Properties of Materials and Nanoparticles
Linking Structure to Properties
The physical properties of a substance — its melting point, electrical conductivity, hardness and solubility — depend on the type of bonding and the structure it forms. This is one of the most important relationships in chemistry: understanding the structure allows you to predict and explain the properties. Exam questions very frequently ask you to explain a property in terms of bonding and structure, so you must be able to link every answer back to the forces involved.
Giant Ionic Lattices
Ionic compounds (such as sodium chloride and magnesium oxide) form giant ionic lattices. They have high melting and boiling points because breaking up the lattice requires a large amount of energy to overcome the many strong electrostatic forces between the ions. They do not conduct electricity when solid because the ions are held in fixed positions in the lattice and cannot move. However, they do conduct electricity when molten or when dissolved in water because the ions become free to move and carry charge. Most ionic compounds are soluble in water because water molecules can surround and separate the individual ions. Ionic compounds tend to be brittle: if a force shifts layers of ions so that like charges line up, repulsion between them causes the crystal to shatter.
Ionic compounds: high melting points (strong electrostatic forces in the lattice), conduct electricity only when molten or dissolved (ions must be free to move), do not conduct as solids (ions fixed in the lattice).
Simple Molecular Substances
Simple molecular substances (such as water, oxygen and carbon dioxide) have strong covalent bonds within each molecule but only weak intermolecular forces between molecules. When a simple molecular substance melts or boils, the intermolecular forces are overcome — not the covalent bonds. This gives simple molecular substances low melting and boiling points. They do not conduct electricity in any state because they have no free electrons or ions to carry charge. Larger molecules have more electrons, creating stronger intermolecular forces and therefore higher boiling points. A very common exam error is writing that the covalent bonds are weak — within the molecule, they are strong; the weak forces are those between molecules.
Giant Covalent Structures
Giant covalent structures contain billions of atoms bonded by strong covalent bonds throughout the whole structure. They have very high melting points because many strong covalent bonds must be broken to separate the atoms. Carbon forms several giant covalent allotropes:
In diamond, each carbon atom forms four covalent bonds arranged in a rigid tetrahedral lattice. This makes diamond extremely hard and gives it a very high melting point. Diamond does not conduct electricity because all four outer electrons of each carbon are used in covalent bonds — none are free to move.
In graphite, each carbon atom bonds to three others in flat hexagonal layers. The fourth electron on each carbon is delocalised within the layer, allowing graphite to conduct electricity. The layers are held together by weak intermolecular forces and can slide over each other, making graphite soft and useful as a lubricant or pencil material.
Graphene is a single layer of graphite — one atom thick. It is exceptionally strong for its mass, flexible, and an excellent conductor of electricity. It is being researched for uses in electronics, composites and membranes. Fullerenes are hollow carbon molecules. The most studied is buckminsterfullerene (\(C_{60}\)), shaped like a football. Fullerenes are being investigated as drug delivery vehicles and in nanotechnology. Their cage-like structure can trap other molecules.
Graphite conducts electricity because it has delocalised electrons within each layer. Diamond does not conduct because all four of each carbon’s outer electrons are used in covalent bonds. Graphene (a single graphite layer) also conducts. Fullerenes (e.g. \(C_{60}\)) are hollow cage-like molecules.
Nanoparticles
Nanoparticles have dimensions in the range of roughly 1 to 100 nanometres (nm) and contain between a few hundred and a few thousand atoms. At this scale, materials often behave very differently from how they behave in bulk form. This is because nanoparticles have a very large surface area to volume ratio compared to bulk materials. A larger surface area means more atoms are exposed at the surface, which can dramatically alter reactivity, optical properties and electrical behaviour.
There are several established uses of nanoparticles. Silver nanoparticles are incorporated into some wound dressings and antibacterial coatings because silver at the nanoscale has greater antibacterial effectiveness than bulk silver. Titanium dioxide nanoparticles are used in some sunscreen formulations because they provide UV protection while remaining transparent to visible light, unlike larger particles which appear white. Nanoparticles of various metals and metal oxides are used as catalysts in industrial processes because their large surface area increases reaction rate.
Because nanoparticles are new materials, our knowledge of their long-term effects on health and the environment is still limited. Their very small size means they can potentially enter cells, tissues and even the bloodstream in ways that larger particles cannot. Some research suggests that certain nanoparticles may cause inflammation or other harmful effects in the body or in ecosystems, but long-term data are still being gathered. This is a genuine scientific uncertainty — it does not mean nanoparticles are definitely harmful, but it does mean decisions about their use must weigh potential benefits (improved performance, new medicines) against possible risks (unknown health or ecological impacts). This type of risk-benefit analysis under uncertainty is an important part of how science informs decision-making about new technologies.
Polymers and Metals
Polymers are very long chain molecules. Strong covalent bonds hold the chain together; weaker intermolecular forces act between the chains. Their properties vary depending on the repeating unit and whether the chains are cross-linked or not. Thermoplastic polymers can be remoulded on heating because the intermolecular forces between chains are overcome. Thermosetting polymers cannot be remoulded because they have extensive cross-links between chains. Metals have high melting points, are malleable and are good conductors of electricity and heat. These properties all arise from the sea of delocalised electrons within the metal lattice. Alloys — mixtures of metals — often have different properties from the pure metals they contain, because the differently-sized atoms disrupt the regular lattice and prevent layers sliding as easily.