Properties of Metals
1. Physical Properties of Metals
Learning outcomes
- I can identify the common physical properties of metals.
- I can explain why metals are good conductors of heat and electricity.
- I can describe the properties of malleability and ductility.
- I can compare metals with non-metals based on physical characteristics.
- I can relate the physical properties of metals to their uses.
What Are Metals?
Metals are elements that share a number of characteristic physical properties.
Most elements in the periodic table are metals. They include familiar elements such as:
- Iron (Fe)
- Copper (Cu)
- Aluminium (Al)
- Gold (Au)
- Silver (Ag)
- Zinc (Zn)
- Magnesium (Mg)
Most metals are found on the left-hand side and centre of the periodic table.
Metals can have very different appearances and uses, but many share similar physical properties because of the way their particles are arranged and bonded.
Common Physical Properties of Metals
Most metals have several properties in common.
They are generally:
- Good conductors of electricity
- Good conductors of heat
- Strong
- Hard
- Malleable
- Ductile
- Shiny when polished
- High in density
- High in melting and boiling points
There are exceptions to some of these properties, so we should describe them as general properties of metals rather than rules that apply to every metal.
Metals Are Good Electrical Conductors
One of the most important properties of metals is their ability to conduct electricity.
For example, copper is widely used in electrical wiring.
To understand why, we need to consider the structure of a metal.
In a metal, positive metal ions are arranged in a closely packed structure. Their outer electrons become delocalised.
Delocalised electrons are not attached to one particular atom. They are able to move throughout the metal.
When a potential difference is applied, these mobile electrons can move through the structure and carry electrical charge.
Therefore:
Mobile delocalised electrons allow metals to conduct electricity.Why Are Metals Good Thermal Conductors?
Metals are also good conductors of thermal energy.
If one end of a metal rod is heated, thermal energy can quickly travel through the metal.
There are two important reasons for this.
Delocalised Electrons
The mobile electrons gain kinetic energy in the hotter part of the metal.
They move through the structure and transfer energy to other particles.
Vibrations of the Metal Ions
When the metal is heated, its particles vibrate more strongly.
These vibrations transfer energy through the closely packed structure.
As a result, metals can transfer thermal energy efficiently.
This is why metals such as aluminium and copper are commonly used in cookware and heat-transfer equipment.
Metallic Structure and Properties
The physical properties of metals are closely connected to metallic bonding.
A simple model of a metal consists of:
- Positive metal ions
- Closely packed in a regular structure
- Surrounded by delocalised electrons
The attraction between the positive ions and negative electrons forms strong metallic bonds.
This structure helps explain many of the properties of metals.
Strength
Many metals are strong because metallic bonds hold the particles together.
A large amount of force may be required to pull the particles apart.
This makes metals useful for structures that must support heavy loads.
For example:
- Steel is used in buildings.
- Iron and steel are used in bridges.
- Aluminium alloys are used in aircraft.
However, different metals have different strengths, and pure metals can often be strengthened by forming alloys.
Malleability
Malleability is the ability of a material to be hammered or pressed into different shapes without breaking.
Most metals are malleable.
For example, aluminium can be rolled into extremely thin sheets to make:
- Aluminium foil
- Food containers
- Packaging
Gold is also highly malleable and can be formed into extremely thin sheets.
Why Are Metals Malleable?
The particles in a metal are arranged in layers.
When a force is applied, these layers can slide past one another.
The metallic bonding is maintained because the positive ions remain attracted to the surrounding delocalised electrons.
Therefore, the metal changes shape instead of immediately breaking.
This makes metals very different from many brittle materials.
Ductility
Ductility is the ability of a material to be drawn into a wire.
Metals such as copper are highly ductile.
This allows copper to be made into long, thin electrical wires without breaking.
Ductility is particularly important in:
- Electrical cables
- Communication wires
- Metal fibres
- Manufacturing
Malleability vs. Ductility
These two properties are related but have different meanings.
Malleability
A material can be:
hammered or rolled into sheetsDuctility
A material can be:
drawn into wiresA useful way to remember them is:
- Malleable → sheets
- Ductile → wires
Metals Are Usually Shiny
Many freshly cut or polished metals have a shiny surface.
This property is called lustre.
Examples include:
- Silver
- Gold
- Copper
- Aluminium
This shiny appearance makes some metals useful for:
- Jewellery
- Decorations
- Reflective surfaces
- Coins
However, metal surfaces may become dull when they react with substances in the environment.
For example, iron can rust and copper can develop a surface coating over time.
Density
Many metals have relatively high densities.
Density describes how much mass is contained in a given volume.
For example, a small piece of iron may feel much heavier than a similar-sized piece of plastic.
However, not all metals have high densities.
Aluminium is relatively low in density compared with metals such as iron or lead.
This makes aluminium particularly useful when a strong but lightweight material is needed.
Melting and Boiling Points
Many metals have relatively high melting and boiling points.
This is because considerable energy may be required to overcome the strong metallic bonding between particles.
For example, iron remains solid at temperatures where many other materials would melt.
This can make metals useful in:
- Engines
- Machinery
- Cooking equipment
- Industrial equipment
Again, there are exceptions. For example, mercury is a metal that is liquid at room temperature.
Metals and Non-Metals
Metals and non-metals generally have very different physical properties.
| Property | Metals | Non-Metals |
|---|---|---|
| Electrical conductivity | Usually good | Usually poor |
| Thermal conductivity | Usually good | Usually poor |
| Appearance | Often shiny | Often dull |
| Malleability | Usually malleable | Usually brittle when solid |
| Ductility | Usually ductile | Usually not ductile |
| Density | Often high | Often lower |
| Melting point | Often high | Often lower |
| State at room temperature | Usually solid | Solid, liquid or gas |
These are general trends, and there are important exceptions.
Important Exceptions
The properties of metals and non-metals should not be treated as absolute rules.
For example:
Mercury
Mercury is a metal, but it is liquid at room temperature.
Graphite
Graphite is a form of the non-metal carbon, but it can conduct electricity.
Aluminium
Aluminium is a metal but has a relatively low density.
Recognising these exceptions is important when comparing materials scientifically.
Properties Determine Uses
Materials are chosen because their properties make them suitable for particular jobs.
Engineers often ask:
What properties does this object need?
They can then choose an appropriate metal.
Copper in Electrical Wiring
Copper is widely used for electrical wires.
Important properties include:
- Excellent electrical conductivity
- High ductility
- Good thermal conductivity
Its ductility allows it to be drawn into long, thin wires.
Its conductivity allows electric current to pass through efficiently.
Therefore:
Conductivity + ductility→electrical wiringAluminium in Aircraft
Aluminium and aluminium alloys are widely used in aircraft.
Important properties include:
- Relatively low density
- Good strength when alloyed
- Malleability
- Resistance to corrosion
Reducing the mass of an aircraft reduces the amount of energy required to keep it in flight.
Therefore, aluminium's relatively low density is particularly useful.
Metals in Cooking Equipment
Metals such as aluminium, copper, and stainless steel are commonly used in cookware.
An important property is:
good thermal conductivityThermal energy can move efficiently from the heat source through the cooking vessel.
However, saucepan handles are often made from plastic, wood, or another insulating material.
Why?
These materials are poor thermal conductors and therefore help reduce the transfer of thermal energy to the user's hand.
Iron and Steel in Construction
Iron-based materials such as steel are widely used in:
- Buildings
- Bridges
- Vehicles
- Tools
- Machinery
Important properties include:
- High strength
- High melting point
- Durability
- Ability to be formed into useful shapes
The combination of these properties makes steel an important structural material.
Gold in Jewellery
Gold is commonly used in jewellery because it is:
- Shiny
- Highly malleable
- Ductile
- Relatively unreactive
Its malleability allows it to be shaped into detailed designs.
Its low reactivity means that it does not easily corrode or tarnish.
Choosing a Metal for a Purpose
Suppose an engineer needs to choose a material for an electrical cable.
The material should have:
- High electrical conductivity
- High ductility
Copper is therefore a suitable choice.
Now suppose the engineer needs a material for the body of an aircraft.
Important properties might include:
- Low density
- Strength
- Resistance to corrosion
An aluminium alloy may be more appropriate.
There is no single "best metal." The best material depends on the properties required for the job.
Did You Know?
Many objects that we casually describe as being made of a particular metal are actually made from alloys.
An alloy is a mixture containing a metal and one or more other elements.
For example, steel is mainly iron mixed with carbon and sometimes other elements.
By changing the composition of an alloy, scientists and engineers can modify properties such as:
- Strength
- Hardness
- Corrosion resistance
- Density
This allows metals to be designed for specific applications.
Key Vocabulary
Metal – An element that generally has properties such as good conductivity, malleability and ductility.
Electrical conductor – A material that allows electric charge to move through it easily.
Thermal conductor – A material that transfers thermal energy efficiently.
Delocalised electron – An electron that is free to move throughout a metallic structure.
Metallic bonding – The attraction between positive metal ions and delocalised electrons.
Malleable – Able to be hammered or rolled into sheets without breaking.
Ductile – Able to be drawn into wires.
Lustre – The shiny appearance of a surface.
Density – Mass per unit volume.
Alloy – A mixture containing a metal and one or more other elements.
Key Takeaways
- Metals generally share a number of characteristic physical properties.
- Most metals are good conductors of electricity and thermal energy.
- Electrical conductivity is possible because metals contain mobile delocalised electrons.
- Strong metallic bonding helps explain the strength and high melting points of many metals.
- Malleability is the ability to be hammered or rolled into sheets.
- Ductility is the ability to be drawn into wires.
- Metals are often shiny, strong, dense, and solid at room temperature.
- Non-metals are generally poorer conductors and are usually not malleable or ductile.
- There are important exceptions to the general properties of both metals and non-metals.
- The physical properties of a metal determine which applications it is suitable for.
- Engineers select materials by matching their properties to their intended use.