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Unit 4: Metallic Bonding

Unit 4: Metallic Bonding. Metallic Bonds are…. How metal 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. +. +. +. +. +. +. +. +. +. +. +. +.

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Unit 4: Metallic Bonding

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  1. Unit 4: Metallic Bonding

  2. Metallic Bonds are… • How metal 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

  3. + + + + + + + + + + + + Examples of Metallic Bonding

  4. + + + + + + + + + + + + Sea of Electrons • Electrons are free to move through the solid, so .. . . . • Metals conduct electricity. • Vacant “d” orbitals overlap and allow electrons to move.

  5. Properties of Metals • Excellent conductors of heat and electricity • Solids at room temperature (except Hg) • Extremely high melting points (200˚C and above) • Malleable • Ductile

  6. 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

  7. + + + + + + + + + + + + Malleable Force

  8. + + + + + + + + + + + + Malleable • Mobile electrons allow atoms to slide by, sort of like ball bearings in oil. Force

  9. - + - + + - + - - + - + + - + - Properties of ionic compounds • Crystalline structures (repeated patterns of + and – ions) • Are crystalline solids at room temperature • Have very high melting points • Are hard and brittle – they break under pressure.

  10. Do they Conduct? • Conducting electricity means allowing charges to move. • In a solid, the ions are locked in place. • Ionic solids are insulators. • When melted, the ions can move around. • Melted ionic compounds conduct. • NaCl: must get to about 800 ºC. • Dissolved in water, they also conduct (ions separate and are free to move in solutions)

  11. - Page 198 The ions are free to move when they are molten (or in aqueous solution), and thus they are able to conduct the electric current.

  12. + - + - - + - + + - + - - + - + Ionic solids are brittle Force

  13. - + - + - + - + + - + - - + - + Ionic solids are brittle • Strong Repulsion breaks a crystal apart, due to similar ions being next to each other. Force

  14. Structure of Ionic Compounds

  15. Bond Strength • Determined by Lattice Energy – the energy released when one mole of an ionic crystalline compound is formed from gaseous ions. • Lattice Energy = k (Q1)(Q2) r • r = bond length • Q = ion charge

  16. Comparing Bond Strength • Higher charged ions = stronger L.E. = stronger bond = higher melting point • L.E for NaF = 923 kJ • L.E. for MgO = 3916 kJ • The reason for this big difference is . . . • Charge is +2 and -2 in MgO • Higher Charge = Stronger Bond = Higher Melting Point!

  17. Metallic bond Strength • The higher the positive charge for the metal = smaller ions = stronger bond = higher melting point • Which will have a stronger bond Mg or Al ? Why? • Al will be higher. • It forms a +3 charge vs. +2 for Mg • Smaller ion = shorter bond = stronger bond!

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