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.