A1.2 Structure, Bonding and Properties
Structure: the way that the particles of a material are arranged in (usually) three dimensions.
You should know the difference between a giant structure, where a vast number of individual particles are joined in one huge structure with strong bonds throughout (e.g. diamond, sodium chloride) and a molecular structure where individual, strongly covalently bonded molecules are held together loosely by weak London forces.
Figure 1.1 Diamond has a giant structure (Image from Shutterstock/Andris Torms)
Figure 1.2 Individual polymer chains in a molecular structure do not join each other
Bonding: the type of interaction that holds the particles together to assemble the structure – ionic, covalent or metallic bonding.
Properties: physical attributes as a result of combinations of structure and bonding. Some properties include:
- Melting point: the stronger the bonding, the higher the melting point.
- Permeability: a solid will be permeable to moisture if there are gaps between the particles so there is room for water molecules to pass through.
Polymers: woven fabrics made from man-made or natural fibres are permeable because there are gaps between the fibres. Many (but not all solid polymers) are impermeable because their polymer chains are non-polar and are usually closely packed together.
Paper and card: made of wood or plant fibres. It is permeable because of the gaps between the fibres.
Rocks: permeability depends on how they were formed. Limestone and chalk are permeable. Marble and granite are impermeable. In general sedimentary rocks are permeable but once they have been subjected to metamorphic processes become impermeable. This is why limestone is permeable but marble is impermeable.
Concrete is often permeable, with a porous structure. It can absorb water like some traditional ceramics (e.g. terracotta, unglazed pottery).
Metals and most ceramics have tightly packed particles that make them impermeable.
- Conductivity: if electrons can flow within a structure, a material can conduct electricity. All metals have a network of delocalised outer shell electrons, so the electrons can be passed from atom to atom and the metal conducts. The layers of carbon atoms in graphite also have delocalised electrons so it conducts, just like a metal.
- Elasticity: if a material is deformed when stretched or compressed by force but it returns to its original shape when the force is removed, it is elastic. It returns to its original shape because the forces that hold the particles of the material in place pull or push them back in to their original positions.
- Malleability/ductility: is a property that allows a material to take on a new shape when a force is applied or removed. The material does not break or return to its original shape because its particles slide into new positions. Ductility, specifically, is the ability to be drawn out to form wires, without breaking.
- Brittleness: is when material breaks easily when a force is applied.
conducting polymers and ceramics have both been developed
The range of properties of polymers are wide and various because they have been developed for particular uses – so a simple list of “typical properties” is inappropriate.
Glass: Glass has a unique structure. It is amorphous as the particles it consists of are not in regular (crystalline) positions. It is a solid but its particles are randomly arranged like that of a liquid. Other than that, its properties are most like those of a ceramic.
Composite Materials: Concrete is a good example of a composite material. Concrete has lots of small separate particles of rock (gravel) suspended in a solid matrix (cement mixture) – similar in structure to a fruit cake. Composites are said to be made of two distinct phases.
Other common examples of composite materials are: carbon fibre (graphite in a polymer resin), reinforced concrete (steel cables in concrete), and fibreglass (which is an example of glass reinforced plastic).
Natural composites include many rocks, and materials like wood and bone. These materials combine the properties of the materials they are made of, often making them both strong and lightweight.
Figure 1.3 Bonding Triangle
Figure 1.4 Detailed bonding triangle
Any binary substance (a substance with only two elements) can be located on the diagram according to the difference in electronegativity between its two elements.
By looking at the position of the compound on the triangle you can classify the substance. If it is near one of the corners, it is mostly either ionic
are both covalent but their polar nature moves their position away from the “pure covalent corner” of the diagram.
is mostly ionic but less so than
is mostly covalent but less so than
The main point here is that there is no clear division, but a continuum in the classification between the three types of bonding types – so a substance can be mostly ionic but with covalent character etc.