Orbitalsand Covalent Bonds. Atomic Orbitals Don’t Work n to explain molecular geometry. n In methane, CH 4, the shape s tetrahedral. n The valence electrons.

Orbitals Orbitals

and and

Covalent BondsCovalent Bonds

Atomic Orbitals Don’t WorkAtomic Orbitals Don’t Work to explain molecular geometry.to explain molecular geometry. In methane, CHIn methane, CH44 , the shape s , the shape s

tetrahedral.tetrahedral. The valence electrons of carbon should The valence electrons of carbon should

be two in s, and two in p.be two in s, and two in p. the p orbitals would have to be at right the p orbitals would have to be at right

angles.angles. The atomic orbitals change when The atomic orbitals change when

making a molecule making a molecule

HybridizationHybridization We blend the s and p orbitals of the We blend the s and p orbitals of the

valence electrons and end up with the valence electrons and end up with the tetrahedral geometry.tetrahedral geometry.

We combine one s orbital and 3 p We combine one s orbital and 3 p orbitals.orbitals.

spsp33 hybridization has tetrahedral hybridization has tetrahedral geometry.geometry.

In terms of energyIn terms of energyIn terms of energyIn terms of energyE

nerg

y

2p

2s

Hybridization sp3

How we get to hybridizationHow we get to hybridization We know the geometry from experiment.We know the geometry from experiment. We know the orbitals of the atomWe know the orbitals of the atom hybridizing atomic orbitals can explain hybridizing atomic orbitals can explain

the geometry.the geometry. So if the geometry requires a tetrahedral So if the geometry requires a tetrahedral

shape, it is spshape, it is sp33 hybridized hybridized This includes bent and trigonal pyramidal This includes bent and trigonal pyramidal

molecules because one of the spmolecules because one of the sp33 lobes lobes holds the lone pair.holds the lone pair.

spsp22 hybridization hybridization CC22HH44

Double bond acts as one pair.Double bond acts as one pair. trigonal planartrigonal planar Have to end up with three blended Have to end up with three blended

orbitals.orbitals. Use one s and two p orbitals to make Use one s and two p orbitals to make

spsp22 orbitals. orbitals. Leaves one p orbital perpendicular.Leaves one p orbital perpendicular.

In terms of energyIn terms of energyIn terms of energyIn terms of energyE

nerg

y

2p

2s

sp2Hybridization

2p

Where is the P orbital?Where is the P orbital? PerpendicularPerpendicular The overlap of The overlap of

orbitals makes a orbitals makes a sigma bond (sigma bond ( bond)bond)

Two types of BondsTwo types of Bonds Sigma bonds from overlap of orbitals.Sigma bonds from overlap of orbitals. Between the atoms.Between the atoms. Pi bond (Pi bond ( bond) above and below atoms bond) above and below atoms Between adjacent p orbitals.Between adjacent p orbitals. The two bonds of a The two bonds of a

double bond.double bond.

CCH

H

H

H

spsp22 hybridization hybridization When three things come off atom.When three things come off atom. trigonal planartrigonal planar 120º120º on on one one bond bond

What about twoWhat about two When two things come off.When two things come off. One s and one p hybridize.One s and one p hybridize. linearlinear

sp hybridizationsp hybridization End up with two lobes 180º End up with two lobes 180º

apart.apart. p orbitals are at right p orbitals are at right

anglesangles Makes room for two Makes room for two

bonds and two sigma bonds and two sigma bonds.bonds.

A triple bond or two double A triple bond or two double bonds.bonds.

In terms of energyIn terms of energyIn terms of energyIn terms of energyE

nerg

y

2p

2s

Hybridizationsp

2p

COCO22

C can make two C can make two and two and two O can make one O can make one and one and one

CCOO OO

Breaking the octetBreaking the octet PClPCl55 The model predicts that we must use The model predicts that we must use

the d orbitals.the d orbitals. dspdsp33 hybridization hybridization There is some controversy about how There is some controversy about how

involved the d orbitals are.involved the d orbitals are.

dspdsp33

Trigonal Trigonal bipyrimidalbipyrimidal

can only can only bond. bond. can’t can’t bond. bond. basic shape for basic shape for

five things.five things.

PClPCl55

Can’t tell the hybridization of Cl

Assume sp3 to minimize repulsion of electron pairs.

dd22spsp33

gets us to six things aroundgets us to six things around octahedraloctahedral

Molecular Orbital ModelMolecular Orbital Model Localized Model we have learned explains Localized Model we have learned explains

much about bonding.much about bonding. It doesn’t deal well with the ideal of It doesn’t deal well with the ideal of

resonance, unpaired electrons, and bond resonance, unpaired electrons, and bond energy.energy.

The MO model is a parallel of the atomic The MO model is a parallel of the atomic orbital, using quantum mechanics.orbital, using quantum mechanics.

Each MO can hold two electrons with Each MO can hold two electrons with opposite spinsopposite spins

Square of wave function tells probabilitySquare of wave function tells probability

What do you get?What do you get? Solve the equations for HSolve the equations for H22

HHA A H HBB

get two orbitalsget two orbitals

MOMO11 = 1s = 1sAA - 1s - 1sBB

MOMO22 = 1s = 1sAA + 1s + 1sBB

The Molecular Orbital ModelThe Molecular Orbital ModelThe molecular orbitals are centered on The molecular orbitals are centered on a line through the nucleia line through the nuclei

– MOMO11 the greatest probability is the greatest probability is

between the nucleibetween the nuclei

– MOMO22 it is on either side of the nuclei it is on either side of the nuclei

– this shape is called a sigma molecular this shape is called a sigma molecular orbitalorbital

The Molecular Orbital ModelThe Molecular Orbital ModelIn the molecule only the molecular orbitals In the molecule only the molecular orbitals exist, the atomic orbitals are goneexist, the atomic orbitals are gone

MOMO11 is lower in energy than the 1s orbitals is lower in energy than the 1s orbitals they came from.they came from.– This favors molecule formationThis favors molecule formation– Called an bonding orbitalCalled an bonding orbital

MOMO22 is higher in energy is higher in energy– This goes against bondingThis goes against bonding– antibonding orbital antibonding orbital

The Molecular Orbital ModelThe Molecular Orbital ModelE

nerg

y

MO2

MO1

ssss

The Molecular Orbital ModelThe Molecular Orbital ModelWe use labels to indicate shapes, and We use labels to indicate shapes, and whether the MO’s are bonding or whether the MO’s are bonding or antibonding.antibonding.

– MOMO1 1 = = 1s1s

– MOMO2 2 = = 1s1s* (* indicates antibonding)* (* indicates antibonding)

Can write them the same way as atomic Can write them the same way as atomic orbitalsorbitals

– HH22 = = 1s1s22

The Molecular Orbital ModelThe Molecular Orbital ModelEach MO can hold two electrons, but Each MO can hold two electrons, but they must have opposite spinsthey must have opposite spins

Orbitals are conserved. The number of Orbitals are conserved. The number of molecular orbitals must equal the molecular orbitals must equal the number atomic orbitals that are used to number atomic orbitals that are used to make them.make them.

HH22--

Ene

rgy

1s1s

1s1s**

ssss

Bond OrderBond Order The difference between the number of The difference between the number of

bonding electrons and the number of bonding electrons and the number of antibonding electrons divided by twoantibonding electrons divided by two

Bond Order = # bonding-#antibonding

2