4.1 Introducing and Classifying Materials

4.1.1: Define atom, molecule, alloy and composite

Atom: the smallest component of an element in a chemical element
Molecule: a group of atoms bonded together, representing the smallest fundamental part of a chemical compound
Alloy: a metal made by combining 2 or more metallic elements
Composite: made up of various parts or elements

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

The atoms of all materials are held or bonded together by electric forces.
i.e. Forces of attraction between positively and negatively charged particles and forces of repulsion between two positively or two negatively charged particles
Bonds can be classified into 2 groups:

  • Primary bonds – ionic, covalent, metallic bonds (strongest bond)
  • Secondary bonds – Van der Waals and hydrogen bonds (relatively weak – about 1% the strength of primary bonds)

Materials are generally bonded by a combination of bond types however the predominate bonds are:

  • For metals = metallic
  • Polymers = covalent (with secondary bonds between chains)
  • Ceramics = ionic and/or covalent bonding depending on the type of ceramic material

Primary Bonds

Ionic Bonds:

  • In an ionic bond atoms either lose electrons (+ve ion) or gain electrons (-ve ion) so they have a complete outer electron shell (a complete outer shell is one in which there are 2 electrons if it is the first shell or 8 electrons for other shells)
  • ‘Transfer’ is a key word used to describe ionic bonding
  • The attraction and repulsion of the charged particles (= ions) creates the bond
    • e.g. Sodium has a shell structure 2,8, 1 electrons while chlorine has 2,8, 7 shell structure – sodium gives up one electron to form a +ve ion and chlorine receives the electron to form a –ve ion; the compound sodium chloride (NaCl) results from the ionic bonding of Na+ and Cl-
  • Ionically bonded materials are poor conductors because the electrons are tightly held in the bonds; they also tend to have high melting points because a lot of energy is required to break the bonds.

Covalent Bonds:

  • In covalent bonds the atoms share their outer electrons to achieve the ‘full outer shell’ configuration
    • e.g. Carbon has 4 electrons in its outer shell and hydrogen has only 1; thus 4 atoms of hydrogen share their 1 electron with 1 carbon atom so both atoms achieve a ‘full shell’
  • The bond is created by the shared electrons mainly being concentrated between the atoms thereby ‘gluing’ the atoms together
  • Covalently bonded materials have high melting points due to the high strength of the bond and they are also good insulators due to the electrons (=charge carriers) being securely held in the bonds of the material.

Metallic Bonds:

  • Predominant bond in metals
  • It is the bond between the positive ions and the negative electrons in the 'electron cloud'
  • The free electrons explain why metals are good conductors of electricity and heat; the high strength of metals is explained by the repulsive forces of the +ve ions (compression) and the attractive forces between the –ve electron cloud and the +ve ions (tension)

Secondary Bonds

Molecular or van der Waals bonds:

  • Produced by the concentration of –ve electrons on one side of an atom at one particular time which leaves the rest of the atom with a +ve charge (the charge can vary with time and can easily break down with heat)

Hydrogen Bonding:

  • Water molecule is covalently bonded which results in the shared electrons being closer to the oxygen atom leaving the 2-hydrogen-atom-end of the molecule being slightly positive relative to the oxygen atom; each water molecule is said to be a dipole
  • Occurs as a result of electrostatic attraction between these molecular diploes; the molecules of wood cells (cellulose) are bound to teach other by hydrogen bonding.

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


  • Electrical conductivity
  • Thermal conductivity
  • Ductility- ability to deform under tensile stress- e.g. form wires by stretching
  • Malleability- ability to deform under compressive stress- e.g. form thin sheets by hammering or rolling
  • High density
  • Generally high melting points


  • Transparency
  • Flexibility
  • Elasticity
  • Electrical Resistance
  • Thermal Stability


  • Hard and brittle
  • High compressive and low tensile strength
  • Low electrical and thermal conductivity
  • Chemically inert- does not react with chemicals
  • Examples: glasses, cements, fired clay, etc


  • Bonding of two or more materials


  • Density of timbers varies between different species but are generally low
  • Strength of the timber depends on the specie and depends on whether the timber is measured parallel or perpendicular to the grain of the timber


4.1.4: Explain that several classifications are recognised but that no single classification is "perfect"

As stated above, several classifications are recognised. However even though for example, timber is classified as wood, there are different types of wood thus a single classification is not “perfect”. There are subdivisions and each type of wood is different in a variety of ways and can only ever be similar but never identical.
This happens because it is convenient to be able to classify materials into categories (albeit crude in nature – simplistic in nature) that have characteristic combinations of properties.

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

Metals are classified as:
i) Ferrous alloys (=iron based alloys) which include:

  • Plain carbon steels
  • Alloy steels
  • Cast irons

ii) Non-ferrous alloys (= do not contain iron) which include:

  • Light alloys of aluminium, magnesium, titanium, zinc.
  • Heavy alloys of copper, lead, nickel
  • Heat resisting metals (=refractory metals) such as molybdenum, tungsten
  • Precious metals such as gold, silver, platinum

i) Thermosoftening plastics (=soften with heat and are generally flexible and relatively soft)
ii) Thermosetting plastics (=do not soften with heat and are rigid and hard)

i) Glasses
ii) Cements
iii) Fired clays (=pottery)
iv) Electronic ceramics (=semiconductor and superconductors)
v) Engineering ceramics (for cutting edges of tools = alumina, carbides)

Composites are made by bonding two or more materials so that the final material has a combination of the good properties of the bonded materials. e.g
i) Glass reinforced polymer (GRP)
ii) Reinforced concrete
iii) Asphalt
iv) Wood

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