Formation of chemical Formation of chemical bondsbonds

The role of electronsThe role of electrons

Learning objectivesLearning objectives

Describe the octet ruleDescribe the octet rule Predict number of valence electrons on atomPredict number of valence electrons on atom Write Lewis dot structures for atoms and ionsWrite Lewis dot structures for atoms and ions Predict ionic chargesPredict ionic charges Predict composition of simple ionic compoundsPredict composition of simple ionic compounds Describe difference between ionic and covalent bondsDescribe difference between ionic and covalent bonds Describe differences between ionic and covalent Describe differences between ionic and covalent

compoundscompounds Predict when ionic or covalent compounds are formedPredict when ionic or covalent compounds are formed Describe a polar bondDescribe a polar bond

Driving force for bondsDriving force for bonds

Chemical bonds make atoms more stable Chemical bonds make atoms more stable than they are if non-bondedthan they are if non-bonded

Bond formation involves changes in the Bond formation involves changes in the electrons on two atomselectrons on two atoms

Electron transferElectron transfer Electron sharingElectron sharing

Noble gases provide cluesNoble gases provide clues

The Octet Rule: The Octet Rule: Happiness is a filled shellHappiness is a filled shell

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

Filling the outer shell – 8 electrons Filling the outer shell – 8 electrons Achieve this by adding electronsAchieve this by adding electrons Or taking them awayOr taking them away

The valence shellThe valence shell

Valence electrons:Valence electrons: Only the electrons in the outer shell matterOnly the electrons in the outer shell matter

Core electrons:Core electrons: Filled inner shells don’t participateFilled inner shells don’t participate

Filled valence shells means no bond Filled valence shells means no bond formation – noble gasesformation – noble gases

Unfilled shells mean reactivityUnfilled shells mean reactivity

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

represented by the element symbolrepresented by the element symbol The valence electrons are represented by The valence electrons are represented by

dots – one for each electrondots – one for each electron Number of dots in Lewis model is equal Number of dots in Lewis model is equal

to group number (in 1 – 8 system)to group number (in 1 – 8 system)

Remember this simple rule!!Remember this simple rule!!

Number of dots in Lewis model is equal to Number of dots in Lewis model is equal to group number (in 1 – 8 numbering system)group number (in 1 – 8 numbering system)

Filled shells by gain or loss of Filled shells by gain or loss of electronselectrons

Lose electron – inner shell full

Gain electron – outer shell full

Rules to predict ion chargeRules to predict ion charge

CationCation Charge = group numberCharge = group number

AnionAnion Charge = - (8 - group number)Charge = - (8 - group number)

Practice with a few examplesPractice with a few examples

Ionic bondingIonic bonding

TransferTransfer of electrons of electrons Create one positive ion – cationCreate one positive ion – cation One negative ion – anionOne negative ion – anion

Cation Cation losesloses electrons electrons Anion Anion gainsgains electrons electrons

Sodium chloride provides exampleSodium chloride provides exampleNa loses electron

Cl gains electron

Composition depends on ionic Composition depends on ionic charge (group number)charge (group number)

Charges must balance: compounds are Charges must balance: compounds are neutral – ions are chargedneutral – ions are charged

Total cation charges = total anion chargesTotal cation charges = total anion charges One CaOne Ca2+ 2+ two Ftwo F--

Determining formulaeDetermining formulae

Overall charge must be neutralOverall charge must be neutral Metal ion first, nonmetal ion secondMetal ion first, nonmetal ion second

Coefficient of metal = charge on nonmetalCoefficient of metal = charge on nonmetal Coefficient of nonmetal = charge on metalCoefficient of nonmetal = charge on metal x and y are shown with lowest common denominator in x and y are shown with lowest common denominator in

most cases. Calcium oxide is CaO not Camost cases. Calcium oxide is CaO not Ca22OO22

A Bx y

x-y+

Properties of ionic compoundsProperties of ionic compounds

Hard, rigid solids at room temperatureHard, rigid solids at room temperature High melting pointHigh melting point Dissolve in polar solvents (if soluble)Dissolve in polar solvents (if soluble) Solutions conduct electricitySolutions conduct electricity Melts conduct electricityMelts conduct electricity Closely packed dense structuresClosely packed dense structures

The ionic model works well for The ionic model works well for metals and non-metalsmetals and non-metals

What about compounds between non-What about compounds between non-metals CO, PClmetals CO, PCl33 and diatomic elements like and diatomic elements like

HH22, N, N22, O, O22, F, F22

Formation of negative ion is favourableFormation of negative ion is favourable Formation of a positive ion will be very Formation of a positive ion will be very

unfavourable (remember Lewis dot unfavourable (remember Lewis dot structure)structure)

Covalent bonding involves electron Covalent bonding involves electron sharingsharing

Covalent bond is net result of attractive and Covalent bond is net result of attractive and repulsive electrostatic forces. Nucleus – electron repulsive electrostatic forces. Nucleus – electron attractions (blue arrows) are greater than nucleus attractions (blue arrows) are greater than nucleus – nucleus and electron – electron repulsions (red – nucleus and electron – electron repulsions (red arrows).arrows).

Sharing two electrons effectively Sharing two electrons effectively doubles the countdoubles the count

Each atom wants 8Each atom wants 8 Alone each has sevenAlone each has seven Together each one has eight Together each one has eight 14 electrons 14 electrons appearappear to become 16 to become 16 Single covalent bondSingle covalent bond

Covalent bonds between unlike Covalent bonds between unlike elementselements

Oxygen requires eight – shares two with H Oxygen requires eight – shares two with H atomsatoms

Hydrogen requires two – each shares one Hydrogen requires two – each shares one with Owith O

Lewis dot structuresLewis dot structures

In going from G4 – G7, a H In going from G4 – G7, a H atom is replaced by a lone atom is replaced by a lone pair of electronspair of electrons

The total number of The total number of electrons is equal to the electrons is equal to the sum of all the valence sum of all the valence electronselectrons

The The totaltotal number of number of electrons remains the electrons remains the same – 8same – 8

Each atom has a complete Each atom has a complete octetoctet

FH OHH

NH

H HC

H

HH H

Multiple bonds are a featureMultiple bonds are a feature

OO22 and N and N22 do not achieve octets by sharing do not achieve octets by sharing

twotwo Must share more electronsMust share more electrons OO22 has double bond has double bond

NN22 has triple bond – one of the strongest in has triple bond – one of the strongest in

chemistrychemistry NN22 is very stable and unreactive – also the is very stable and unreactive – also the

major product from explosivesmajor product from explosives

Bond order increases as electron Bond order increases as electron total decreasestotal decreases

MoleculeMolecule Group Group numbernumber

Total number Total number of electronsof electrons

Bond Bond orderorder

F-FF-F 77 1414 11

O=OO=O 66 1212 22

NN≡N≡N 55 1010 33

Properties of covalent compoundsProperties of covalent compounds

Gases, liquids and solids at room Gases, liquids and solids at room temperaturetemperature

May be hard or soft (diamond is a covalent May be hard or soft (diamond is a covalent solid)solid)

Dissolve in Dissolve in polarpolar and and non-polarnon-polar solvents, solvents, depending on molecule’s depending on molecule’s polaritypolarity

Solutions and melts do not conduct Solutions and melts do not conduct electricityelectricity

Most covalent compounds are molecular Most covalent compounds are molecular

What is this polarity?What is this polarity?

The ionic bond and the equally shared The ionic bond and the equally shared covalent bond are the two extremes of covalent bond are the two extremes of chemical bondingchemical bonding Ionic - complete transfer of charge Ionic - complete transfer of charge Covalent - equal sharing of chargeCovalent - equal sharing of charge

Many bonds are somewhere betweenMany bonds are somewhere between Atoms of different elements have different Atoms of different elements have different

attraction for electronsattraction for electrons

Enter electronegativityEnter electronegativity

The degree to which an atom attracts The degree to which an atom attracts electrons towards itself in a bond with electrons towards itself in a bond with another atomanother atom

Highly electronegative atoms attract Highly electronegative atoms attract electrons; weakly electronegative atoms do electrons; weakly electronegative atoms do notnot

Table of electronegativityTable of electronegativityMost

electronegative

Least electronegative

Polar bonds and polar moleculesPolar bonds and polar molecules

Any bond containing different elements will Any bond containing different elements will be polar to some degreebe polar to some degree

Molecules contain several bondsMolecules contain several bonds Molecular polarity depends on how the Molecular polarity depends on how the

bonds are arrangedbonds are arranged A molecule may contain polar bonds and be A molecule may contain polar bonds and be

itself itself nonnon-polar-polar We need to understand the molecular We need to understand the molecular

structure…structure…

Diatomic molecules are easyDiatomic molecules are easy

Beyond diatomicsBeyond diatomics

More complicated molecules demand More complicated molecules demand knowledge of the molecular shapeknowledge of the molecular shape

The next frontier…The next frontier…