STRUCTURE OF MATERIALS

Three types of atomic bonds: Covalent bonds Ionic bonds Metallic bonds

Covalent bonds Known as primary bonding. A pair of atoms are Shared by two or more atoms.

Ionic bonds Cations (+) and anions (-) are attracted to each other in an ionic bond. Electrons may be transferred from one atom to another in ionisation.

Metallic bond Cations in a ‘sea’ of electrons. The movement of these electrons makes metals good conductors of heat and electricity.

Properties of metals Conduct heat and electricity Malleable and ductile Electron donors (form oxides) Can take a shine High density High tensile strength Solid at room temperature (except Mercury)

Crystalline structures Body-centred cubic (BCC) Face-centred cubic (FCC) Close-packed hexagonal (CPH)

Slip in BCC and FCC structures As atoms in FCC are more closely packed than BCC, slip will occur more easily. FCC metals are ductile and BCC metals are more brittle.

Crystal defects There are two categories of defects in a crystal Line defects – dislocations. Point defects vacancy substitution interstitial

Vacancy point defect

Substitution point defect

Interstitial point defect