Properties of materials
What you'll learn
- How bonding and structure explain the properties of different materials.
- Why carbon forms so many different natural and synthetic compounds.
- How diamond, graphite, graphene and fullerenes differ.
- How nanoparticles behave differently because of their very small size.
Starting point: atoms, elements and compounds
You already know that an atom is the smallest particle of an element that can take part in chemical reactions.
An element contains only one type of atom. For example, carbon contains only carbon atoms.
A compound contains atoms of two or more different elements chemically bonded together. For example, carbon dioxide contains carbon and oxygen atoms bonded together.
A material’s properties depend on what particles it contains, how they are bonded, and how those particles are arranged.
Bulk properties
Bulk properties are the properties of a material you can observe in a sample, such as melting point, hardness, electrical conductivity, strength, flexibility and solubility.
A key GCSE idea is that individual atoms do not usually have the same properties as a bulk material. One iron atom is not “shiny” or “malleable” by itself — those properties come from many iron atoms arranged in a metallic structure.
Giving atoms the material’s properties
Do not say “carbon atoms are hard” to explain diamond. Say diamond is hard because each carbon atom is joined to four others by strong covalent bonds in a giant covalent structure.
Carbon: a very special element
Carbon has four electrons in its outer shell, so it can form four covalent bonds.
Covalent bond
A covalent bond is a strong chemical bond formed when two atoms share a pair of electrons.
Because carbon can make four covalent bonds, carbon atoms can join together in many different ways:
- long chains
- branched chains
- rings
- networks
- small molecules
- giant structures
This is why there is a vast range of organic compounds — compounds based on carbon, often also containing hydrogen, oxygen, nitrogen or halogens.
Some organic compounds are natural, such as sugars, oils and proteins. Others are synthetic, meaning made by humans, such as many plastics, medicines and dyes.
Why carbon makes so many compounds
Carbon forms four covalent bonds and can bond to other carbon atoms, so it can make chains, rings and families of similar compounds with related properties.
Different structures of carbon
Carbon can exist in different forms where the atoms are arranged differently. These forms include diamond, graphite, graphene and fullerenes.
Allotrope
An allotrope is a different structural form of the same element, where the atoms are bonded and arranged in a different way.
The diagram shows how the same element, carbon, can have very different structures.
Diamond
Diamond is a giant covalent structure. Each carbon atom forms four strong covalent bonds to four other carbon atoms.
This makes diamond:
- very hard
- very high melting point
- unable to conduct electricity
Diamond does not conduct electricity because all the outer-shell electrons are used in covalent bonds. There are no delocalised electrons free to move and carry charge.
Graphite
Graphite is also made only of carbon atoms, but its structure is different.
Each carbon atom forms three covalent bonds in flat layers of hexagons. The fourth outer-shell electron becomes delocalised.
Delocalised electron
A delocalised electron is an electron that is not fixed between two specific atoms and can move through part of a structure.
Graphite:
- has a high melting point because there are strong covalent bonds within each layer
- conducts electricity because delocalised electrons can move through the layers
- is soft and slippery because weak forces between layers allow the layers to slide
This is why graphite is used in pencil “lead” and as a lubricant.
Graphene
Graphene is a single layer of graphite, only one atom thick.
It is:
- very strong
- very thin and light
- a good electrical conductor
- useful in electronics and advanced materials research
Graphene’s strength comes from strong covalent bonds in the sheet. Its conductivity comes from delocalised electrons.
Fullerenes
Fullerenes are molecules made from carbon atoms arranged in hollow shapes, such as spheres or tubes.
A well-known fullerene is C60, where 60 carbon atoms form a hollow cage.
Fullerenes can have useful properties because they are tiny, hollow and have a large surface area compared with their volume. They may be used in drug delivery, lubricants and catalysts.
Explaining conductivity in carbon structures
Graphite conducts electricity, but diamond does not. Explain why.
- Compare the bonding in each structure: in diamond, each carbon atom forms four covalent bonds; in graphite, each carbon atom forms three covalent bonds.
- Work out what happens to the outer-shell electrons: diamond has no free outer electrons, but graphite has one delocalised electron per carbon atom.
- Link this to electrical conduction: graphite conducts because delocalised electrons can move and carry charge; diamond does not conduct because it has no mobile charged particles.
Bonding, structure and properties
Different types of materials have different properties because their particles are bonded and arranged differently.
Ionic compounds
An ionic compound contains positive and negative ions held together by strong electrostatic forces in a giant lattice.
Ionic compounds usually:
- have high melting and boiling points
- are solid at room temperature
- do not conduct electricity when solid
- conduct electricity when molten or dissolved in water
Solid ionic compounds do not conduct because the ions are fixed in place. When molten or aqueous, the ions can move and carry charge.
Simple molecular substances
A simple molecule is a small group of atoms held together by covalent bonds.
Examples include water, oxygen, carbon dioxide and methane.
Simple molecular substances often have low melting and boiling points. This is because the forces between molecules are weak compared with covalent bonds.
Bonds inside molecules vs forces between molecules
When a simple molecular substance melts or boils, the covalent bonds inside each molecule are not broken. Only the weak intermolecular forces between molecules are overcome.
Giant covalent structures
A giant covalent structure contains many atoms joined by strong covalent bonds in a large network.
Examples include diamond, graphite and silicon dioxide.
They usually have very high melting points because many strong covalent bonds must be broken.
Polymers
A polymer is a very large molecule made from many repeating units joined together.
Polymers have strong covalent bonds along their chains. Their properties also depend on the forces between chains.
Many polymers are:
- solid at room temperature
- flexible
- poor electrical conductors
Longer chains and stronger forces between chains usually make a polymer stronger and give it a higher softening or melting temperature.
Metals
A metal has a giant metallic structure. Positive metal ions are arranged in layers, with delocalised electrons moving through the structure.
Metals usually:
- conduct electricity and heat
- are malleable, meaning they can be hammered into shape
- have high melting points
- are strong
They conduct because delocalised electrons can move and carry energy or charge.
Energy transfers and changes of state
A change of state happens when a substance changes between solid, liquid and gas. Melting, freezing, boiling and condensing are changes of state.
During a change of state, energy is transferred. For example:
- melting requires energy to overcome attractions holding particles in fixed positions
- boiling requires energy to separate particles into a gas
- freezing and condensing release energy to the surroundings
The stronger the attractions between particles, the higher the temperature needed for melting or boiling.
Intermolecular forces
Intermolecular forces are forces of attraction between molecules. They are much weaker than covalent bonds within molecules.
For simple molecular substances, melting and boiling involve overcoming intermolecular forces.
For ionic compounds, metals and giant covalent substances, melting involves overcoming much stronger attractions or bonds in giant structures. This is why their melting points are often much higher.
Predicting state from melting and boiling points
A substance has a melting point of −20 °C and a boiling point of 85 °C. Predict its state at 25 °C.
- Compare the temperature with the melting point: 25 °C is above −20 °C, so the substance is not solid.
- Compare the temperature with the boiling point: 25 °C is below 85 °C, so the substance has not boiled.
- Place the temperature between the two changes of state: between melting point and boiling point, the substance is a liquid.
State prediction shortcut
Below melting point = solid. Between melting point and boiling point = liquid. Above boiling point = gas.
Nanoparticles
This nanoparticle section is for separate Chemistry J248, not Combined Science.
A nanoparticle is a particle with a diameter from about 1 nanometre to 100 nanometres.
Nanometre
A nanometre, symbol nm, is one billionth of a metre: 1 nm=1×10−9 m1 \text{ nm} = 1 \times 10^{-9} \text{ m}1 nm=1×10−9 m.
Atoms are usually about 0.1 nm across, which is around 1×10−10 m1 \times 10^{-10} \text{ m}1×10−10 m. Nanoparticles are bigger than individual atoms and many small molecules, but much smaller than cells, grains of sand or dust particles.
For scale:
- an atom is roughly 0.1 nm
- a small molecule may be around 1 nm
- a nanoparticle is about 1–100 nm
- a human hair is tens of thousands of nanometres wide
Comparing atomic and nanoparticle sizes
A nanoparticle is 50 nm wide. An atom is about 0.1 nm wide. How many times wider is the nanoparticle than the atom?
- Use a ratio of the two sizes: 50 nm0.1 nm\frac{50 \text{ nm}}{0.1 \text{ nm}}0.1 nm50 nm.
- Divide the numbers and cancel the same units: 50÷0.1=50050 \div 0.1 = 50050÷0.1=500.
- The nanoparticle is 500 times wider than the atom.
Surface area to volume ratio
Small particles have a larger surface area to volume ratio than large particles.
Surface area to volume ratio
The surface area to volume ratio compares how much outside surface a particle has compared with the amount of material inside it.
This matters because reactions and dissolving happen at surfaces. If more particles are exposed at the surface, the material may react faster, dissolve faster or act as a better catalyst.
For a cube:
surface area=6l2\text{surface area} = 6l^2surface area=6l2 volume=l3\text{volume} = l^3volume=l3where lll is the side length.
Calculating surface area to volume ratio
Compare one cube of side length 2 cm with one cube of side length 1 cm.
- Calculate the surface area and volume of the 2 cm cube: surface area is 6×22=24 cm26 \times 2^2 = 24 \text{ cm}^26×22=24 cm2 and volume is 23=8 cm32^3 = 8 \text{ cm}^323=8 cm3.
- Find its ratio: 248=3\frac{24}{8} = 3824=3, so the surface area to volume ratio is 3:1.
- Calculate the surface area and volume of the 1 cm cube: surface area is 6×12=6 cm26 \times 1^2 = 6 \text{ cm}^26×12=6 cm2 and volume is 13=1 cm31^3 = 1 \text{ cm}^313=1 cm3.
- Find its ratio: 61=6\frac{6}{1} = 616=6, so the surface area to volume ratio is 6:1.
- Compare the results: the smaller cube has the larger surface area to volume ratio.
Uses of nanoparticulate materials
Nanoparticles can have different properties from larger particles of the same substance. This is because of their small size and high surface area to volume ratio.
Examples include:
- sunscreens: titanium dioxide or zinc oxide nanoparticles block ultraviolet radiation but may appear transparent on skin
- catalysts: nanoparticles provide a large surface area, so less material can give a faster reaction
- medicine: nanoparticles may help deliver drugs to specific parts of the body
- antibacterial materials: silver nanoparticles can be used in wound dressings or fabrics
- electronics and composites: carbon nanotubes and graphene can be strong, light and conductive
Possible risks of nanoparticles
Nanoparticles can be useful, but their risks are still being researched.
Because they are so small, some nanoparticles may enter the body through the lungs, skin or digestive system. They may also pass into cells or enter the bloodstream.
They could be harmful if they:
- damage cells
- build up in organs
- cause inflammation
- harm aquatic organisms after being washed into waterways
- behave differently from larger particles of the same substance
Not all nanoparticles have the same risk
The risk depends on the substance, particle size, shape, coating, concentration, exposure time and how it enters the body.
In the exam
- Link each property to structure and bonding: name the particles, the bonds or forces, and whether anything can move.
- For melting and boiling questions, decide whether strong chemical bonds or weaker intermolecular forces are being overcome.
- For nanoparticle questions, mention small size, high surface area to volume ratio, useful properties and possible unknown risks.
Check yourself
- Why does graphite conduct electricity but diamond does not?
- How would you predict whether a substance is solid, liquid or gas from its melting and boiling points?
- Why do nanoparticles often make better catalysts than larger particles of the same material?
