chemical bonding &
DESCRIPTION
CHEMICAL BONDING &. MOLECULAR STRUCTURE. What is a Chemical Bond?. It will take us the next two chapters to answer this question!. Bonds are forces that hold groups of atoms together and make them function as a unit. [25 words or less, but leaves out LOTS of details!]. - PowerPoint PPT PresentationTRANSCRIPT
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 them of atoms together and make 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 requires That means being bound 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
rQQnmJxE 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
rQQnmJxE 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 happen: each other, 2 bad E things 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 spatial strongly perturb or change the 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 than nuclei, is lower 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 PolaritiesPolaritiesOrder 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 profound Ion-ion attractions have a 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.
rQQkrgyLatticeEne 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 A dash ( - ) is used to represent 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 and Obviously, combinations of σ 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 BondsBondsSome 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 covalent the formation of a coordinate 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 each Give the Lewis structure for each
of the following: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 IWrite 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 chlorine a. The 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 to We use the double edged arrows 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 StructuresDescribe 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 THE THIS IS “BACKWARDS” FROM THE THERMODYNAMICS “BIG MAMMA” THERMODYNAMICS “BIG MAMMA” EQUATION. EQUATION.
We’re back to that misconception We’re back to that misconception if 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 ChargesGive 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 PairRepulsions !!!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 GeometryGeometry1. 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 IDescribe 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 IIWhen 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) with [tetrahedral (see Table 8.2) with the carbon atom at the center], and the carbon atom at the 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 IIIBecause 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 of Predict the molecular structure of the sulfur dioxide molecule. the sulfur dioxide molecule.
Is this molecule expected to have Is this molecule expected to have a 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 centersb. attraction between metal ions b. attraction between metal ions and electronsand electronsc. 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 strengthens, When lattice energy 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 compoundscompoundsdissolve 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.