Bonding in Metals

1. Bonding in Metals Section 7.3

2. 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.

3. 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.

4. Metallic Bonds • Metals are made up of closely packed cationsrather 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. Summary • Metals are made up of closely packed cationsrather 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

16. 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.