Objectives
When you complete this presentation, you will be able to … Model the valence electrons of metal
atoms.
Describe the arrangement of atoms in a metal.
Explain the importance of alloys.
Introduction
We are already familiar with metals
We know that metals are Tough
Ductile
Malleable
Conductive
These properties come from the way metal ions form bonds with one another.
Metallic Bonds
Metals are made up of closely packed cations rather than neutral atoms.
The valence electrons of metal atoms can best be modeled as a sea of electrons. These electrons are not associated with
any particular metal atom (they are not localized).
They are mobile and are free to drift from one part of the metal to another.
Metallic Bonds
Metallic bonds consist of the attraction of the free-floating valence electrons for the
positively charged metal ions.
These are the forces of attraction that hold metals together.
Metallic Properties
The “sea of electrons” model explains many of the properties of metals we are familiar with.
Conductivity: As we introduce an electron into the end of
a metal wire (for example)
electrons are free to move throughout the metal and will allow another electron to exit out the other end.
Metallic Properties
Ductility and Malleability: The sea of electrons isolates the metal ions
from one another.
As pressure is applied to the bulk metal, the metal ions are free to move and rearrange so that the metal can reform without damage.
This is unlike ionic crystals which are not isolated from each other and will fracture under pressure.
Crystalline Structure
The ions in many metals are arranged in a hexagonal pattern, much like fruits or vegetables in a produce section of a grocery store.
This arrangement is called a “hexagonal close-packed” (hcp) arrangement.
Every ion has twelve neighbors.
Metals with this arrangement include magnesium, Mg, zinc, Zn, and cadmium, Cd.
Crystalline Structure
Other metal ions may form a “body-centered cubic” (bcc) arrangement.
Every ion has eight neighbors.
This arrangement is used by sodium, Na, potassium, K, iron, Fe, chromium, Cr, and tungsten, W.
Crystalline Structure
Yet other metal ions may form a “face-centered cubic” (fcc) arrangement.
Every ion has twelve neighbors.
This arrangement is used by copper, Cu, silver, Ag, gold, Au, aluminum, Al, and lead, Pb.
Alloys
Few metals that we encounter are composed of just one type of metal.
Usually, we use mixtures of metals called alloys.
Alloys are mixtures of two or more metals. Brass is a mixture of copper and zinc.
Bronze is a mixture of copper and tin.
Alloys
Alloys are important because their properties often make them more useful than the pure metals. Sterling silver (92.5% Ag, 7.5% Cu) is
harder and more durable than pure silver.
Stainless steel (80.6% Fe, 18.0% Cr, 0.4% C, 1.0% Ni) is harder and less likely to corrode than pure iron.
Alloys
Alloys can form in many ways. If the ion replacing the pure metal ion is
about the same size, the new ion just fits into the crystal where the original ion would have been. This is called a substantial alloy.
Alloys
Alloys can form in many ways. If the ion is smaller than the pure metal ion
it is replacing, then the ion fits into the spaces between the metal ions. This is called an interstitial alloy.
Summary
Metals are made up of closely packed cations rather than neutral atoms.
The valence electrons of metal atoms can best be modeled as a sea of electrons.
The “sea of electrons” model explains many of the properties of metals we are familiar with. Conductivity
Malleability and Ductility