Ionic bonds and main Ionic bonds and main group chemistrygroup chemistry

Towards the noble gas configurationTowards the noble gas configuration

Noble gases are unreactive – they have filled Noble gases are unreactive – they have filled shellsshells

Shells of reactive elements are unfilledShells of reactive elements are unfilled Achieve noble gas configuration by gaining or Achieve noble gas configuration by gaining or

losing electronslosing electrons Metals lose electrons – form positive ionsMetals lose electrons – form positive ions Nonmetals gain electrons – form negative ionsNonmetals gain electrons – form negative ions

Lewis dot modelLewis dot model The nucleus and all of the core electrons are represented by The nucleus and all of the core electrons are represented by

the element symbolthe element symbol The valence electrons are represented by dots – one for eachThe valence electrons are represented by dots – one for each Number of dots in Lewis model is equal to group number (in Number of dots in Lewis model is equal to group number (in

1 – 8 system)1 – 8 system)

The Octet RuleThe Octet Rule

All elements strive to All elements strive to become a noble gas, at become a noble gas, at least as far as the least as far as the electrons are concerned.electrons are concerned.

Filling the outer shell – Filling the outer shell – 8 electrons 8 electrons

Achieve this by adding Achieve this by adding electronselectrons

Or taking them awayOr taking them away

Predicting ion chargesPredicting ion charges

s and p block elements are easy: s and p block elements are easy: charge = group number for cationscharge = group number for cations charge = -(8 – group number) for anionscharge = -(8 – group number) for anions

Less predictable for transition Less predictable for transition metalsmetals

Occurrence of variable ionic chargeOccurrence of variable ionic charge CrCr2+2+, Cr, Cr3+3+, Cr, Cr4+4+, Cr, Cr6+6+ etc. etc.

4s electrons are lost first and then the 3d4s electrons are lost first and then the 3d Desirable configurations coincide with empty, Desirable configurations coincide with empty,

half-filled or filled 3d orbitalshalf-filled or filled 3d orbitals FeFe2+2+ ([Ar]3d ([Ar]3d66) is less stable than Fe) is less stable than Fe3+3+ ([Ar]3d ([Ar]3d55))

Ionic size and chargeIonic size and charge

Loss of electrons Loss of electrons increases the effective increases the effective nuclear charge – ion nuclear charge – ion shrinksshrinks

Gain of electrons Gain of electrons decreases the effective decreases the effective nuclear charge – ion nuclear charge – ion expandsexpands

Ionization energyIonization energy Energy required to remove an electron from a Energy required to remove an electron from a

neutral gaseous atomneutral gaseous atom Always positiveAlways positive Follows periodic trendFollows periodic trend

Increases across periodIncreases across period Decreases down groupDecreases down group

Removal of electrons from filled or half-filled shells is not as favourableRemoval of electrons from filled or half-filled shells is not as favourable

[He]2s2

[He]2s22p3

[He]2s22p4

[He]2s22p1

Higher ionization energiesHigher ionization energies

Depend on group numberDepend on group number Much harder to remove electrons from a filled shellMuch harder to remove electrons from a filled shell Stepwise trend below illustrates thisStepwise trend below illustrates this Partially filled –

valence electronsCompletely

filled – core electrons

Electron affinityElectron affinity Energy released on adding an electron to a neutral gaseous Energy released on adding an electron to a neutral gaseous

atomatom Values are either Values are either

negative – energy released, meaning negative ion formation is favourablenegative – energy released, meaning negative ion formation is favourable Or zero – meaning can’t be measured and negative ions are not formedOr zero – meaning can’t be measured and negative ions are not formed

Addition of electrons to filled or half-filled shells is not favoured (e.g. He, N)Addition of electrons to filled or half-filled shells is not favoured (e.g. He, N) It is easier to add an electron to Na (3sIt is easier to add an electron to Na (3s11) than to Mg (3s) than to Mg (3s22))

Ionic bondingIonic bonding

Reaction between elements that form positive and Reaction between elements that form positive and negative ionsnegative ions Metals (Metals (positivepositive ions) and nonmetals ( ions) and nonmetals (negativenegative ions) ions)

Neutral Na + Cl Neutral Na + Cl → ionic→ ionic Na Na++ClCl--

[Ne]3s[Ne]3s11 + [Ne]3s + [Ne]3s223p3p55 = [Ne] = [Ne]++ + [Ar] + [Ar]--

Stability of the ionic latticeStability of the ionic lattice

Simply forming ions does not give an energy payout:Simply forming ions does not give an energy payout: EEii(Na) = 496 kJ/mol(Na) = 496 kJ/mol

EEaa(Cl) = -349 kJ/mol(Cl) = -349 kJ/mol

Net energy investment requiredNet energy investment required Formation of a crystal lattice releases energyFormation of a crystal lattice releases energy The The lattice energylattice energy is the energy released on bringing is the energy released on bringing

ions from the gas phase into the solid latticeions from the gas phase into the solid lattice Depends on coulombic attraction between ionsDepends on coulombic attraction between ions

-U = -U = κκzz11zz22/d (/d (κκ = 8.99x10 = 8.99x1099 JmC JmC-2-2

Born-Haber cycle for calculating Born-Haber cycle for calculating energyenergy

The lattice energy can be obtained using other The lattice energy can be obtained using other experimentally determined quantities and the energy experimentally determined quantities and the energy cyclecycle

Lattice energies follow simple trendsLattice energies follow simple trends

As ionic charge As ionic charge increasesincreases, U , U increases increases ((U U z z11zz22)) As ion size As ion size decreasesdecreases, U , U increases increases (U (U 1/d) 1/d)

U(LiF) > U(LiCl) > U(LiBr)U(LiF) > U(LiCl) > U(LiBr) U(NaI) < U(MgIU(NaI) < U(MgI22) < U(AlI) < U(AlI33))

The Octet RuleThe Octet Rule

Main-group elements undergo reactions which Main-group elements undergo reactions which leave them with eight valence electronsleave them with eight valence electrons Group 1 (Group 1 (nsns11) M) M++

Group 2 (Group 2 (nsns22) M) M2+2+

Group 6 (Group 6 (nsns22npnp44) X) X2-2-

Group 7 (Group 7 (nsns22npnp55) X) X--

Works very well for second row (Li – F)Works very well for second row (Li – F) Many violations in heavier Many violations in heavier pp-block elements -block elements

(Pb(Pb2+2+, Tl, Tl++, Sb, Sb3+3+))