Adv chem chapt 8

Bonding: General Bonding: General ConceptsConcepts

Advanced Chemistry Chapter 8Advanced Chemistry Chapter 8

Types of Chemical Types of Chemical BondsBonds

Ionic BondsIonic Bonds

Ionic Bonds are formed when Ionic Bonds are formed when an atom that loses electrons an atom that loses electrons relatively easily reacts with relatively easily reacts with an atom that has a high an atom that has a high attraction for electrons. attraction for electrons.

Ionic Compounds results when a Ionic Compounds results when a metal bonds with a nonmetal.metal bonds with a nonmetal.

Bond EnergyBond EnergyBond energy is the energy required to Bond energy is the energy required to break a bond.break a bond.

The energy of interaction between a The energy of interaction between a pair of ions can be calculated using pair of ions can be calculated using Coulomb’s lawCoulomb’s law

r = the distance between the ions in nm.r = the distance between the ions in nm.

QQ11 and Q and Q2 2 are the numerical ion charges.are the numerical ion charges.

E is in joulesE is in joules

Bond EnergyBond Energy

When the calculated energy When the calculated energy between ions is negative, that between ions is negative, that indicates an attractive force. indicates an attractive force.

A positive energy is a repulsive A positive energy is a repulsive energy.energy.

The distance where the energy is The distance where the energy is minimal is called the bond minimal is called the bond length.length.

Covalent BondsCovalent Bonds

Covalent bonds form between Covalent bonds form between molecules in which electrons molecules in which electrons are shared by nuclei.are shared by nuclei.

The bonding electrons are The bonding electrons are typically positioned between the typically positioned between the two positively charged nuclei. two positively charged nuclei.

Polar Covalent Polar Covalent BondsBonds

Polar covalent bonds are an Polar covalent bonds are an intermediate case in which the intermediate case in which the electrons are not completely electrons are not completely transferred but form unequal transferred but form unequal sharing.sharing.

A δA δ-- or δ or δ+ + is used to show a is used to show a fractional or partial charge on a fractional or partial charge on a molecule with unequal sharing. molecule with unequal sharing. This is called a dipole. This is called a dipole.

ElectronegativityElectronegativity

ElectronegativityElectronegativityElectronegativity is the ability of an atom in Electronegativity is the ability of an atom in

a molecule to attract shared electrons to a molecule to attract shared electrons to itself. (electron love)itself. (electron love)

Relative electronegativities are Relative electronegativities are determined by comparing the measured determined by comparing the measured bond energy with the “expected” bond bond energy with the “expected” bond energy.energy.

Measured in Paulings. After Linus Pauling Measured in Paulings. After Linus Pauling the American scientist who won the Nobel the American scientist who won the Nobel Prizes for both chemistry and peace. Prizes for both chemistry and peace.


Expected H-X bond energy=Expected H-X bond energy=

H −H bond energy+ X −X bond energy2


Electronegativity values generally Electronegativity values generally increase going left to right across increase going left to right across the periodic table and decrease the periodic table and decrease going top to bottom.going top to bottom.

Electronegativity Electronegativity and Bond typeand Bond type

Bond Polarity and Bond Polarity and DipoleDipole

Dipoles and Dipoles and Dipole MomentsDipole Moments

A molecule that has a center of A molecule that has a center of positive charge and a center of positive charge and a center of negative charge is said to be negative charge is said to be dipolardipolar or to have a or to have a dipole dipole moment.moment.

An arrow is used to show this An arrow is used to show this dipole moment by pointing to dipole moment by pointing to the negative charge and the tail the negative charge and the tail at the positive charge.at the positive charge.


Electrostatic Electrostatic potential diagram potential diagram shows variation shows variation in charge. Red is in charge. Red is the most electron the most electron rich region and rich region and blue is the most blue is the most electron poor electron poor region.region.




Dipoles and Dipoles and Dipole MomentsDipole MomentsDipole moments are when Dipole moments are when opposing bond polarities opposing bond polarities don’t cancel out.don’t cancel out.


Example Example ProblemsProblems

For each of the following For each of the following molecules, show the direction molecules, show the direction of the bond polarities and of the bond polarities and indicate which ones have a indicate which ones have a dipole moment: HCl, Cldipole moment: HCl, Cl22, SO, SO33, , CHCH44, H, H22SS

HClHCl

ClCl22

SOSO33

CHCH44

HH22SS

Ions: Electron Ions: Electron Configurations Configurations

and Sizesand Sizes

Electron Electron Configurations of Configurations of

CompoundsCompoundsWhen two nonmetals react to form When two nonmetals react to form a covalent bond, they share a covalent bond, they share electrons in a way that completes electrons in a way that completes the valence electron the valence electron configurations of both atoms. configurations of both atoms. That is, both nonmetals attain That is, both nonmetals attain noble gas electron configurations.noble gas electron configurations.

Electron Electron Configurations of Configurations of

CompoundsCompoundsWhen a nonmetal and a When a nonmetal and a representative-group metal react to representative-group metal react to form a binary ionic compounds, the form a binary ionic compounds, the ions form so that the valence electron ions form so that the valence electron configuration of the nonmetal achieves configuration of the nonmetal achieves the electron configuration of the next the electron configuration of the next noble gas atom and the valence noble gas atom and the valence orbitals of the metal are emptied. In orbitals of the metal are emptied. In this way both ions achieve noble gas this way both ions achieve noble gas electron configurations. electron configurations.

Predicting Ionic Predicting Ionic FormulasFormulas

To predict the formula of the ionic To predict the formula of the ionic compound, we simply recognize compound, we simply recognize that the chemical compounds are that the chemical compounds are always electrically neutral. They always electrically neutral. They have the same quantities of have the same quantities of positive and negative charges.positive and negative charges.

Sizes of IonsSizes of Ions

Size of an ion generally follows the Size of an ion generally follows the same trend as atomic radius. The same trend as atomic radius. The big exception to this trend is big exception to this trend is where the metals become where the metals become nonmetals and the ions switch nonmetals and the ions switch charge.charge.


A positive ion is formed by removing A positive ion is formed by removing one or more electrons from a one or more electrons from a neutral atom, the resulting cation neutral atom, the resulting cation is smaller than the neutral atom.is smaller than the neutral atom.

Less electrons allow for less Less electrons allow for less repulsions and the ion gets repulsions and the ion gets smaller.smaller.


An addition of electrons to a neutral An addition of electrons to a neutral atom produces an anion that is atom produces an anion that is significantly larger than the significantly larger than the neutral atom.neutral atom.

An addition of an electron causes An addition of an electron causes additional repulsions around the additional repulsions around the atom and therefore its size atom and therefore its size increases.increases.

Energy Effects in Energy Effects in Binary Ionic Binary Ionic CompoundsCompounds

Lattice EnergyLattice Energy

Lattice energy is the change in energy Lattice energy is the change in energy that takes place when separated that takes place when separated gaseous ions are packed together to gaseous ions are packed together to form an ionic solid.form an ionic solid.

The lattice energy is often defined The lattice energy is often defined as the energy released when an as the energy released when an ionic solid forms from its ions.ionic solid forms from its ions.

Lattice energy has a negative sign Lattice energy has a negative sign to show that the energy is released. to show that the energy is released.

Lattice Energy Lattice Energy ExampleExample

Estimate the enthalpy of lithium fluoride Estimate the enthalpy of lithium fluoride and the changes of energy and lattice and the changes of energy and lattice energy during formation:energy during formation:

1.1.Break down LiF into its standard state Break down LiF into its standard state elements (use formation reaction): elements (use formation reaction):

LiLi(s) (s) + ½F + ½F2(g) 2(g) LiF LiF(s)(s)

Li+(g) + F-(g) LiF(s)


LiLi(s) (s) + ½F + ½F2(g) 2(g) LiF LiF(s)(s) LiLi++(g) (g) + F + F--

(g) (g) LiF LiF(s)(s)

Use sublimation and evaporation reactions to get Use sublimation and evaporation reactions to get reactants into gas form (since lattice energy reactants into gas form (since lattice energy depends on gaseous state). Find the enthalpies to depends on gaseous state). Find the enthalpies to these reactions:these reactions:

LiLi(s)(s) Li Li(g)(g) 161 kJ/mol161 kJ/mol

LiLi(g) (g) + ½F + ½F2(g) 2(g) LiF LiF(s)(s)


LiLi(g) (g) + ½F + ½F2(g) 2(g) LiF LiF(s)(s) LiLi++(g) (g) + F + F--


Ionize cation to form ions for bonding. Use Ionize cation to form ions for bonding. Use Ionization energy for the enthalpy of the Ionization energy for the enthalpy of the reaction.reaction.

LiLi(g)(g) Li Li++(g)(g) + e + e-- Ionization energy: 520 Ionization energy: 520

kJ/molkJ/mol

LiLi++(g) (g) + ½F + ½F2(g) 2(g) LiF LiF(s)(s)


LiLi++(g) (g) + ½F + ½F2(g) 2(g) LiF LiF(s)(s) LiLi++

(g) (g) + F + F--(g) (g) LiF LiF(s)(s)

Dissociate diatomic gas to individual atoms:Dissociate diatomic gas to individual atoms:

½F½F2(g)2(g) F F(g) (g) ½ Bond dissociation energy of ½ Bond dissociation energy of F-FF-F

= 154 kJ/ 2 = 77 kJ/mol= 154 kJ/ 2 = 77 kJ/mol

LiLi++(g) (g) + F + F(g) (g) LiF LiF(s)(s)


LiLi++(g) (g) + F + F(g) (g) LiF LiF(s)(s) LiLi++

(g) (g) + F + F--(g) (g) LiF LiF(s)(s)

Electron addition to fluorine is the electron Electron addition to fluorine is the electron affinity of fluorine:affinity of fluorine:

FF(g)(g) + e + e-- F F--(g)(g) -328 kJ/mol-328 kJ/mol

LiLi++(g) (g) + F + F--



LiLi++(g) (g) + F + F--

(g) (g) LiF LiF(s)(s) LiLi++(g) (g) + F + F--


Formation of solid lithium fluoride from the Formation of solid lithium fluoride from the gaseous ions corresponds to its lattice gaseous ions corresponds to its lattice energy:energy:

LiLi++(g) (g) + F + F--

(g) (g) LiF LiF(s)(s) -1047 kJ/mol-1047 kJ/mol

Lattice Energy Lattice Energy ExampleExampleThe sum of these five processes yields the The sum of these five processes yields the

overall reaction and the sum of the overall reaction and the sum of the individual energy changes gives the individual energy changes gives the

overall energy change or lattice energy:overall energy change or lattice energy:

LiLi(s)(s) Li Li(g)(g)

LiLi(g)(g) Li Li++(g)(g) + e + e--

½F½F2(g)2(g) F F(g)(g)

FF(g)(g) + e + e-- F F--(g)(g)

LiLi++(g) (g) + F + F--

(g) (g) LiFLiF(s)(s)

161 kJ161 kJ

520 kJ520 kJ

77 kJ77 kJ

-328 kJ-328 kJ

-1047 kJ-1047 kJ

Total = -617 Total = -617 kJ/molkJ/mol

Lattice EnergyLattice Energy

Lattice EnergyLattice EnergyLattice energy can be calculated with Lattice energy can be calculated with

at form of Coulomb’s law:at form of Coulomb’s law:

Q is the charges on the ions and r is Q is the charges on the ions and r is the shortest distance between the the shortest distance between the centers of the cations and anions. k centers of the cations and anions. k is a constant that depends on the is a constant that depends on the structure of the solid and the structure of the solid and the electron configurations of the ions. electron configurations of the ions.

LatticeEnergy =kQ1Q2

r⎛⎝⎜

⎞⎠⎟

Partial Ionic Partial Ionic Character of Character of

Covalent BondsCovalent Bonds

Bond CharacterBond CharacterCalculations of ionic character:Calculations of ionic character:

Even compounds with the maximum Even compounds with the maximum possible electronegativity differences are possible electronegativity differences are not 100% ionic in the gas phase. not 100% ionic in the gas phase. Therefore the operational definition of Therefore the operational definition of ionic is any compound that conducts an ionic is any compound that conducts an electric current when melted will be electric current when melted will be classified as ionic.classified as ionic.

Percent ionic character of a bond =dipolemoment of x−ydipolemoment of x+yy

⎛

⎝⎜⎞

⎠⎟x100%

Bond CharacterBond Character

The Covalent The Covalent Chemical BondChemical Bond

Chemical Bond Chemical Bond ModelModel

A chemical bond can be viewed as A chemical bond can be viewed as forces that cause a group of atoms forces that cause a group of atoms to behave as a unit. to behave as a unit.

Bonds result from the tendency of Bonds result from the tendency of a system to seek its lowest a system to seek its lowest possible energy.possible energy.

Individual bonds act relatively Individual bonds act relatively independent.independent.

ExampleExampleIt takes 1652 kJ of energy required to It takes 1652 kJ of energy required to break the bonds in 1 mole of methane. break the bonds in 1 mole of methane. 1652 kJ of energy is released when 1 1652 kJ of energy is released when 1 mole of methane is formed from mole of methane is formed from gaseous atoms.gaseous atoms.Therefore, 1 mole of methane in gas Therefore, 1 mole of methane in gas phase has 1652 kJ lower energy than phase has 1652 kJ lower energy than the total of the individual atoms.the total of the individual atoms.One mole of methane is held together One mole of methane is held together with 1652 kJ of energy.with 1652 kJ of energy.Each of the four C-H bonds contains Each of the four C-H bonds contains 413 kJ of energy. 413 kJ of energy.

ExampleExampleEach of the four C-H bonds Each of the four C-H bonds contains 413 kJ of energy.contains 413 kJ of energy.CHCH33Cl contains 1578 kJ of Cl contains 1578 kJ of energy:energy:

1 mol of C-Cl bonds + 3 mol (C-H 1 mol of C-Cl bonds + 3 mol (C-H bonds)=1578 kJbonds)=1578 kJ

C-Cl bond energy + 3 (413 kJ/mol) C-Cl bond energy + 3 (413 kJ/mol) = 1578 kJ= 1578 kJ

C-Cl bond energy = 339 kJ/mol C-Cl bond energy = 339 kJ/mol

Properties of Properties of ModelsModels

A model doesn’t equal reality; A model doesn’t equal reality; they are used to explain they are used to explain incomplete understanding of how incomplete understanding of how nature works.nature works.

Models are often oversimplified Models are often oversimplified and are sometimes wrong.and are sometimes wrong.

Models over time tend to get over Models over time tend to get over complicated due to “repairs”.complicated due to “repairs”.

Properties of Properties of ModelsModels

Remember that simple models Remember that simple models often require restrictive often require restrictive assumptions. Best way to use assumptions. Best way to use models is to understand their models is to understand their strengths and weaknesses. strengths and weaknesses.

We often learn more when models We often learn more when models are incorrect than when they are are incorrect than when they are right.right.

Cu and Cr.Cu and Cr.

Covalent Bond Covalent Bond Energies and Energies and

Chemical Chemical ReactionsReactions

Bond EnergiesBond Energies

Bond energy averages are used for Bond energy averages are used for individual bond dissociation energies to individual bond dissociation energies to give approximate energies in a give approximate energies in a particular bond.particular bond.

Bond energies vary due to several Bond energies vary due to several reasons:reasons:

multiple bonds, 4 C-H bonds in methane multiple bonds, 4 C-H bonds in methane

different elements in the molecule, C-H different elements in the molecule, C-H bond in Cbond in C22HH66 or C-H bond in HCCl or C-H bond in HCCl33

Bond Energy Bond Energy ExampleExample

CHCH4(g)4(g)CHCH3(g) 3(g) + + HH(g)(g)

CHCH3(g)3(g)CHCH2(g) 2(g) + + HH(g)(g)

CHCH2(g)2(g)CHCH(g) (g) + + HH(g)(g)

CHCH(g)(g)CC(g) (g) + H+ H(g)(g)

435 kJ435 kJ

453 kJ453 kJ

425 kJ425 kJ

339 kJ339 kJ

Total 1652 kJTotal 1652 kJ

Average 413 kJAverage 413 kJ


HCBrHCBr33

HCClHCCl33

HCFHCF33

CC22HH66

380 kJ380 kJ

380 kJ380 kJ

430 kJ430 kJ

410 kJ410 kJ

Average Bond Average Bond EnergiesEnergies


A relationship also exists A relationship also exists between the number of between the number of shared electron pairs.shared electron pairs.

single bond – 2 electronssingle bond – 2 electrons

double bond – 4 electronsdouble bond – 4 electrons

triple bond – 6 electronstriple bond – 6 electrons

Bond EnergyBond EnergyBond energy values can be used to Bond energy values can be used to

calculate approximate energies for calculate approximate energies for reactions.reactions.

Energy associated with bond breaking Energy associated with bond breaking have positive signshave positive signs

Endothermic processEndothermic process

Energy associated with forming bonds Energy associated with forming bonds releases energy and is negative.releases energy and is negative.

Exothermic processExothermic process


A relationship exists between the A relationship exists between the number of shared electron pairs number of shared electron pairs and the bond length. and the bond length.

• As the number of electrons shared As the number of electrons shared goes up the bond length shortens.goes up the bond length shortens.


Bond EnergyBond EnergyΔH = sum of the energies required to ΔH = sum of the energies required to

break old bonds (positive signs) plus break old bonds (positive signs) plus the sum of the energies released in the sum of the energies released in the formation of new bonds the formation of new bonds (negative signs).(negative signs).

• D represents bond energies per mole D represents bond energies per mole and always has positive signand always has positive sign

• n is number of molesn is number of moles


HH2(g)2(g) + F + F2(g) 2(g) 2HF2HF(g)(g)

1 H-H bond, F-F bond and 2 H-F 1 H-H bond, F-F bond and 2 H-F bondsbonds

ΔH = DΔH = DH-HH-H + D + DF-F F-F – 2D– 2DH-FH-F

• ΔH= (1mol x 432 kJ/mol) + (1mol x ΔH= (1mol x 432 kJ/mol) + (1mol x 154 kJ/mol) – (2mol x 565 kJ/mol)154 kJ/mol) – (2mol x 565 kJ/mol)

ΔH = -544 kJΔH = -544 kJ

The Localized The Localized Electron Bonding Electron Bonding

ModelModel

Localized Electron Localized Electron ModelModel

• The localized electron model The localized electron model assumes that a molecule is composed assumes that a molecule is composed of atoms that are bound together by of atoms that are bound together by sharing pairs of electrons using the sharing pairs of electrons using the atomic orbitals of the bound atoms.atomic orbitals of the bound atoms.

• Electrons are assumed to be localized Electrons are assumed to be localized on a particular atom individually or in on a particular atom individually or in the space between atoms.the space between atoms.


• Pairs of electrons that are Pairs of electrons that are localized on an atom are called localized on an atom are called lone pairs.lone pairs.

• Pairs of electrons that are found Pairs of electrons that are found in the space between the atoms in the space between the atoms are called bonding pairs are called bonding pairs


Three parts of the LE Model:Three parts of the LE Model:

1.1.Description of the valence electron Description of the valence electron arrangement in the molecule using Lewis arrangement in the molecule using Lewis structures.structures.

2.2.Prediction of the geometry of the molecule Prediction of the geometry of the molecule using VSEPR modelusing VSEPR model

3.3.Description of the type of atomic orbitals Description of the type of atomic orbitals used by the atoms to share electrons or hold used by the atoms to share electrons or hold lone pairs. lone pairs.

Lewis StructuresLewis Structures

Lewis StructuresLewis StructuresThe Lewis structure of a molecule show how the The Lewis structure of a molecule show how the

valence electrons are arranged among the valence electrons are arranged among the atoms in the molecule.atoms in the molecule.

• Named after G. N. LewisNamed after G. N. Lewis

• Rules are based on observations of thousands Rules are based on observations of thousands of molecules.of molecules.

• Most important requirement for the formation Most important requirement for the formation of a stable compound is that the atoms of a stable compound is that the atoms achieve noble gas electron configurations. achieve noble gas electron configurations.

Lewis StructuresLewis Structures• Only the valence electrons are Only the valence electrons are

included. included.

• The duet rule: diatomic molecules can The duet rule: diatomic molecules can find stability in the sharing of two find stability in the sharing of two electrons.electrons.

• The octet rule: since eight electrons The octet rule: since eight electrons are required to fill these orbitals, are required to fill these orbitals, these elements typically are these elements typically are surrounded by eight electrons.surrounded by eight electrons.

Lewis Structure Lewis Structure StepsSteps

1.1. Sum the valence electrons from all Sum the valence electrons from all the atoms. Total valence electrons.the atoms. Total valence electrons.

2.2. Use a pair of electrons to form a Use a pair of electrons to form a bond between each pair of bound bond between each pair of bound atoms.atoms.

3.3. Arrange the remaining electrons to Arrange the remaining electrons to satisfy the duet rule for hydrogen satisfy the duet rule for hydrogen and the octet rule for the others.and the octet rule for the others.a)a) Terminal atoms first. Terminal atoms first.

b)b) Check for happinessCheck for happiness

Examples Examples • HFHF

• NN22

• NHNH33

• CHCH44

• CFCF44

• NONO++

Exceptions to the Exceptions to the Octet RuleOctet Rule


• Incomplete: An odd number of Incomplete: An odd number of electrons are available for bonding. electrons are available for bonding. One lone electron is left unpaired.One lone electron is left unpaired.

• Suboctet: Less than 4 pairs of electrons Suboctet: Less than 4 pairs of electrons are assigned to the central atomare assigned to the central atom– Suboctets tend to form coordinate Suboctets tend to form coordinate

covalentbondscovalentbonds– BHBH33 + NH + NH33


• Extended: The central atom has more Extended: The central atom has more than 4 pairs of electrons. than 4 pairs of electrons. – At the third energy level and At the third energy level and

higher, atoms may have empty d higher, atoms may have empty d orbitals that can be used for orbitals that can be used for bonding.bonding.

General RulesGeneral Rules• The second row elements C, N, O, and F The second row elements C, N, O, and F

always obey the octet rulealways obey the octet rule

• The second row elements B and Be often The second row elements B and Be often have fewer than eight electrons around have fewer than eight electrons around them in their compounds. They are them in their compounds. They are electron deficient and very reactive.electron deficient and very reactive.

• The second row elements never exceed The second row elements never exceed the octet rule, since their valence orbitals the octet rule, since their valence orbitals can only hold 8.can only hold 8.

General RulesGeneral Rules• Third-row and heavier elements often Third-row and heavier elements often

satisfy the octet rule but can exceed the satisfy the octet rule but can exceed the octet rule by using their empty valence d octet rule by using their empty valence d orbitals.orbitals.

• When writing the Lewis structure for a When writing the Lewis structure for a molecule, satisfy the octet rule for the molecule, satisfy the octet rule for the atoms first. If electrons remain after the atoms first. If electrons remain after the octet rule has been satisfied, then place octet rule has been satisfied, then place them on the elements having available d them on the elements having available d orbitals orbitals

ResonanceResonance

ResonanceResonance

• Resonance is when more than on Resonance is when more than on valid Lewis structure can be valid Lewis structure can be written for a particular molecule. written for a particular molecule. The resulting electron structure of The resulting electron structure of the molecule is given by the the molecule is given by the average of these resonance average of these resonance structures.structures.

ResonanceResonance• The concept of resonance is necessary The concept of resonance is necessary

because the localized electron model because the localized electron model postulates that electrons are localized postulates that electrons are localized between a given pair of atoms. However, between a given pair of atoms. However, nature does not really operate this way. nature does not really operate this way. Electrons are really delocalized- they Electrons are really delocalized- they move around the entire molecule. The move around the entire molecule. The valence electrons in a resonance valence electrons in a resonance structure distribute themselves equally structure distribute themselves equally and produce equal bonds. and produce equal bonds.

Formal ChargeFormal Charge

Some molecules or polyatomic ions Some molecules or polyatomic ions can have several non-equivalent can have several non-equivalent Lewis structures.Lewis structures.

• Example: SOExample: SO442-2-

Because of this we assign atomic Because of this we assign atomic charges to the molecules in order charges to the molecules in order to find the right structure. to find the right structure.


The formal charge of an atom in a The formal charge of an atom in a molecule is the difference between the molecule is the difference between the number of valence electrons on the number of valence electrons on the free atom and the number of valence free atom and the number of valence electrons assigned to the atom in the electrons assigned to the atom in the moleculemolecule

Formal charge = (# of valence electrons Formal charge = (# of valence electrons on neutral ‘free atom’) – (# of valence on neutral ‘free atom’) – (# of valence electrons assigned to the atom in the electrons assigned to the atom in the molecule)molecule)


Assumptions on electron Assumptions on electron assignment:assignment:

• Lone pair electrons belong Lone pair electrons belong entirely to the atom in question.entirely to the atom in question.

• Shared electrons are divided Shared electrons are divided equally between the two sharing equally between the two sharing atoms.atoms.

Formal Charge Formal Charge ExampleExample

• SOSO442-2-: All single bonds: All single bonds

• Formal charge on each O is -1Formal charge on each O is -1

• Formal charge on S is 2 Formal charge on S is 2

Formal Charge Formal Charge ExampleExample

• SOSO442-2-: two double bonds, two single: two double bonds, two single

• Formal charge on single bonded O is -1Formal charge on single bonded O is -1

• Formal charge on double bonded O is 0Formal charge on double bonded O is 0

• Formal charge on S is 0 Formal charge on S is 0

Formal ChargesFormal Charges1.1. Atoms in molecules try to achieve formal Atoms in molecules try to achieve formal

charges as close to zero as possible.charges as close to zero as possible.

2.2. Any negative formal charges are expected Any negative formal charges are expected to reside on the most electronegative to reside on the most electronegative atoms.atoms.

If nonequivalent Lewis structures exist for a If nonequivalent Lewis structures exist for a species, those with formal charges closest species, those with formal charges closest to zero and with any negative formal to zero and with any negative formal charges on the most electronegative atoms charges on the most electronegative atoms are considered to best describe the are considered to best describe the bonding in the molecule or ion.bonding in the molecule or ion.

Molecular Molecular Structure: The Structure: The VSEPR ModelVSEPR Model

VSEPRVSEPR

Valence shell electron repulsion Valence shell electron repulsion model is useful in predicting the model is useful in predicting the geometries of molecules formed geometries of molecules formed from nonmetals.from nonmetals.

• The structure around a given atom The structure around a given atom is determined principally by is determined principally by minimizing electron – pair minimizing electron – pair repulsion.repulsion.

VSEPRVSEPR• From the Lewis structure, count the From the Lewis structure, count the

electron pairs around the central atom. electron pairs around the central atom.

• Lone pairs require more room than Lone pairs require more room than bonding pairs and tend to compress the bonding pairs and tend to compress the angles between the bonding pairs.angles between the bonding pairs.

• Multiple bonds should be counted as one Multiple bonds should be counted as one effective pair.effective pair.

• With a molecule with resonance, all With a molecule with resonance, all structures should yield the same shape.structures should yield the same shape.

LinearLinear

• 180°180°

Trigonal PlanerTrigonal Planer

• 120°120°

TetrahedralTetrahedral

• 109.5°109.5°

Trigonal Trigonal PyramidalPyramidal

• 107°107°

Bent/VBent/V

• 104.5°104.5°

Tetrahedral Tetrahedral ArrangementsArrangements

BipyramidaBipyramidal l

ArrangemeArrangementsnts

Octahedral Octahedral ArrangementsArrangements

Molecules without Molecules without a central atoma central atom

• The molecular structure of more The molecular structure of more complicated atoms can be complicated atoms can be predicted from the arrangement predicted from the arrangement of pairs around the center atoms. of pairs around the center atoms. A combination of shapes will A combination of shapes will result that allows for minimum result that allows for minimum repulsion throughout.repulsion throughout.

Molecules without Molecules without a central atoma central atom

The EndThe End

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Adv chem chapt 8