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Classifying Materials

4.1.1 Define atom, molecule, alloy and composite.

The smallest part of an element that can exist chemically.
Two or more atoms that are normally bonded together covalently.
A mixture that contains at least one metal. This can be a mixture of metals or a mixture of metals and non-metals.
A mixture composed of two or more substances (materials) with one substance acting as the matrix or glue.
Atom Molecule
An atom is the smallest particle characterizing a chemical element. An atom consists of an electron cloud surrounding a dense nucleus. This nucleus contains positively charged protons and electrically neutral neutrons, whereas the surrounding cloud is made up of negatively charged electrons. When the number of protons in the nucleus equals the number of electrons, the atom is electrically neutral; otherwise it is an ion and has a net positive or negative charge. An atom is classified according to its number of protons and neutrons: the number of protons determines the chemical element and the number of neutrons determines the isotope of that element. A molecule is as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds. Basically it is composed of a group of atoms covalently bonded to form an element such as Water (H2O).
Alloy Composite
* An alloy is a homogeneous mixture of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. The resulting substance usually has different properties (sometimes substantially different) from those of its components.
* Unlike pure metals, most alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases.
* Alloying one metal with others usually improves on the properties of other elements.
Composite materials (or composites for short) are engineered materials made from two or more constituent materials with significantly different physical or chemical properties and which remain separate and distinct on a macroscopic level within the finished structure. For example the most primitive composite materials comprised straw and mud in the form of bricks for building construction. One way to form composites is using many moulding methods.

4.1.2 Describe a bond as a force of attraction between atoms.

Consider and differentiate between the three main types of bond: ionic, covalent and metallic.

There are three types of inter atomic bonds. Ionic, covalent and metallic.

Ionic Bonds
Ionic bonds occur between a metal and a non metal. All atoms strive for a full outer shell of electrons. Metals have an empty outer shell but a full shell beneath it while non metals have a few electrons on their outer shell. The metal becomes positively charged as it gives its extra electrons to the non-metal, which leaves them negatively charged. Opposite charges attract and so the atoms stay connected through electrostatic forces. The ionic bonds are fragile; small bumps can cause alterations in the position of the ions causing two like charges which then causes them to repel. When ions dissociate (separate), whether in a solvent such as water or through melting, they can conduct electricity.
Covalent Bonds
Covalent bonds are formed between two non-metals. It is characterized by the sharing of electrons between atoms. These bonds are the strongest type of bonds because the outer electron shell of two or more atoms overlap and create a completely new shell around both of them. There are two types of covalent bonds: molecular and network. A molecular bond, such as water, creates molecules that are weakly bonded to each other. They are usually in the gaseous or liquid state at room temperature. Network covalent bonds, such as diamond, can make crystals of virtually any size. Diamonds are known to be the toughest element. They are usually hard because of the strength between covalent bonds.
Metallic bonds
Metallic bonds are formed between metals. In a metallic bond, the valence electrons from each atom form a 'sea' of electrons that works as glue to keep the metals together. The sea of valence electrons gives the metals two important properties, the first being malleability: metals are malleable because the lattice ions are still attracted to the electrons even after being moved. The second property is conductivity. The electrons have enough freedom to move, which is exactly what electricity is. The electrons' freedom also means that they can start vibrating easily (and so can the lattice ions because they have quite a bit of freedom too) and that makes them good heat conductors because temperature is a measure of the average kinetic energy of the substance.

4.1.3 Describe how materials are classified into groups according to similarities in their microstructures and properties.

Steel TPU under an electron microscope
Wood Iron

The microstructure of a material is its structure when perceived through a microscope. Materials are classified depending on their crystal structure, their size, composition, orientation, formation and interaction. Materials will be classified in terms of their physical properties such as strength, toughness, ductility, hardness, corrosion resistance, high / low temperature behaviour, durability, etc… In the case of metals, the microstructures must be viewed with a microscope. Each polygon , usually hexagons, represents a single crystal of element. Metals are classified by their microstructures and properties (ex. this is why steel is different from iron).

4.1.4 Explain that several classifications are recognised but that no single classification is “perfect”.

It is convenient to be able to classify materials into categories (albeit crude in nature) that have characteristic combinations of properties.

Classification of different materials is not considered perfect due to the variance of different material's microstructures. For example, the microstructures of different plastics, thermoplastics and thermoset plastics, have distinct differences. The microstructure of a thermoplastic is composed of strings or threads of plastic particles. This gives it special characteristics such as being easily reformed or malleable. On the other hand, thermoset plastics are composed of stronger covalent bonds, which make it stronger but less malleable. It is for reasons such as this, that classifications of materials cannot be considered perfect. In essence, there is no such "perfect" classification due to all classifications of materials having advantages and disadvantages. Ultimately, the faults, whether minor or major, in all classifications prevents the ideology of a "perfect" classification for a certain material.

Below is a list of some of the commonly classification of materials within these four general groups of materials.


* Ferrous metals and alloys (irons, carbon steels, alloy steels, stainless steels, tool and die steels)
* Nonferrous metals and alloys (aluminium, copper, magnesium, nickel, titanium, precious metals, refractory metals, superalloys)
* Thermoplastics plastics
* Thermoset plastics
* Elastomers
* Porcelain
* Glasses
* Graphite
* Diamond
* Reinforced plastics
* Metal-matrix composites
* Ceramic-matrix composites
* Sandwich structures
* Concrete

4.1.5 Describe that, for this course, materials are classified into groups: timber, metals, plastics, ceramics, food and composites; and that some of these groups have subdivisions.

In each group there can be subdivisions, for example, for timber (natural wood and man-made), metals (ferrous and non-ferrous), plastics (thermoplastics, thermosets), ceramics (earthenware, porcelain, stoneware, glass), textile fibres (natural or synthetic), food (vegetable or animal origin) and composites (difficult to classify due to variability and continual development of new composite materials). Food is included here for completeness, although it is dealt with in detail as an option.

Each group of materials is classified by a certain trait of that material and each material is in turn categorised into sub-categories of that certain material, based on the different properties that a single type of material can have. An example for a material is Ceramics. Ceramics is a combination of earth materials such as mud or sand. Yet, examples of a sub-category of ceramics is earthenware and glass. The difference between these two, are things such as the process in which they are formed, glass being man-made from the melting of sand and the result of other processes, while earthenware can just be created by putting some wet dirt or mud together and letting it dry out. These two examples have major differences, as glass can be much more resistant than earthenware. Another thing to consider, is that there are also some sub-categories of the sub-categories, such as in the case of glass, there can be plastic added to glass to make it bulletproof, even though most of these sub-sub categories are composites.

Natural wood e.g. Cherry or Oak Man-made e.g. plywood
Ferrous Metals e.g. Stainless steel watches or cutlery. Non Ferrous Metals - e.g. copper, brass, aluminium.
Thermoplastics e.g. ice cube trays or some drink bottles Thermoset plastics e.g. some plastic dishes
Natural fibre e.g. wool or cotton Synthetic fibre e.g nylon socks or rayon
Vegetable Origin e.g carrots, apples etc Animal Origin Fish, steaks etc
Glass Earthenware e.g. pottery
Porcelain e.g Mums special porcelain dinner sets Earthenware e.g the Sake pot
Used in aeroplanes


Bulleted list and italicised paragraphs are excerpted from Design Technology: guide. Cardiff Wales, UK: International Baccalaureate Organization, 2007.

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Page last modified on August 25, 2013, at 10:26 PM