Unit 4: Metallic Unit 4: Metallic BondingBonding

Metallic Bonds are…Metallic Bonds are…

How metal atoms are held How metal atoms are held together in the solid.together in the solid.

Metals hold on to their valence Metals hold on to their valence electrons electrons very weaklyvery weakly..

Think of them as positive ions Think of them as positive ions (cations) floating in a (cations) floating in a sea of sea of electronselectrons

Examples of Metallic BondingExamples of Metallic Bonding

+ + + ++ + + +

+ + + +

Sea of ElectronsSea of Electrons

+ + + ++ + + +

+ + + +

Electrons are free to move through Electrons are free to move through the solid, so .. . . .the solid, so .. . . .Metals conduct electricity.Metals conduct electricity.

Vacant “d” orbitals overlap and allow Vacant “d” orbitals overlap and allow electrons to move.electrons to move.

Properties of MetalsProperties of Metals

Excellent Excellent conductors conductors of heat and electricityof heat and electricity

Solids at room Solids at room temperature (except temperature (except Hg)Hg)

Extremely high Extremely high melting pointsmelting points (200(200˚C and above)˚C and above)

Malleable Malleable

DuctileDuctile

Metals are Metals are MalleableMalleable

Hammered into shape (bend).Hammered into shape (bend).

Also Also ductileductile - drawn into wires. - drawn into wires.

Both malleability and ductility Both malleability and ductility explained in terms of the explained in terms of the mobilitymobility of the valence of the valence electronselectrons

MalleableMalleable

+ + + ++ + + +

+ + + +

Force

MalleableMalleable

+ + + +

+ + + ++ + + +

Mobile electrons allow atoms to slide by, Mobile electrons allow atoms to slide by, sort of like ball bearings in oil.sort of like ball bearings in oil.

Force

Properties of ionic compoundsProperties of ionic compounds

Crystalline structures Crystalline structures (repeated patterns of + and – (repeated patterns of + and – ions)ions)

Are crystalline solids at room Are crystalline solids at room temperaturetemperatureHave very high melting Have very high melting pointspointsAre hard and brittle – they Are hard and brittle – they break under pressure.break under pressure.

- + - +-+ -+

- + - +-+ -+

Do they Conduct?Do they Conduct?Conducting electricity means allowing Conducting electricity means allowing charges to move.charges to move.In a solid, the ions are locked in place.In a solid, the ions are locked in place.Ionic solids are insulators.Ionic solids are insulators.When When meltedmelted,, the ions can move around. the ions can move around.

Melted ionic compounds conduct.Melted ionic compounds conduct.– NaCl: must get to about 800 ºC.NaCl: must get to about 800 ºC.

Dissolved in waterDissolved in water,, they also conduct they also conduct (ions separate and are free to move in (ions separate and are free to move in solutions)solutions)

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

Ionic solids are brittleIonic solids are brittle

+ - + -+- +-

+ - + -+- +-

Force

Ionic solids are brittleIonic solids are brittle

+ - + -

+- +-+ - + -

+- +-

Strong RepulsionStrong Repulsion breaks a crystal apart, breaks a crystal apart, due to similar ions being next to each due to similar ions being next to each other.other.

Force

Structure of Ionic CompoundsStructure of Ionic Compounds

Bond StrengthBond Strength

Determined by Determined by Lattice EnergyLattice Energy – the – the energy released when one mole of an energy released when one mole of an ionic crystalline compound is formed ionic crystalline compound is formed from gaseous ions.from gaseous ions.

Lattice Energy = Lattice Energy = k (Qk (Q11)(Q)(Q22))

rr r = bond lengthr = bond length Q = ion chargeQ = ion charge

Comparing Bond StrengthComparing Bond Strength

Higher charged ions = stronger L.E. = Higher charged ions = stronger L.E. = stronger bond = higher melting pointstronger bond = higher melting point

L.E for NaF = 923 kJL.E for NaF = 923 kJ

L.E. for MgO = 3916 kJL.E. for MgO = 3916 kJ

The reason for this big difference is . . . The reason for this big difference is . . .

Charge is +2 and -2 in MgOCharge is +2 and -2 in MgO

Higher Charge = Stronger BondHigher Charge = Stronger Bond

= Higher Melting Point!= Higher Melting Point!

Metallic bond StrengthMetallic bond StrengthThe The higherhigher the positive charge for the the positive charge for the metal = metal = smallersmaller ions = ions = strongerstronger bond = bond = higher higher melting pointmelting pointWhich will have a stronger bond Mg or Which will have a stronger bond Mg or Al ? Why?Al ? Why?Al will be higher.Al will be higher.It forms a +3 charge vs. +2 for MgIt forms a +3 charge vs. +2 for MgSmaller ion = shorter bond = stronger Smaller ion = shorter bond = stronger bond!bond!