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Bonding in Metals

Bonding in Metals. OBJECTIVES: Use the theory of metallic bonds to explain the physical properties of metals. Bonding in Metals. OBJECTIVES: Describe the arrangements of atoms in some common metallic crystal structures. Metallic Bonds. How atoms are held together in the solid.

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Bonding in Metals

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  1. Bonding in Metals • OBJECTIVES: • Use the theory of metallic bonds to explain the physical properties of metals.

  2. Bonding in Metals • OBJECTIVES: • Describe the arrangements of atoms in some common metallic crystal structures.

  3. Metallic Bonds • How atoms are held together in the solid. • Metals hold on to their valence electrons very weakly. • Think of them as positive ions (cations) floating in a sea of electrons:

  4. + + + + + + + + + + + + Sea of Electrons • Electrons are free to move through the solid. • Metals conduct electricity.

  5. Metals are Malleable • Hammered into shape (bend). • Also ductile - drawn into wires. • Both malleability and ductility explained in terms of the mobility of the valence electrons

  6. + + + + + + + + + + + + Malleable

  7. + + + + + + + + + + + + Malleable • Electrons allow atoms to slide by.

  8. + - + - - + - + + - + - - + - + Ionic solids are brittle

  9. - + - + - + - + + - + - - + - + Ionic solids are brittle • Strong Repulsion breaks crystal apart.

  10. Crystalline structure of metal • If the metal is made of one kind of atom (i.e. it is an element) metals are among the simplest crystals • Body-centered cubic: • every atom has 8 neighbors • Na, K, Fe, Cr, W

  11. Crystalline structure of metal 2. Face-centered cubic: • every atom has 12 neighbors • Cu, Ag, Au, Al, Pb 3. Hexagonal close-packed • every atom also has 12 neighbors • different pattern due to hexagonal • Mg, Zn, Cd

  12. Alloys • We use lots of metals every day, but few are pure metals • Alloys - mixtures of 2 or more elements, at least 1 is a metal • made by melting a mixture of the ingredients, then cooling • Brass: an alloy of Cu and Zn • Bronze: Cu and Sn

  13. Why use alloys? • Properties often superior to element • Sterling silver (92.5% Ag, 7.5% Cu) is harder and more durable than pure Ag, but still soft enough to make jewelry and tableware • Steels are very important alloys • corrosion resistant, ductility, hardness, toughness, cost

  14. Why use alloys? • Look up alloys in your text book • Types? a) substitutional alloy- the atoms in the components are about the same size • b) interstitial alloy- the atomic sizes quite different; smaller atoms fit into the spaces between larger • Amalgam- dental use, contains Hg

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