Chapter 8 Concepts of Chemical Bonding

Chapter 8Chapter 8Concepts of Concepts of Chemical Chemical BondingBonding

Chemical BondsChemical Bonds

Three basic types Three basic types of bonds:of bonds: IonicIonic

Electrostatic Electrostatic attraction between attraction between ionsions

CovalentCovalent Sharing of electronsSharing of electrons

MetallicMetallic Metal atoms bonded Metal atoms bonded

to several other to several other atomsatoms

Ionic Ionic BondingBonding

Energetics of Ionic Energetics of Ionic BondingBonding

As we saw in the last As we saw in the last chapter, it takes 495 chapter, it takes 495 kJ/mol to remove electrons kJ/mol to remove electrons from sodium.from sodium.


We get 349 We get 349 kJ/mol back kJ/mol back by giving by giving electrons to electrons to chlorine.chlorine.


But these But these numbers don’t numbers don’t explain why the explain why the reaction of reaction of sodium metal sodium metal and chlorine gas and chlorine gas to form sodium to form sodium chloride is so chloride is so exothermic!exothermic!


There must be a There must be a third piece to the third piece to the puzzle.puzzle.

What is as yet What is as yet unaccounted for unaccounted for is the is the electrostatic electrostatic attraction attraction between the newly between the newly formed sodium formed sodium cation and cation and chloride anion.chloride anion.

Lattice EnergyLattice Energy

This third piece of the puzzle is the This third piece of the puzzle is the lattice lattice energy:energy:The energy required to completely separate The energy required to completely separate

a mole of a solid ionic compound into its a mole of a solid ionic compound into its gaseous ions.gaseous ions.

The energy associated with electrostatic The energy associated with electrostatic interactions is governed by Coulomb’s law:interactions is governed by Coulomb’s law:

Eel = Q1Q2

d

Lattice EnergyLattice Energy

Lattice energy, then, increases with the Lattice energy, then, increases with the charge on the ions.charge on the ions.

• It also increases with decreasing size of ions.


By accounting for By accounting for all three energies all three energies (ionization (ionization energy, electron energy, electron affinity, and affinity, and lattice energy), lattice energy), we can get a good we can get a good idea of the idea of the energetics energetics involved in such a involved in such a process.process.


These phenomena These phenomena also helps explain also helps explain the “octet rule.”the “octet rule.”

• Metals, for instance, tend to stop losing electrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies.

Covalent BondingCovalent Bonding

In these bonds atoms In these bonds atoms share electrons.share electrons.

There are several There are several electrostatic electrostatic interactions in these interactions in these bonds:bonds: Attractions between Attractions between

electrons and nucleielectrons and nuclei Repulsions between Repulsions between

electronselectrons Repulsions between nucleiRepulsions between nuclei

Polar Covalent BondsPolar Covalent Bonds

Although atoms often Although atoms often form compounds by form compounds by sharing electrons, sharing electrons, the electrons are not the electrons are not always shared always shared equally.equally.

• Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does.

• Therefore, the fluorine end of the molecule has more electron density than the hydrogen end.

Electronegativity:Electronegativity:

The ability of atoms The ability of atoms in a molecule to in a molecule to attract electrons to attract electrons to itself.itself.

On the periodic On the periodic chart, chart, electronegativity electronegativity increases as you go…increases as you go… ……from left to right from left to right

across a row.across a row. ……from the bottom to from the bottom to

the top of a column.the top of a column.


When two atoms share When two atoms share electrons unequally, a electrons unequally, a bond dipole results.bond dipole results.

The dipole moment, The dipole moment, , , produced by two equal produced by two equal but opposite charges but opposite charges separated by a separated by a distance, distance, rr, is , is calculated:calculated:

= = QQrr It is measured in It is measured in

debyes (D).debyes (D).


The greater the The greater the difference in difference in electronegativielectronegativity, the more ty, the more polar is the polar is the bond.bond.

Lewis StructuresLewis Structures

Lewis structures are Lewis structures are representations of molecules representations of molecules showing all electrons, bonding showing all electrons, bonding and nonbonding.and nonbonding.

Writing Lewis StructuresWriting Lewis Structures

1.1. Find the sum of Find the sum of valence electrons valence electrons of all atoms in the of all atoms in the polyatomic ion or polyatomic ion or molecule.molecule.

If it is an anion, add If it is an anion, add one electron for one electron for each negative each negative charge.charge.

If it is a cation, If it is a cation, subtract one subtract one electron for each electron for each positive charge.positive charge.

PClPCl33

5 + 3(7) = 26


2.2. The central atom The central atom is the is the leastleast electronegative electronegative element that element that isn’t hydrogen. isn’t hydrogen. Connect the Connect the outer atoms to it outer atoms to it by single bonds.by single bonds.Keep track of the electrons:

26 6 = 20


3.3. Fill the octets of Fill the octets of the outer atoms.the outer atoms.

Keep track of the electrons:

26 6 = 20 18 = 2


4.4. Fill the octet of Fill the octet of the central the central atom.atom.

Keep track of the electrons:

26 6 = 20 18 = 2 2 = 0


5.5. If you run out of If you run out of electrons before electrons before the central atom the central atom has an octet…has an octet…

……form multiple form multiple bonds until it does.bonds until it does.


Then assign formal charges.Then assign formal charges. For each atom, count the electrons in lone pairs For each atom, count the electrons in lone pairs

and half the electrons it shares with other atoms.and half the electrons it shares with other atoms. Subtract that from the number of valence Subtract that from the number of valence

electrons for that atom: The difference is its electrons for that atom: The difference is its formal charge.formal charge.


The best Lewis structure…The best Lewis structure… ……is the one with the fewest charges.is the one with the fewest charges. ……puts a negative charge on the puts a negative charge on the

most electronegative atom.most electronegative atom.

ResonanceResonance

This is the This is the Lewis Lewis structure we structure we would draw for would draw for ozone, Oozone, O33.. -

+

ResonanceResonance

But this is at But this is at odds with the odds with the true, observed true, observed structure of structure of ozone, in which…ozone, in which… ……both Oboth O——O O

bonds are the bonds are the same length.same length.

……both outer both outer oxygens have a oxygens have a charge of charge of 1/2.1/2.

ResonanceResonance One Lewis structure cannot accurately One Lewis structure cannot accurately

depict a molecule such as ozone.depict a molecule such as ozone. We use multiple structures, resonance We use multiple structures, resonance

structures, to describe the molecule.structures, to describe the molecule.

ResonanceResonance

Just as green is a Just as green is a synthesis of blue and synthesis of blue and yellow…yellow…

……ozone is a synthesis ozone is a synthesis of these two of these two resonance structures.resonance structures.

ResonanceResonance

In truth, the electrons that form the second CIn truth, the electrons that form the second C——O bond in the double bonds below do not O bond in the double bonds below do not always sit between that C and that O, but always sit between that C and that O, but rather can move among the two oxygens and rather can move among the two oxygens and the carbon.the carbon.

They are not They are not localizedlocalized, but rather are , but rather are delocalizeddelocalized..

ResonanceResonance

The organic compound The organic compound benzene, Cbenzene, C66HH66, has two , has two resonance structures.resonance structures.

It is commonly It is commonly depicted as a hexagon depicted as a hexagon with a circle inside to with a circle inside to signify the delocalized signify the delocalized electrons in the ring.electrons in the ring.

Exceptions to the Octet Exceptions to the Octet RuleRule

There are three types of ions or There are three types of ions or molecules that do not follow the molecules that do not follow the octet rule:octet rule: Ions or molecules with an odd Ions or molecules with an odd

number of electrons.number of electrons. Ions or molecules with less than an Ions or molecules with less than an

octet.octet. Ions or molecules with more than Ions or molecules with more than

eight valence electrons (an expanded eight valence electrons (an expanded octet).octet).

Odd Number of ElectronsOdd Number of Electrons

Though relatively rare and usually Though relatively rare and usually quite unstable and reactive, there quite unstable and reactive, there are ions and molecules with an odd are ions and molecules with an odd number of electrons.number of electrons.

Fewer Than Eight Fewer Than Eight ElectronsElectrons

Consider BFConsider BF33:: Giving boron a filled octet places a Giving boron a filled octet places a

negativenegative charge on the boron and a charge on the boron and a positivepositive charge on fluorine. charge on fluorine.

This would not be an accurate picture This would not be an accurate picture of the distribution of electrons in BFof the distribution of electrons in BF33..


Therefore, structures that put a Therefore, structures that put a double bond between boron and double bond between boron and fluorine are much less important fluorine are much less important than the one that leaves boron with than the one that leaves boron with only 6 valence electrons.only 6 valence electrons.


The lesson is: If filling the octet of the The lesson is: If filling the octet of the central atom results in a negative central atom results in a negative charge on the central atom and a charge on the central atom and a positive charge on the more positive charge on the more electronegative outer atom, don’t fill electronegative outer atom, don’t fill the octet of the central atom.the octet of the central atom.

More Than Eight More Than Eight ElectronsElectrons

The only way PClThe only way PCl55 can exist is if can exist is if phosphorus has 10 phosphorus has 10 electrons around it.electrons around it.

It is allowed to It is allowed to expand the octet of expand the octet of atoms on the 3rd row atoms on the 3rd row or below.or below. Presumably Presumably dd orbitals orbitals

in these atoms in these atoms participate in bonding.participate in bonding.


Even though we can draw a Lewis structure Even though we can draw a Lewis structure for the phosphate ion that has only 8 for the phosphate ion that has only 8 electrons around the central phosphorus, electrons around the central phosphorus, the better structure puts a double bond the better structure puts a double bond between the phosphorus and one of the between the phosphorus and one of the oxygens.oxygens.


This eliminates the charge on the This eliminates the charge on the phosphorus and the charge on one of the phosphorus and the charge on one of the oxygens.oxygens.

The lesson is: When the central atom is on The lesson is: When the central atom is on the 3rd row or below and expanding its octet the 3rd row or below and expanding its octet eliminates some formal charges, do so.eliminates some formal charges, do so.

Covalent Bond StrengthCovalent Bond Strength

Most simply, the strength of a bond is Most simply, the strength of a bond is measured by determining how much measured by determining how much energy is required to break the bond.energy is required to break the bond.

This is the This is the bond enthalpybond enthalpy.. The bond enthalpy for a ClThe bond enthalpy for a Cl——Cl bond,Cl bond,

DD(Cl(Cl——Cl), is measured to be 242 kJ/mol.Cl), is measured to be 242 kJ/mol.

Average Bond EnthalpiesAverage Bond Enthalpies

This table lists the This table lists the average bond average bond enthalpies for enthalpies for many different many different types of bonds.types of bonds.

Average bond Average bond enthalpies are enthalpies are positive, because positive, because bond breaking is bond breaking is an endothermic an endothermic process.process.

Average Bond EnthalpiesAverage Bond Enthalpies

NOTE: These are NOTE: These are averageaverage bond bond enthalpies, not enthalpies, not absolute bond absolute bond enthalpies; the Centhalpies; the C——H bonds in H bonds in methane, CHmethane, CH44, will , will be a bit different be a bit different than thethan theCC——H bond in H bond in chloroform, CHClchloroform, CHCl33..

Enthalpies of ReactionEnthalpies of Reaction

Yet another way to Yet another way to estimate estimate HH for a for a reaction is to reaction is to compare the bond compare the bond enthalpies of bonds enthalpies of bonds broken to the bond broken to the bond enthalpies of the enthalpies of the new bonds formed.new bonds formed.

• In other words, Hrxn = (bond enthalpies of bonds broken)

(bond enthalpies of bonds formed)


CHCH44((gg)) + Cl + Cl22((gg))

CHCH33ClCl((gg)) + HCl + HCl((gg))

In this example, oneIn this example, one

CC——H bond and oneH bond and one

ClCl——Cl bond are Cl bond are broken; one Cbroken; one C——Cl Cl and one Hand one H——Cl bond Cl bond are formed.are formed.


So,So,

HHrxnrxn = [ = [DD(C(C——H) + H) + DD(Cl(Cl——Cl) Cl) [ [DD(C(C——Cl) + Cl) + DD(H(H——Cl)Cl)

= [(413 kJ) + (242 kJ)] = [(413 kJ) + (242 kJ)] [(328 kJ) + [(328 kJ) + (431 kJ)](431 kJ)]

= (655 kJ) = (655 kJ) (759 kJ) (759 kJ)

= = 104 kJ104 kJ

Bond Enthalpy and Bond Bond Enthalpy and Bond LengthLength

We can also measure an average bond We can also measure an average bond length for different bond types.length for different bond types.

As the number of bonds between two As the number of bonds between two atoms increases, the bond length atoms increases, the bond length decreases.decreases.

Chapter 8 Concepts of Chemical Bonding

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