

Metals are excellent conductors of electricity.

(Image: freyer, Pixabay)

 

Thermal Conductivity

Description

  • Thermal conductivity refers to the ability of a substance to transfer heat.
  • Heat travels readily from one end of a metal object to another.

Explanation

  • Free electrons readily acquire heat energy and transfer it to neighbouring electrons and cations.

 
thermal heat conductivity iron metal

Metals are excellent conductors of heat.

(Image: Martinelle, Pixabay)

 

Malleability and Ductility

Description

  • Malleability refers to the ability of a material to be bent or hammered into different shapes or flattened sheets.
  • 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.

Explanation

  • 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.

 

metal malleability    metal ductility

Metals can be moulded, flattened and drawn into wires.

(Images: Mark Fergus, CSIRO Science Image; aitoff, Pixabay)

 

Physical Properties that Vary Among Metals

Melting Point

Description

  • 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.

Exceptions

  • 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.

Explanation

  • Bond strength (and therefore the energy required to overcome these bonds) is generally high in metals, due to the strong 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.

 
melting point liquid mercury metal

Mercury is the only metal that is not a solid at room temperature.

(Image: Bionerd, Wikimedia Commons)

 

Hardness

Description

  • 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.

Examples

  • Chromium and tungsten (group 6) are two of the hardest metals.
    Sodium and potassium (group 1) are two of the softest metals – they can be easily cut with a knife.

Explanation

  • (See explanation for melting point.)

 

hard metal chromium    soft metal lithium

Chromium (left) is the hardest metal – 8.5 on Mohs Hardness Scale.
Group 1 metals, such as lithium (right), can be cut with a knife.

(Images: Laitche, Wikimedia Commons; Dnn87, Wikimedia Commons)

 

Strength

Description

  • 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.

Examples

  • Iron, titanium and tungsten are high strength metals.
    Aluminium, zinc and gold are low strength metals.

Explanation

  • (See explanation for melting point.)

 
high strength steel metal

Steel has a very high strength.

(Image: nikko, Pixabay)

 

Density

Description

  • Density refers to the amount of mass in a given volume of space.
  • Most metals have a high or very high density, but there is a wide range.

Examples

  • 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.

Explanation

  • Density of metals is affected by how closely cations are packed in the lattice, as well as the density of atoms.
    Cations are generally tightly packed in metals.
    Atomic density generally increases down periodic groups, as the neutron-proton ratio increases.

 

iron high density metal    aluminium low density metal

Most metals, such as iron (left), have a high density.
Aluminium (right) is a low density metal.

(Image: nikko, Pixabay)

 

Alloys

  • 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 properties compared to their components.
    For example, alloys are generally harder and stronger structures than pure metals, as the different sized atoms make it more difficult for layers within the lattice to slide over each other.
  • 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 in substitutional alloys are of a similar size.
    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 interstices (spaces) in the lattice, rather than displace atoms of the main metal.
    In interstitial alloys, the atoms of the added element are significantly smaller than the atoms of the main metal.
    Steel is an example of an interstitial alloy – carbon atoms occupy spaces within the iron lattice.
    Stainless steel is an example of an alloy containing substitutional elements (chromium and nickel) and interstitial elements (carbon).

 

substitutional metal alloy            interstitial metal alloy

In substitutional alloys, atoms within the metallic lattice are replaced.
In interstitial alloys, additional atoms occupy spaces within the metallic lattice.

 

Common Alloys

Bronze

  • 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.

 

bronze alloy shield    bronze alloy weapons    bronze metal statue

Bronze was first used almost 5000 years ago.

(Images: BabelStone, Pixabay; Zde, Wikimedia Commons; Dirk Beyer, Wikimedia Commons)

 

Steel

  • Steel is the most abundantly produced metal in the world, formed by mixing varying amounts of carbon with iron.
    The result is a stronger, harder and more durable material.
    Steel is used widely in the construction and transportation industries.
  • Steel can be mixed with other elements to produce different alloys, such as stainless steel, which contains chromium, nickel and smaller amount of other elements.
    Stainless steel has a shiny, polished surface which is much more resistant to corrosion.

 

steel beam metal alloy    steel railway tracks    stainless steel tap metal alloy

Steel is used more than any other metal.

(Images: 1st Lt Jay Ostrich, US Air Force; TheDigitalArtist, Pixabay; RonPorter, Pixabay)

 

Brass

  • Brass is also a copper alloy, containing 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.

 

brass musical instrument    brass padlock fitting    brass clock gears

Brass has a shiny, durable appearance.

(Images: hpgruesen, Pixabay; ComMkt, Pixabay; music4life, Pixabay)

 

Aluminium Alloys

  • 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.

 

aluminium alloy aeroplane wing    aluminium alloy wheel rim    aluminium alloy phone case

Aluminium alloys are lightweight, durable and strong.

(Images: bottlein, Pixabay; Hatsukari715, Pixabay; hurk, Pixabay)

 

Titanium Alloys

  • 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.

 

titanium alloy plane engine    titanium alloy watch    titanium alloy medical implant

Titanium alloys have many superior properties.

(Images: Cory W. Watts, Wikimedia Commons; Tuxyso, Wikimedia Commons; Max Pixel)

 

Summary

  • 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.
    • Hardness.
    • High strength.
    • High density.
  • Alloys are mixtures containing a metal and one or more other elements.
    Alloys are metallic structures with enhanced properties compared to their components.

 
metallic-structure-lattice

(Image: TheDigitalArtist, Pixabay)

 

(Header image: Homar, Pixabay)

 

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