Transcript STRUCTURE OF MATERIALS - St Oliver's Community College

STRUCTURE OF
MATERIALS
The make up of an atom
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The simple idea of an atom is that an atom
has a nucleus and negatively charged
electrons whirling around the nucleus.
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Within the nucleus, there are
protons (positively charged)
and neutrons (no charge)
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Electrostatic forces hold the
nucleus and the electrons together.
Three types of atomic bonds:
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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
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Cations (+) and
anions (-) are attracted
to each other in an
ionic bond.
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Electrons may be
transferred from one
atom to another in
ionisation.
Metallic bond
(+) Cations in a ‘sea’ of (-) electrons.
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The movement of these electrons makes
metals good conductors of heat and electricity.
Crystalline structures
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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
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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
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Line defects
-Dislocations.
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Point defects -Vacancy
-Substitution
-Interstitial
Vacancy point defect
If 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 defect
In this case, a much larger atom has been substituted in the
lattice and distorts the structure.
Interstitial point defect
In this case, a foreign atom has moved into the space between
the atoms of the lattice.