Introducing and classifying materials

Atom Definition: The smallest part of an element that can exist chemically This image shows the smallest portion into which an element can be divided (atom) and still retain its properties, made up of a dense, positively charged nucleus surrounded by a system of electrons.

Molecule Definition: Two or more atoms that are normally bonded together covalently. This image shows a number of atoms bonded together with a chemical bond in which the attractive force between atoms is created by the sharing of electrons (covalently).

Alloy Definition: A mixture that contains at least one metal. This can be a mixture of metals or a mixture of metals and non-metals. These Motocross goggles are made of an alloy mix of metal and plastic. Alloy rims are a mixture of metal and other materials

Composite Definition: A mixture composed of two or more substances (materials) with one substance acting as the matrix or glue. Many tennis rackets, racing bicycles, and skis are now fabricated from a carbon fibre-epoxy composite that is strong, light, and only moderately expensive. A cross section view of a carbon-epoxy composite showing the strong and stiff graphite fibres embedded in the tough epoxy matrix.

Bonds Bonds are a force of attraction between atoms. There are three main types of bond: Ionic Covalent Metallic

Bonds Ionic: An ionic bond (or electrovalent bond) is a type of chemical bond based on electrostatic forces between two oppositely-charged ions. In ionic bond formation, a metal donates an electron, due to a low electro negativity to form a positive ion or cation (a cation is an ion with a positive charge. It is the inverse anion). In ordinary table salt (NaCl), the bonds between the sodium and chloride ions are ionic bonds.

Bonds Covalent: Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. In short, attraction-to-repulsion stability that forms between atoms when they share electrons is known as covalent bonding.

Bonds Metallic Metallic bonding is the bonding between atoms within metals. It involves the delocalised sharing of free electrons among a lattice of metal atoms. Thus, metallic bonds may be compared to molten salts. Metallic bonding is the electrostatic attraction between the metal atoms or ions and the delocalized electrons, also called conduction electrons. This is why atoms or layers are allowed to slide past each other, resulting in the characteristic properties of malleability and ductility.

Introducing and classifying materials

Materials can be classified into groups according to similarities in their microstructures and properties. Several classifications are recognised, but no single classification is perfect. It is convenient to be able to classify materials into groups which have characteristic combinations of properties.

For this course materials are classified into the following groups: Timber Metals Ceramics Plastic Textile Fibres Food Composites Some of these groups have subdivisions

Subdivisions: Timber: Natural Wood or Composite. Hardwood or softwood. Metals: Ferrous or Nonferrous. Ceramics: Stoneware, Earthenware or Porcelain. Plastics: Thermosets and Thermoplastics. Textile Fibres: Natural or synthetic. Food: Animal or Vegetable origin. Composites are difficult to classify due to variability and the development of new ones.