Download Topic 4: Materials - Education Umbrella

Topic 4: Materials
Metals are often described as positively
charged nuclei in a sea of electrons.
The outer electrons of the metal atom nuclei are free and can flow through the crystalline structure. The bonding is caused by attraction between the positively charged metallic atom nuclei and the negatively charged cloud of free electrons. Specific arrangements of metal atoms are not required.
Metallic bond
Metallic bonding involves sharing outer electrons but these are freer and they can flow through the crystalline structure. The bonding is caused by attraction between the positively charged metal atom nuclei and the negatively charged cloud of free electrons, and is spread throughout the lattice – “positively charged nuclei in a sea of electrons”.
How the movement of free electrons makes metals
very good electrical and thermal conductors
Movement of free electrons
In a metal, the outer electrons are shared among all the atoms in the solid. Each atom gives up its outer electrons and becomes slightly positively charged. The negatively charged electrons hold the metal atoms together. Since the electrons are free to move, they lead to good thermal and electrical conductivity. With the metallic bond, atoms are also held together by their outermost electrons, but instead of being
shared by a couple of atoms, these electrons act like a cloud that is able to move through the structure of the
crystal. This cloud, which is negatively charged (electrons have a negative charge), pulls on the atoms, which
are positively charged (each atom is missing one or more of its electrons). The attraction between the atoms
extends beyond the crystal, pulling in on adjacent crystals.
The electron cloud moves freely when
+
‐
a voltage is applied. This movement of
electrons is an electric current. The
electron cloud can also transfer
thermal energy, making it a good
conductor of heat. Also, the electron
cloud absorbs then radiates most of
the light that falls on it, which gives
metal its lustrous appearance.
Crystal structure
Pure and alloyed metals exist as crystals
Metals, many ceramics and some polymers have a uniform, geometrical arrangement of atoms or ions that is
repeated throughout the material and are therefore said to be crystalline. This regular 3‐D atomic pattern is
known as the space lattice and the unit cell is the smallest unit of the lattice that retains the overall characteristic
of the lattice. The crystal structure refers to the size, shape and atomic arrangement of the lattice unit cell and
varies from one substance to another. Crystals are regular arrangements of particles (atoms, ions or molecules).
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