Bonding in Metals Section 7.3

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

Summary  The ions in many metals are arranged patterns.  Hexagonal-close packed  Body-centered cubic  Face-centered cubic  Alloys are mixtures of metals.  Alloys are important because their properties often make them more useful than the pure metals.