Bonding

Special Topics

Metallic Bonding

Model must account for metallic properties: Malleability Ductility Conduction of heat and electricity in all directions High melting points

Metallic bonds are strong and nondirectional.

Metallic Bonding

Metals have 1, 2, or 3 valence electrons. “electron sea” model – a sea of delocalized

electrons surrounding a positively charged metal center

Valence electrons delocalized – free to move around – shared by all atoms

Positive ions arranged in regular, repeating pattern, stationary - crystal

A - Outermost electrons wander freely through metal. Metal consists of cations held together by negatively-charged electron "glue."

B - Free electrons can move rapidly in response to electric fields, that's why metals are a good conductor of electricity.

C - Free electrons can transmit kinetic energy rapidly, hence metals are good conductors of heat.

D - The layers of atoms in metal are hard to pull apart because of the electrons holding them together, that's why metals are tough. But individual atoms are not held to any other specific atoms, it's why atoms slip easily past one another. Thus metals are ductile.

Metallic luster

Metal atoms contain many orbitals separated by extremely small energy differences

Absorb and emit wide range of light frequencies

Emission responsible for shiny appearance

Metallic Bonding

Metallic bond strength and melting point correlate with number of valence electrons

More valence electrons = more “glue”

Alkali metals softest, lowest melting point

Heats of Vaporization of Some Metals (kJ/mol)

period element

second Li

147

Be

297

third Na

97

Mg

128

Al

294

fourth K

77

Ca

155

Sc

333

fifth Rb

76

Sr

137

Y

365

sixth Cs

64

Ba

140

La

402

Alloys

Metallic bonding can occur with like atoms or with different kinds of metal atoms

Alloy – a substance that contains a mixture of elements and has metallic properties

Types of Alloys

Substitutional alloy – some of host metal atoms replaced by other metal atoms of similar size

Interstitial alloy – some of interstices (holes) in metal structure occupied by smaller atoms

Examples of alloys

Bronze (copper and tin) Brass (copper and zinc) Steel (iron and carbon) Sterling silver (silver and copper) Pewter (tin, copper, bismuth, antimony) Solder (tin and antimony)

Covalent Network Solids

Contain strong covalent bonds Can be viewed as “giant molecule” – size

limited by number of atoms Two allotropes of carbon are examples of

covalent network solids.

Allotropes of Carbon: Diamond Each carbon bonded to 4 other

carbons Each atom is sp3 hybridized and

has tetrahedral geometry mp 4500°C – C-C bonds very

strong solubility – insoluble in all solvents

– solvent molecules can’t penetrate lattice of strong C-C bonds

hardness – very hard – rigid tetrahedral arrangement of covalent bonds

conductivity – no mobile electrons so no conductivity

Allotropes of Carbon: Graphite

Each C bonded to 3 others

Each C is sp2 hybridized – trigonal planar

Flat sheets of carbon stack up – vdw forces hold sheets together

Allotropes of Carbon: Graphite

mp - 3730°C – high – strong C-C bonds

solubility – insoluble in all solvents

hardness – soft and has lubricative properties – weak vdw forces between layers

conductivity – conducts electricity – has delocalized electrons in network of π bonds

Covalent Network Solids: SiO2

Silica – empirical formula SiO2

Quartz (some types of sand) based on network of SiO4 tetrahedra

Quartz and Glass

When silica heated above mp (~1600°C) and cooled rapidly, amorphous solid called glass results

Glass Additives

Properties of glass can be varied greatly by adding different substances to the melt before cooling.

Adding B2O3 produces borosilicate glass – expands/contracts very little with changes in temp – cooking and lab glassware – brand name Pyrex

Ionic Solids and Lattice Energy

Modified Coulomb’s law gives lattice energy

lattice energy = k(Q1Q2)/r Lattice energy – change in energy when

separated gaseous ions are packed together to form an ionic solid

Born-Haber Cycle A method for calculating lattice energy from thermodynamic data Consider all energy changes that must occur when elements in standard

state form one mole of ionic solid

Lattice Energy Problem

Calculate the lattice energy for LiF(s) given the following:

sublimation energy for Li(s) 166 kJ/molbond energy for F2(g) 154 kJ/molfirst IE for Li(g) 520. kJ/molEA for F(g) -328 kJ/molenthalpy of formation of LiF(s) -617 kJ/mol

a. 182 kJ/molb. -1129 kJ/molc. -1052 kJ/mold. -105 kJ/mole. 724 kJ/mol