STRUCTURE OF MATERIALS
The make up of an atom
The simple idea of an atom is that an atom has a nucleus and negatively charged electrons whirling around the nucleus.
Within the nucleus, there are protons (positively charged) and neutrons (no charge)
Electrostatic forces hold the nucleus and the electrons together.
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 elements.
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.
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.
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)
Crystal defects
There are two categories of defects in a crystal
Line defects -Dislocations.
Point defects -Vacancy -Substitution -Interstitial
Vacancy point defectIf there is an atom missing from the lattice, then the whole lattice is distorted as other atoms are forced into the vacant space.
Substitution point defectIn this case, a much larger atom has been substituted in the lattice and distorts the structure.
Interstitial point defectIn this case, a foreign atom has moved into the space between the atoms of the lattice.
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