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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…