CHEMICAL BONDING & MOLECULAR STRUCTURE. What is a Chemical Bond? It will take us the next two...

CHEMICAL BONDINGCHEMICAL BONDING &&

MOLECULAR STRUCTUREMOLECULAR STRUCTURE

What is a Chemical Bond?What is a Chemical Bond?

It will take us the next two It will take us the next two chapters chapters

to answer this question! to answer this question!

Bonds are forces that hold groups Bonds are forces that hold groups

of atoms together and make of atoms together and make them them

function as a unit. function as a unit.

[25 words or less, but leaves out [25 words or less, but leaves out

LOTS of details!]LOTS of details!]

Bonding relates to physical Bonding relates to physical properties such as melting point, properties such as melting point, hardness and electrical and thermal hardness and electrical and thermal conductivity as well as solubility conductivity as well as solubility characteristics. characteristics. The system is The system is achieving the lowest possible energy achieving the lowest possible energy by bonding.by bonding...

If you think about it, most of the If you think about it, most of the

chemical substances you can chemical substances you can name name

or identify are NOT elements. or identify are NOT elements.

They are compounds.They are compounds.

That means being bound That means being bound requires requires

less energy than existing in the less energy than existing in the

elemental form. It also means elemental form. It also means that that

energy energy was releasedwas released from the from the

system. system.

This is a HUGE misconception most This is a HUGE misconception most

students have—it takes energy to students have—it takes energy to

break a bond, not make a bond! break a bond, not make a bond!

Energy is RELEASED when a bond is Energy is RELEASED when a bond is

formed, therefore, it REQUIRES formed, therefore, it REQUIRES

energy to break a bond.energy to break a bond.

Bond EnergyBond Energy

energy required to break the energy required to break the bondbond

TYPES OF CHEMICAL TYPES OF CHEMICAL BONDSBONDS

Ionic BondsIonic Bonds

an electrostatic attraction between an electrostatic attraction between

ions -- usually the reaction between ions -- usually the reaction between a a

metal and nonmetal. Cause very metal and nonmetal. Cause very

high melting points and usually a high melting points and usually a

solid state since the attraction is SO solid state since the attraction is SO

strong that the ions are VERY close strong that the ions are VERY close

together in a crystal formation. together in a crystal formation.

Covalent BondsCovalent Bonds

Electrons are shared by nuclei.Electrons are shared by nuclei.

[careful, sharing is hardly ever [careful, sharing is hardly ever 50-50!] 50-50!]

Coulomb’s LawCoulomb’s Law

used to calculate the Energy of used to calculate the Energy of an an

ionic bondionic bond

r

QQnmJxE 21191031.2

r is the distance between the ion r is the distance between the ion

centers in nanometers [size matters!]centers in nanometers [size matters!]

J is the energy in JoulesJ is the energy in Joules

QQ11 and Q and Q22 are the numerical ion are the numerical ion

chargescharges

r

QQnmJxE 21191031.2

There will be a negative sign on There will be a negative sign on the the

Energy once calculated. It Energy once calculated. It indicates indicates

an attractive force so that the an attractive force so that the ion ion

pair has lower energy than the pair has lower energy than the

separated ions.separated ions.

You can also use Coulomb’s Law You can also use Coulomb’s Law to to

calculate the repulsive forces calculate the repulsive forces

between like charges. between like charges.

What sign will that calculation What sign will that calculation have?have?

VALENCE ELECTRONSVALENCE ELECTRONS

Valence ElectronsValence Electrons

outermost electronsoutermost electrons

Focus on Focus on ns, np ns, np and and dd electrons of electrons of

transition elements. transition elements.

Once d is filled it doesn’t play Once d is filled it doesn’t play

anymore!anymore!

Lewis Dot StructuresLewis Dot Structures

G.N. Lewis, 1916 (usually main G.N. Lewis, 1916 (usually main

group elements)group elements)

Emphasizes rare gas configurations, Emphasizes rare gas configurations,

ss22pp66, as a stable state. All rare gasses , as a stable state. All rare gasses

except Heexcept He have 8 valence electrons have 8 valence electrons octet ruleoctet rule

CHEMICAL BOND CHEMICAL BOND FORMATIONFORMATION

Level 1Level 1

When 2 hydrogen atoms approach When 2 hydrogen atoms approach

each other, 2 bad E things each other, 2 bad E things happen: happen:

electron/electron repulsion electron/electron repulsion

and and

proton/proton repulsion. proton/proton repulsion.

One good E thing happens: One good E thing happens:

proton/electron attraction proton/electron attraction

When the attractive forces offset When the attractive forces offset

the repulsive forces, the energy of the repulsive forces, the energy of

the two atoms decreases and a the two atoms decreases and a

bondbond is formed. is formed.

Remember, nature is always Remember, nature is always striving striving

for a for a LOWER ENERGY STATELOWER ENERGY STATE..

Bond LengthBond Length

the distance between the 2 the distance between the 2 nuclei nuclei

where the energy is minimum where the energy is minimum

between the two nucleibetween the two nuclei

energy decrease is small--energy decrease is small--van der van der WaalsWaals IMforces [another chapter!] IMforces [another chapter!]

energy decrease is larger--energy decrease is larger--chemical bondschemical bonds

TooFAR

TooCLOS

E

Getting

BETTER

JustRIGHT

Level 2 -- Orbital TheoryLevel 2 -- Orbital Theory

Electrons and nucleus of one atom Electrons and nucleus of one atom

strongly perturb or change the strongly perturb or change the spatial spatial

distribution of the distribution of the otherother atom’s atom’s

valence electrons. A new orbital valence electrons. A new orbital

(wave function) is needed to describe (wave function) is needed to describe

the distribution of the bonding the distribution of the bonding

electrons electrons bond orbital.bond orbital.

Bond OrbitalBond Orbital

describes the motion of the 2 describes the motion of the 2

electrons of electrons of opposite spinopposite spin

Lone Pair OrbitalLone Pair Orbital

The orbitals of electrons on a The orbitals of electrons on a

bonded atom that are distorted bonded atom that are distorted

awayaway from the bond region also from the bond region also

have new descriptions (wave have new descriptions (wave

functions).functions).

The new bond orbital is “built” from The new bond orbital is “built” from

the atomic orbitals of the two the atomic orbitals of the two bonded bonded

atoms. Looks a lot like the original.atoms. Looks a lot like the original.

BUT, the bond orbital is concentrated BUT, the bond orbital is concentrated

in the region between the bonded in the region between the bonded

nuclei.nuclei.

The energy of the electrons in a bond The energy of the electrons in a bond

orbital, where the electrons are orbital, where the electrons are

attracted by attracted by twotwo nuclei, is lower nuclei, is lower than than

their energy in valence electron their energy in valence electron

orbitals where the electrons are orbitals where the electrons are

attracted to attracted to only oneonly one nucleus. nucleus.

[ZAPPED!!][ZAPPED!!]

Ionic BondIonic Bond

The bonding orbital is strongly The bonding orbital is strongly

displaced toward one nuclei.displaced toward one nuclei.

(metal from the left side of the (metal from the left side of the

periodic table + nonmetal from periodic table + nonmetal from

right side of the periodic table)right side of the periodic table)

Covalent BondCovalent Bond

Bond orbital is more or less (polar Bond orbital is more or less (polar or non-polar) evenly distributed and or non-polar) evenly distributed and the electrons are the electrons are sharedshared by two by two nuclei. nuclei.

(Elements lie close to one another (Elements lie close to one another on the table.)on the table.)

Most chemical bonds are in fact Most chemical bonds are in fact

somewhere between purely ionic somewhere between purely ionic

and purely covalent.and purely covalent.

Recall the information you’ve Recall the information you’ve already already

learned about electronegativity…learned about electronegativity…

Electronegativity (EElectronegativity (Enn))

The ability of an atom IN A The ability of an atom IN A

MOLECULE [meaning it’s MOLECULE [meaning it’s

participating in a BOND] to attract participating in a BOND] to attract

shared electrons to itself. Think “tug shared electrons to itself. Think “tug

of war”. Now you know why they of war”. Now you know why they

teach you such games in elementary teach you such games in elementary

school!school!

Linus Pauling’s ScaleLinus Pauling’s ScaleNobel Prize for Chemistry & Nobel Prize for Chemistry &

PeacePeace

Fluorine is the most EFluorine is the most Enn

and and

Francium is the least EFrancium is the least Enn

Why is F the most?Why is F the most?

Highest ZHighest Zeffeff and smallest so that and smallest so that

the nucleus is closest to the the nucleus is closest to the “action”. “action”.

Why is Fr the least?Why is Fr the least?

Lowest ZLowest Zeffeff and largest so that and largest so that the the

nucleus is farthest from the nucleus is farthest from the

““action”.action”.

We’ll use this concept a great We’ll use this concept a great deal deal

in our discussions about bonding in our discussions about bonding

since this atomic trend is since this atomic trend is only only

used when atoms form used when atoms form

molecules.molecules.

Use the difference in EUse the difference in Enn to to

determine the type of bond determine the type of bond formed.formed.

Ionic -- EIonic -- Enn difference > 1.67 difference > 1.67

Covalent -- ECovalent -- Enn difference < 1.67 difference < 1.67

NONpolar -- ENONpolar -- Enn difference < 0.4 difference < 0.4

Exercise 1Exercise 1 Relative Bond Relative Bond PolaritiesPolarities

Order the following bonds Order the following bonds according according

to polarity: to polarity:

H—HH—H O—H Cl—HO—H Cl—H

S—HS—H F—H F—H

SolutionSolution

Bond Polarity and Bond Polarity and ElectronegativityElectronegativity

EEnn ( () determines polarity since ) determines polarity since it it

measures a nucleus’ pull on the measures a nucleus’ pull on the

bonded electron pair. bonded electron pair.

EEnn ranges from 0 -- 4.0. ranges from 0 -- 4.0.

When 2 nuclei When 2 nuclei

are the same, are the same,

the sharing is the sharing is

equal.equal.

NONPOLAR (a) NONPOLAR (a)

When the 2 nuclei When the 2 nuclei

are different, the are different, the

electrons are not electrons are not

shared equally, shared equally,

setting up slight setting up slight

+/- poles. +/- poles.

POLAR (b) POLAR (b)

When the When the

electrons are electrons are

shared unequally shared unequally

to a greater to a greater

extent.extent.

IONIC (c)IONIC (c)

The polarity of a bond can be The polarity of a bond can be

estimated from Δestimated from Δ/Σ/Σ. .

Range is 0 for pure covalent Range is 0 for pure covalent bonds, bonds,

to 1 for completely ionic bonds.to 1 for completely ionic bonds.

Ionic BondingIonic Bonding

The final result of ionic bonding is The final result of ionic bonding is a a

solid, regular array of cations and solid, regular array of cations and

anions called a anions called a crystal latticecrystal lattice

You can see the energy changes You can see the energy changes

involved in forming LiF from the involved in forming LiF from the

elements Li and F2.elements Li and F2.

Enthalpy of DissociationEnthalpy of Dissociation

energy required to decompose an energy required to decompose an

ion pair (from a lattice) into ions ion pair (from a lattice) into ions

a measure of the strength of the a measure of the strength of the

ionic bond from Coulomb’s law…ionic bond from Coulomb’s law…

where nwhere n++ is the charge on the is the charge on the

positive ion and npositive ion and n-- is the charge on is the charge on

the negative ion and d is the the negative ion and d is the

distance between the ion centers in distance between the ion centers in

the crystal lattice.the crystal lattice.

dnn H

-ondissociati

+

Energy of attraction depends Energy of attraction depends

directly on the magnitude of the directly on the magnitude of the

charges (higher the charges the charges (higher the charges the

greater the attractive energy) and greater the attractive energy) and

inversely on the distance between inversely on the distance between

them (greater the distance, the them (greater the distance, the

smaller the attractive energy).smaller the attractive energy).

The larger the ion the smaller the The larger the ion the smaller the

ΔHΔHdissociationdissociation (it’s a distance thing). (it’s a distance thing).

Ion-ion attractions have a Ion-ion attractions have a profound profound

effect on effect on melting points melting points and and

solubilities.solubilities.

Water must overcome the ion-ion Water must overcome the ion-ion

attractions to dissolve an ionic attractions to dissolve an ionic

substance. substance.

Size affects this as does charge. Size affects this as does charge.

HOW??HOW??

The crystal The crystal

lattice for LiF lattice for LiF is is

shown at the shown at the

left.left.

Lattice energy can be represented Lattice energy can be represented by a modified form of Coulomb’s by a modified form of Coulomb’s Law: k is a proportionality Law: k is a proportionality

constant constant that depends on the structure of that depends on the structure of the solid and the electron the solid and the electron configurations of the ions.configurations of the ions.

r

QQkrgyLatticeEne 21

Covalent BondingCovalent Bonding

Most compounds are covalently Most compounds are covalently

bonded, especially carbon bonded, especially carbon compounds.compounds.

We have 3 major bonding theories to We have 3 major bonding theories to

discuss. Only one for this chapter discuss. Only one for this chapter

though!though!

Localized Electron [LE] Localized Electron [LE] Bonding ModelBonding Model

Assumes that a molecule is Assumes that a molecule is

composed of atoms that are composed of atoms that are bound bound

together by sharing pairs of together by sharing pairs of

electrons using the electrons using the atomicatomic orbitals orbitals

of the bound atoms. of the bound atoms.

Electron pairs are assumed to Electron pairs are assumed to be be

localized localized onon a particular atom a particular atom [[lone lone

pairspairs] or in the space ] or in the space betweenbetween

two atoms [two atoms [bonding pairsbonding pairs].].

Lewis Structures describe the Lewis Structures describe the valence electron arrangementvalence electron arrangement

Geometry of the molecule is Geometry of the molecule is predicted with VSEPRpredicted with VSEPR

Description of the type of atomic Description of the type of atomic orbitals used by the atoms to orbitals used by the atoms to share electrons or hold lone share electrons or hold lone pairs [hybrids—next chapter].pairs [hybrids—next chapter].

Number of Bond Pairs: Number of Bond Pairs: The Octet RuleThe Octet Rule

““noble is good”noble is good”

Predict # of bonds by counting Predict # of bonds by counting the the

number of unpaired electrons number of unpaired electrons in a in a

Lewis structure.Lewis structure.

A dash ( - ) is used to represent A dash ( - ) is used to represent a a

pairpair of shared electrons. of shared electrons.

A colon ( : ) is used to represent A colon ( : ) is used to represent a a

lone pairlone pair of electrons. of electrons.

SINGLE AND MULTIPLE SINGLE AND MULTIPLE BONDSBONDS

Single BondSingle Bond

one pair of electrons sharedone pair of electrons shared

sigma (σ) bondsigma (σ) bond

Multiple BondsMultiple Bonds

Are most often formed by Are most often formed by

C, N, O, P and S atoms.C, N, O, P and S atoms.

Say “C-NOPS”Say “C-NOPS”

Double BondDouble Bond

two pairs of electrons sharedtwo pairs of electrons shared

one σ bond and one one σ bond and one bond bond

Triple BondTriple Bond

three pairs of electrons sharedthree pairs of electrons shared

one σ bond and two one σ bond and two bonds bonds

Obviously, combinations of σ Obviously, combinations of σ and and

are stronger than σ alone. are stronger than σ alone.

Pi bonds are weaker than sigma Pi bonds are weaker than sigma but but

never exist alone.never exist alone.

Multiple bonds increase the Multiple bonds increase the electron electron

density between two nuclei and density between two nuclei and

therefore decrease the nuclear therefore decrease the nuclear

repulsions while enhancing the repulsions while enhancing the

nucleus to electron density nucleus to electron density

attractions.attractions.

Either way, the nuclei move Either way, the nuclei move closer closer

together and the bond length is together and the bond length is

shorter for a double than a shorter for a double than a single,single,

and triple is shortest of all!and triple is shortest of all!

Coordinate Covalent Coordinate Covalent BondsBonds

Some atoms such as N and P, tend to Some atoms such as N and P, tend to

share a share a lone pairlone pair with another atom with another atom

that is short of electrons, leading to that is short of electrons, leading to

the formation of a coordinate the formation of a coordinate covalent covalent

Bond. These bonds are in all Bond. These bonds are in all

coordination compounds and Lewis coordination compounds and Lewis

A/B.A/B.

Ammonium Ion Ammonium Ion FormationFormation

We show that N is sharing the We show that N is sharing the lone lone

PAIR of electrons by drawing an PAIR of electrons by drawing an

arrow from it to the Harrow from it to the H++. .

Remember HRemember H++ has NO electrons to has NO electrons to

contribute to the bond. Note that all contribute to the bond. Note that all

four bonds are actually identicalfour bonds are actually identical..

EXCEPTIONS TO THE EXCEPTIONS TO THE OCTET RULEOCTET RULE

Fewer than EightFewer than Eight

H at most only 2 electrons! H at most only 2 electrons!

BeHBeH22, only 4 valence electrons , only 4 valence electrons around Be! around Be!

Boron compounds, only 6 Boron compounds, only 6 valence valence

electrons! electrons!

ammonia + boron trifluorideammonia + boron trifluoride

Expanded ValenceExpanded Valence

3rd or higher period [periods 4, 3rd or higher period [periods 4, 5, 5,

6…] can be surrounded by more 6…] can be surrounded by more than than

four valence pairs in certain four valence pairs in certain

compounds.compounds.

# of bonds depends on the # of bonds depends on the balance balance

between the ability of the between the ability of the nucleus nucleus

to attract electrons and the to attract electrons and the

repulsion between the pairs. repulsion between the pairs.

Odd-electron CompoundsOdd-electron Compounds

A A fewfew stable compounds. stable compounds.

Contain an odd number of valence Contain an odd number of valence electrons and thus cannot obey the electrons and thus cannot obey the octet rule. octet rule.

NO NONO NO22 ClO ClO22

Drawing Lewis Drawing Lewis StructuresStructures

((predicting predicting

molecular shape)molecular shape)

To predict arrangement of To predict arrangement of

atoms within the molecule atoms within the molecule use use

the following rules:the following rules:

(1) H is always a terminal atom. (1) H is always a terminal atom.

ALWAYS connected to only one ALWAYS connected to only one

other atom!!other atom!!

(2) LOWEST E(2) LOWEST Enn is central atom in is central atom in

molecule [not just the oddball molecule [not just the oddball

element]element]

(3) Find the total # of valence (3) Find the total # of valence

electrons by adding up group electrons by adding up group

#’s of the elements. FOR IONS, #’s of the elements. FOR IONS,

addadd for for negativenegative and and subtractsubtract for for

positivepositive charge. charge. Divide by two Divide by two

to get the number of electron to get the number of electron

PAIRS.PAIRS.

(4) Place one pair of electrons, a σ (4) Place one pair of electrons, a σ

bond, between each pair of bond, between each pair of

bonded atoms.bonded atoms.

(5) Subtract from the total the (5) Subtract from the total the

number of bonds you just used.number of bonds you just used.

(6) Place lone pairs about each (6) Place lone pairs about each

terminal atom (EXCEPT H) to terminal atom (EXCEPT H) to

satisfy the octet rule. Left satisfy the octet rule. Left over over

pairs are assigned to the pairs are assigned to the central central

atom. atom.

If the central atom is from the If the central atom is from the 3rd 3rd

or higher period, it can or higher period, it can accommodate accommodate

more than four electron pairs.more than four electron pairs.

(7) If the central atom is not yet (7) If the central atom is not yet

surrounded by four electron surrounded by four electron

pairs, convert one or more pairs, convert one or more

terminal atom lone pairs to terminal atom lone pairs to pi pi

bonds pairs. bonds pairs.

NOT ALL ELEMENTS FORM pi NOT ALL ELEMENTS FORM pi

BONDS!! BONDS!!

only C, N, O, P, and S only C, N, O, P, and S !!!!

Exercise 6Exercise 6 Writing Lewis Writing Lewis StructuresStructuresGive the Lewis structure for Give the Lewis structure for

each of the following:each of the following:

a. a. HFHF d. d. CHCH44

b. b. NN22 e. e. CFCF44

c.c. NH NH33 f.f. NO NO++

SolutionSolution

Exercise 7Exercise 7 Lewis Lewis Structures for Molecules Structures for Molecules That Violate the Octet That Violate the Octet Rule IRule I

Write the Lewis structure for Write the Lewis structure for PClPCl55..

SolutionSolution

Exercise 8Exercise 8 Lewis Lewis Structures for Molecules Structures for Molecules That Violate the Octet That Violate the Octet Rule IIRule II

Write the Lewis structure for each Write the Lewis structure for each

molecule or ion.molecule or ion.

a. ClFa. ClF33 b. XeO b. XeO33 c. c. RnClRnCl22

d. BeCld. BeCl22 e. ICl e. ICl44--

SolutionSolution

a. The a. The chlorine chlorine

atom (third atom (third row) row)

accepts the accepts the extra extra

electrons.electrons.

SolutionSolution

b. All atoms b. All atoms

obey the octet obey the octet

rule.rule.

SolutionSolution

c. Radon, a noble gas in c. Radon, a noble gas in

Period 6, accepts the extra Period 6, accepts the extra

electrons.electrons.

SolutionSolution

d. Beryllium is d. Beryllium is

electron-deficient.electron-deficient.

SolutionSolution

e. Iodine exceeds the octet e. Iodine exceeds the octet rule.rule.

Resonance StructuresResonance Structures

Ozone, OOzone, O33 has equal bond has equal bond lengths, lengths,

implying that there is an equal implying that there is an equal

number of bond pairs on each number of bond pairs on each side side

of the central O atom.of the central O atom.

Resonance StructuresResonance Structures

We draw it as having a double bond We draw it as having a double bond

and a single bond [the dashes are and a single bond [the dashes are

another way of representing lone another way of representing lone

pairs].pairs].

BUT, since there are equal bond BUT, since there are equal bond

lengths and strengths, they are lengths and strengths, they are

clearly NOT as pictured. clearly NOT as pictured.

The bonds are more equivalent to a The bonds are more equivalent to a

““bond and ½” in terms of length and bond and ½” in terms of length and

strength.strength.

We use the double edged arrows We use the double edged arrows to to

indicate resonance. indicate resonance.

We also bracket the structures just We also bracket the structures just as we do for polyatomic ions.as we do for polyatomic ions.

In an attempt In an attempt to improve the to improve the drawing, we drawing, we sometimes use sometimes use a single a single composite composite picture.picture.

Carbonate IonCarbonate Ion

NOTE: These all 3 need NOTE: These all 3 need brackets brackets

and the charge shown in the and the charge shown in the

upper upper

right right

corner…corner…

……like this like this

composite, composite,

to gain full to gain full

credit on credit on

the AP the AP

Exam!!!!Exam!!!!

Notice…Notice…

1) resonance structures differ only 1) resonance structures differ only in in

the assignment of electron pair the assignment of electron pair positions, NEVER atom positions. positions, NEVER atom positions.

2) resonance structures differ in 2) resonance structures differ in the the

number of bond pairs between a number of bond pairs between a given pair of atoms.given pair of atoms.

Exercise 9Exercise 9 Resonance Structures Resonance Structures

Describe the electron Describe the electron arrangement arrangement

in the nitrite anion (NOin the nitrite anion (NO22--) using ) using

the the

localized electron model.localized electron model.

SolutionSolution

Bond PropertiesBond Properties

Bond OrderBond Order

# of bonding electron pairs # of bonding electron pairs shared shared

by two atoms in a molecule.by two atoms in a molecule.

1--only a sigma bond between the 2 1--only a sigma bond between the 2

bonded atomsbonded atoms

2--2 shared pairs between two 2--2 shared pairs between two

atoms; one sigma and one pi (COatoms; one sigma and one pi (CO22

and ethylene)and ethylene)

3--3 shared pairs between two 3--3 shared pairs between two

atoms; one sigma and two pi (c-c atoms; one sigma and two pi (c-c

acetylene and CO and cyanide)acetylene and CO and cyanide)

fractional--resonance; ozone 3/2; fractional--resonance; ozone 3/2;

carbonate -2/3carbonate -2/3

bond order = bond order =

# of shared pairs linking X and Y# of shared pairs linking X and Y

number of X-Y links number of X-Y links

Bond LengthBond Length

distance between the nuclei of two distance between the nuclei of two

bonded atoms bonded atoms

C-N < C-C < C-P C-N < C-C < C-P

Effect of bond order is evident when Effect of bond order is evident when

you compare bonds between the you compare bonds between the

samesame two atoms. two atoms.

Bond C-O C=O C≡O Bond C-O C=O C≡O

Bond order 1 2 3 Bond order 1 2 3

Bond length (pm) 143 122 113 Bond length (pm) 143 122 113

Bond length is reduced by adding Bond length is reduced by adding

multiple bonds.multiple bonds.

Variations in neighboring parts of a Variations in neighboring parts of a

molecule can affect the length of a molecule can affect the length of a

particular bond--as much as 10%.particular bond--as much as 10%.

Bond EnergyBond Energy

The greater the # of bonding The greater the # of bonding

electron pairs between a pair of electron pairs between a pair of

atoms, the shorter the bond. atoms, the shorter the bond.

This implies that atoms are held This implies that atoms are held

together more tightly when there together more tightly when there

are multiple bonds, so there is a are multiple bonds, so there is a

relation between bond order and relation between bond order and

the energy required to separate the energy required to separate

them.them.

Bond Dissociation Energy Bond Dissociation Energy (D)(D)

gaseous atom gaseous atom

E supplied to break a chemical E supplied to break a chemical bond bond

(bond energy for short!).(bond energy for short!).

D is + and breaking bonds is D is + and breaking bonds is

endothermicendothermic. The converse is also . The converse is also

true.true.

D in table 10.4 is +D in table 10.4 is +

D is an average with a +/- 10%D is an average with a +/- 10%

What is the connection What is the connection between between

bond energy and bond order???bond energy and bond order???

Bonds in reactants are broken Bonds in reactants are broken while while

bonds in products are formed. bonds in products are formed.

Calculating reaction Calculating reaction energies from bond energies from bond energies:energies:

Energy released is greater than Energy released is greater than

energy absorbed in EXOthermic energy absorbed in EXOthermic

reactions. reactions.

The converse is also true.The converse is also true.

= H reactiono

ΔHoreaction = reactants - products

made) nD(bonds

) D(bonds brokenm

NOTE…NOTE…

THIS IS “BACKWARDS” FROM THIS IS “BACKWARDS” FROM THE THE

THERMODYNAMICS “BIG THERMODYNAMICS “BIG MAMMA” MAMMA”

EQUATION. EQUATION.

We’re back to that We’re back to that misconception if misconception if

you are confused by this. you are confused by this.

It takes energy to break bonds It takes energy to break bonds NOT NOT

make bonds! make bonds!

First, we must break the bonds First, we must break the bonds of of

the reactants [costs energy], the reactants [costs energy], then then

subtract the energy gained by subtract the energy gained by

forming new bonds in the forming new bonds in the products.products.

Exercise 5Exercise 5 ∆H from Bond ∆H from Bond EnergiesEnergies

Using the bond energies listed in Using the bond energies listed in Table 8.4, calculate ∆H for the Table 8.4, calculate ∆H for the reaction of methane with chlorine reaction of methane with chlorine

and fluorine to give Freon-12 (CFand fluorine to give Freon-12 (CF22ClCl22).).

CHCH44(g) + 2Cl(g) + 2Cl22(g) + 2F(g) + 2F22(g) → (g) →

CFCF22ClCl22(g) + 2HF(g) + 2HCl(g)(g) + 2HF(g) + 2HCl(g)

SolutionSolution

∆∆H = -1194 kJH = -1194 kJ

Formal ChargeFormal Charge

Often, many nonequivalent Lewis Often, many nonequivalent Lewis structures may be obtained which structures may be obtained which

all all follow the rules. follow the rules.

Use the idea of formal charge to Use the idea of formal charge to determine the most favored determine the most favored structure.structure.

Physicists tell us that oxidation Physicists tell us that oxidation

states of more than +/- two are states of more than +/- two are

pure fantasy and that formal pure fantasy and that formal

charges are much more charges are much more realistic.realistic.

Formal ChargeFormal Charge

The difference between the The difference between the number number

of valence electrons on the free of valence electrons on the free

element and the number of element and the number of electrons electrons

assigned to the atom in the assigned to the atom in the

molecule. molecule.

atom’s formal charge = atom’s formal charge =

group number – [# of lone group number – [# of lone electrons electrons

– –1/2 (# of bonding electrons)]1/2 (# of bonding electrons)]

The Sum of the Formal ChargesThe Sum of the Formal Charges

Must Equal the Ion’s Charge !!!Must Equal the Ion’s Charge !!!

Use formal charges along with Use formal charges along with the the

following to determine following to determine resonance resonance

structure…structure…

Atoms in molecules (or ions) Atoms in molecules (or ions) should should

have formal charges as small as have formal charges as small as

possible—as close to zero as possible—as close to zero as

possible [principle of possible [principle of electroneutrality].electroneutrality].

A molecule (or ion) is most A molecule (or ion) is most stable stable

when any negative formal when any negative formal charge charge

resides on the most resides on the most electronegative electronegative

atom.atom.

ExampleExample

Draw all possible structures for Draw all possible structures for the the

sulfate ion. sulfate ion.

Decide which is the most Decide which is the most plausible plausible

using formal charges.using formal charges.

** Caution**** Caution**

Although formal charges are Although formal charges are considered closer to the atomic considered closer to the atomic charges than the oxidation states, charges than the oxidation states, they are still only they are still only estimatesestimates and and should not be taken as the actual should not be taken as the actual atomic charges.atomic charges.

Second, using formal charges can Second, using formal charges can

often lead to erroneous structures, often lead to erroneous structures,

so tests based on experiments must so tests based on experiments must

be used to make the final decisions be used to make the final decisions

on the correct description of on the correct description of

bonding. bonding.

Exercise 10Exercise 10 Formal Charges Formal Charges

Give possible Lewis structures for Give possible Lewis structures for

XeOXeO33, an explosive compound of , an explosive compound of

xenon. xenon.

Which Lewis structure or structures Which Lewis structure or structures

are most appropriate according to are most appropriate according to

the formal charges?the formal charges?

SolutionSolution

Structures with the lower Structures with the lower values ofvalues of

formal charge would be most formal charge would be most

appropriate.appropriate.

Molecular ShapeMolecular Shape

Minimize Electron PairMinimize Electron Pair

Repulsions !!!Repulsions !!!

VSEPRVSEPR

Valence Shell Electron Pair Valence Shell Electron Pair

Repulsion TheoryRepulsion Theory

Molecular shape changes with the Molecular shape changes with the

numbers of σ bonds plus lone numbers of σ bonds plus lone pairs pairs

about the central atom.about the central atom.

The VSPER ModelThe VSPER Model and and

Molecular ShapeMolecular Shape

Molecular GeometryMolecular Geometry

the arrangement in space of the the arrangement in space of the

atoms bonded to a central atomatoms bonded to a central atom

Not necessarily the same as the Not necessarily the same as the

structural pair geometrystructural pair geometry ..

Lone pairs have a different Lone pairs have a different

repulsion since they are repulsion since they are

experiencing a “pull” from only experiencing a “pull” from only

one nucleus. one nucleus.

They also take They also take

up more space up more space

around an around an atom atom

as you can see as you can see

on the left.on the left.

Each lone pair or bond pair repels Each lone pair or bond pair repels

all other lone pairs and bond pairs.all other lone pairs and bond pairs.

They try to avoid each other They try to avoid each other making making

as wide an angle as possible.as wide an angle as possible.

Works well for elements of the s Works well for elements of the s

and p-blocks.and p-blocks.

It does It does notnot apply to transition apply to transition

element compounds element compounds (exceptions).(exceptions).

Molecular Shapes for Molecular Shapes for Central Atoms With Normal Central Atoms With Normal ValenceValenceNo more than 4 structural pairs if No more than 4 structural pairs if

the atom obeys the octet rule. the atom obeys the octet rule.

Since no lone pairs are present, Since no lone pairs are present, the the

molecular and structural pair [or molecular and structural pair [or

electronelectronic] geometry is the same.ic] geometry is the same.

Ignore lone pairs Ignore lone pairs

AFTERAFTER you’ve you’ve

determined the determined the

angles angles only the only the

relative positions relative positions

of the atoms are of the atoms are

important in important in

molecular molecular geometry.geometry.

Here is ammonia—3 sigma bonds Here is ammonia—3 sigma bonds

and one lone pair—when lone pairs and one lone pair—when lone pairs

are present, the structural pair or are present, the structural pair or

electronic geometry are electronic geometry are different!!different!!

““electronic geometry”electronic geometry”

tetrahedral while molecular tetrahedral while molecular geometrygeometry

trigonal pyramidaltrigonal pyramidal

Water has 2 lone pairs and still Water has 2 lone pairs and still

obeys the octet rule. Again, the obeys the octet rule. Again, the

electronic and molecular geometries electronic and molecular geometries

will be will be different different . .

The electronic is still The electronic is still tetrahedral tetrahedral

since the octet rule is obeyed. since the octet rule is obeyed.

BUT, the molecular geometry is BUT, the molecular geometry is

described as “bent” or V-described as “bent” or V-shaped.shaped.

Now, let’s discuss the warping of the Now, let’s discuss the warping of the 109.5 degree bond angle. The lone 109.5 degree bond angle. The lone pairs have more repulsive force than pairs have more repulsive force than shared pairs and are “space hogs”. shared pairs and are “space hogs”. They force the shared pairs to They force the shared pairs to squeeze together.squeeze together.

To Determine the To Determine the GeometryGeometry

1. Sketch the Lewis dot structure. 1. Sketch the Lewis dot structure.

2. Describe the structural pair 2. Describe the structural pair geometry.geometry.

3. Describe which pairs are sigma 3. Describe which pairs are sigma & then describe their locations.& then describe their locations.

Molecular Shapes for Molecular Shapes for Central Atoms with Central Atoms with Expanded ValenceExpanded Valence

Only elements with a principal Only elements with a principal

energy level of 3 or higher can energy level of 3 or higher can

expand their valence and violate expand their valence and violate the the

octet rule on the high side. octet rule on the high side.

Why? Why?

d electrons are needed for the d electrons are needed for the expansion to a 5th or 6th bonding expansion to a 5th or 6th bonding location. location.

The combination of 1 s and 3 p’s The combination of 1 s and 3 p’s provides the four bonding sites provides the four bonding sites

that that make up the octet rule.make up the octet rule.

Seems to be a limit of 3 lone Seems to be a limit of 3 lone pairs pairs

about the central atom.about the central atom.

XeFXeF44

There are 2 lone pairs and 4 There are 2 lone pairs and 4 shared shared

pairs. pairs.

Two possible arrangements Two possible arrangements exist.exist.

AxialAxial

Shared pairs are locatedShared pairs are located

““top and bottom”. top and bottom”.

EquatorialEquatorial

Shared pairs surround Xe. Shared pairs surround Xe.

This is preferred since the lone pairs This is preferred since the lone pairs

are 180 degrees apart, which are 180 degrees apart, which

minimizes their repulsion. Lone minimizes their repulsion. Lone

pairs prefer maximum separation—pairs prefer maximum separation—

use this in your determinations!use this in your determinations!

In both molecular In both molecular arrangements the arrangements the

electronic geometry is electronic geometry is octahedral octahedral

with 90 degree angles. with 90 degree angles.

(a) has a molecular geometry known (a) has a molecular geometry known

as “see saw” as “see saw”

(b) has a molecular geometry that is (b) has a molecular geometry that is

more stable—square planarmore stable—square planar

Which of these arrangements do Which of these arrangements do you predict to be the most stable?you predict to be the most stable?

Remember to look at the lone pair-Remember to look at the lone pair-lone pair angles to make your lone pair angles to make your determination. determination.

Exercise 11Exercise 11 Prediction of Prediction of Molecular Structure IMolecular Structure I

Describe the molecular Describe the molecular structure of structure of

the water molecule.the water molecule.

SolutionSolution

Two bonding and two non-Two bonding and two non-bonding bonding

pairs of electrons, pairs of electrons,

Forming a V shape molecule.Forming a V shape molecule.

Exercise 12Exercise 12 Prediction of Prediction of Molecular Structure IIMolecular Structure II

When phosphorus reacts with When phosphorus reacts with excess excess

chlorine gas, the compound chlorine gas, the compound

phosphorus pentachloride (PClphosphorus pentachloride (PCl55) ) is is

formed.formed.

In the gaseous and liquid states, In the gaseous and liquid states,

this substance consists of PClthis substance consists of PCl55

molecules, but in the solid state it molecules, but in the solid state it

consists of a 1:1 mixture of PClconsists of a 1:1 mixture of PCl44++

and PCland PCl66-- ions. ions.

Predict the geometric structures of: Predict the geometric structures of:

PClPCl5 5 PCl PCl44++ PCl PCl66--

Solution - PClSolution - PCl55

Solution - PClSolution - PCl44++

Solution - PClSolution - PCl66--

RECAPRECAP

including bond angles for allincluding bond angles for all

Structural PairsStructural Pairs

σ bond (σ bond ( bond pairs occupy the bond pairs occupy the

same space) pairs about an same space) pairs about an atom.atom.

2 2 180° 180° linear [and of linear [and of course course planar] planar]

3 3 120° 120° trigonal trigonal planarplanar

4 4 109.5° 109.5° tetrahedral tetrahedral

Not Square Planar !Not Square Planar !

55 120° & 90° 120° & 90° trigonal trigonal bipyramidal bipyramidal

6 6 90° 90° octahedral octahedral

MOLECULAR POLARITYMOLECULAR POLARITY

PolarPolar

Bonds can be polar while the Bonds can be polar while the

molecule isn’t and vice versa.molecule isn’t and vice versa.

Dipole MomentDipole Moment

separation of the charge in a separation of the charge in a

moleculemolecule

product of the size of the product of the size of the charge charge

and the distance of separationand the distance of separation

align themselves with an electric align themselves with an electric

fieldfield

align with each other as well, in align with each other as well, in the the

absence of an electric fieldabsence of an electric field

WaterWater

2 lone pairs establish a strong 2 lone pairs establish a strong

negative polenegative pole

ammoniaammonia

has a lone pair which establishes a has a lone pair which establishes a negative polenegative pole

IF octet rule is obeyed [which it is in IF octet rule is obeyed [which it is in

both water and ammonia] AND all both water and ammonia] AND all

the surrounding bonds are the same the surrounding bonds are the same

[even if very polar] then the [even if very polar] then the

molecule is NONpolar since all the molecule is NONpolar since all the

dipole moments cancel each other dipole moments cancel each other

out. out.

Carbon DioxideCarbon Dioxide

Methane is a great example. Methane is a great example.

Replace one H with a halogen and Replace one H with a halogen and it it

becomes polar. becomes polar.

Replace all and it’s nonpolar again!Replace all and it’s nonpolar again!

CHCH4 4 CH CH33Cl CHCl CH22ClCl22 CHCl CHCl3 3 CCl CCl44

Indicate dipole moment where Indicate dipole moment where

necessary.necessary.

Methane based Methane based drawings:drawings:

Exercise 2Exercise 2 Bond Bond Polarity and Dipole Polarity and Dipole MomentMomentFor each of the following For each of the following

molecules, molecules,

show the direction of the bond show the direction of the bond

polarities and indicate which polarities and indicate which ones ones

have a dipole moment:have a dipole moment:

HCL, HCL,

ClCl22, ,

SOSO33 (a planar molecule with the (a planar molecule with the oxygen atoms spaced evenly oxygen atoms spaced evenly around the central sulfur atom), around the central sulfur atom),

CHCH44 [tetrahedral (see Table 8.2) [tetrahedral (see Table 8.2) with the carbon atom at the with the carbon atom at the center], and center], and

HH22S (V-shaped with the sulfur atom S (V-shaped with the sulfur atom at the point).at the point).

SolutionSolution

HCl : HCl :

from H to Cl from H to Cl with with

dipole momentdipole moment

SolutionSolution

SOSO33: :

from S to O from S to O with with

no dipole no dipole

moment moment

SolutionSolution

CHCH44: :

bond polarities bond polarities

cancel and no cancel and no

dipole momentdipole moment

SolutionSolution

HH22S: S:

from H to S from H to S with with

dipole momentdipole moment

SolutionSolution

ClCl22: :

no polarity and no polarity and

no dipole no dipole

momentmoment

Exercise 13Exercise 13 Prediction of Prediction of Molecular Structure IIIMolecular Structure III

Because the noble gases have filled Because the noble gases have filled ss

and and pp valence orbitals, they were not valence orbitals, they were not

expected to be chemically reactive. In expected to be chemically reactive. In

fact, for many years these elements fact, for many years these elements

were called inert gases because of this were called inert gases because of this

supposed inability to form any supposed inability to form any

compounds. compounds.

However, in the early 1960s However, in the early 1960s several several

compounds of krypton, xenon, compounds of krypton, xenon, and and

radon were synthesized. radon were synthesized.

For example, a team at the Argonne For example, a team at the Argonne

National Laboratory produced the National Laboratory produced the

stable colorless compound xenon stable colorless compound xenon

tetrafluoride (XeFtetrafluoride (XeF44). ).

Predict its structure and whether it Predict its structure and whether it

has a dipole moment.has a dipole moment.

SolutionSolution

Atoms form a square planar Atoms form a square planar

structure structure

With no dipole momentWith no dipole moment

Exercise 14 Structures Exercise 14 Structures of Molecules with Multiple of Molecules with Multiple BondsBondsPredict the molecular structure Predict the molecular structure

of of

the sulfur dioxide molecule. the sulfur dioxide molecule.

Is this molecule expected to Is this molecule expected to have a have a

dipole moment?dipole moment?

SolutionSolution

V-shaped with a 120-degree V-shaped with a 120-degree bond bond

angleangle

With a dipole momentWith a dipole moment

ONCE MORE WITH ONCE MORE WITH

FEELING…FEELING…

Chemical BondsChemical Bonds

Forces of attraction that hold Forces of attraction that hold

groups of atoms together and groups of atoms together and make make

them function as a unit.them function as a unit.

TYPES OF BONDS/TYPES OF BONDS/COMPOUNDSCOMPOUNDS

IONIC - Characterized by:IONIC - Characterized by:

a. transfer of electrons between a. transfer of electrons between atoms having high differences in atoms having high differences in electronegativityelectronegativityb. usually involve metals and b. usually involve metals and nonmetals nonmetals c. strong attraction between c. strong attraction between

positive positive and negative ions and negative ions

d. formulas usually given in d. formulas usually given in simplest simplest

ratio of elements (empirical) ratio of elements (empirical)

e. usually crystalline solids at room e. usually crystalline solids at room

temperature (with organized 3-D temperature (with organized 3-D

shape)shape)

f. high melting points f. high melting points

COVALENT- Characterized COVALENT- Characterized by:by:

a. sharing of electrons between a. sharing of electrons between

atoms having small differences atoms having small differences in in

electronegativities electronegativities

b. usually involve metals and b. usually involve metals and

nonmetals nonmetals

c. attraction between nonmetal c. attraction between nonmetal atomsatoms

d. formulas are usually given in d. formulas are usually given in the the

true ratios of atoms true ratios of atoms (molecular)(molecular)

e. usually low melting points e. usually low melting points

f. may be in any form at room f. may be in any form at room

temp. (solid, liquid, or gas) temp. (solid, liquid, or gas)

METALLIC- Characterized METALLIC- Characterized by:by:

a. a "sea" of electrons a. a "sea" of electrons surrounding surrounding

positively charged centerspositively charged centers

b. attraction between metal ions b. attraction between metal ions

and electronsand electrons

c. formulas usually given as an c. formulas usually given as an

atom (Mg, Pb, etc.) atom (Mg, Pb, etc.)

d. a range of melting points d. a range of melting points usually usually

depending on the number of depending on the number of

valence electrons valence electrons

e. usually crystalline solids e. usually crystalline solids

RELATED FACTS AND RELATED FACTS AND

GENERALIZATIONS GENERALIZATIONS

USING USING

BONDING THEORIESBONDING THEORIES

SOLUBILITYSOLUBILITY

Ionic compoundsIonic compounds

When lattice energy When lattice energy strengthens, strengthens,

water solubility decreases. water solubility decreases.

(Ex. KCl, KBr, KI ) (Ex. KCl, KBr, KI )

*KI would be the most soluble in *KI would be the most soluble in water*water*

Ionic and polar covalent Ionic and polar covalent compoundscompounds

dissolve in polar solvents. dissolve in polar solvents.

Nonpolar solutes dissolve in Nonpolar solutes dissolve in

nonpolar solvents. nonpolar solvents.

ExampleExample

CClCCl44 dissolves better in oil than in dissolves better in oil than in

water. The weak forces holding water. The weak forces holding

CClCCl44 together are not strong together are not strong

enough to separate the strong enough to separate the strong

dipoles holding the Hdipoles holding the H22O's O's

together. together.

MELTINGMELTING

Ionic compoundsIonic compounds

When lattice energy strengthens, When lattice energy strengthens,

the melting temperature also the melting temperature also

increases. increases.

(Ex. KCl, KBr, KI) *KCl would (Ex. KCl, KBr, KI) *KCl would

have the higher melting have the higher melting temperature*temperature*

Looking at the lattice energy Looking at the lattice energy

calculation, since all these calculation, since all these

compounds have the same compounds have the same charges charges

involved, lattice energy depends involved, lattice energy depends

upon size. The smaller radius upon size. The smaller radius

results in greater lattice energy.results in greater lattice energy.