1. Orbitals and Covalent Bond
Chapter 9
Orbitals and Covalent Bond

2. Atomic Orbitals Don’t Work
to explain molecular geometry.
In methane, CH4 , the shape s tetrahedral.
The valence electrons of carbon should be two in s, and two in p.
the p orbitals would have to be at right angles.
The atomic orbitals change when making a molecule

3. Hybridization
We blend the s and p orbitals of the valence electrons and end up with the tetrahedral geometry.
We combine one s orbital and 3 p orbitals.
sp3 hybridization has tetrahedral geometry.

4. In terms of energy 2p
Hybridization
sp3
Energy 2s

5. How we get to hybridization
We know the geometry from experiment.
We know the orbitals of the atom
hybridizing atomic orbitals can explain the geometry.
So if the geometry requires a tetrahedral shape, it is sp3 hybridized
This includes bent and trigonal pyramidal molecules because one of the sp3 lobes holds the lone pair.

6. sp2 hybridization C2H4 Double bond acts as one pair. trigonal planar
Have to end up with three blended orbitals.
Use one s and two p orbitals to make sp2 orbitals.
Leaves one p orbital perpendicular.

9. In terms of energy 2p 2p
Hybridization
sp2
Energy 2s

10. Where is the P orbital? Perpendicular
The overlap of orbitals makes a sigma bond (s bond)

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

12. H H C C H H

13. sp2 hybridization When three things come off atom. trigonal planar
120º
on s one p bond

14. What about two When two things come off. One s and one p hybridize.
linear

15. sp hybridization End up with two lobes 180º apart.
p orbitals are at right angles
Makes room for two p bonds and two sigma bonds.
A triple bond or two double bonds.

16. In terms of energy 2p 2p sp
Hybridization
Energy 2s

17. CO2
C can make two s and two p
O can make one s and one p O C O

18. Breaking the octet
PCl5
The model predicts that we must use the d orbitals.
dsp3 hybridization
There is some controversy about how involved the d orbitals are.

19. dsp3 Trigonal bipyrimidal can only s bond. can’t p bond.
basic shape for five things.

20. PCl5 Can’t tell the hybridization of Cl
Assume sp3 to minimize repulsion of electron pairs.

21. d2sp3
gets us to six things around
octahedral

22. Molecular Orbital Model
Localized Model we have learned explains much about bonding.
It doesn’t deal well with the ideal of resonance, unpaired electrons, and bond energy.
The MO model is a parallel of the atomic orbital, using quantum mechanics.
Each MO can hold two electrons with opposite spins
Square of wave function tells probability

23. What do you get? Solve the equations for H2 HA HB get two orbitals
MO1 = 1sA - 1sB
MO2 = 1sA + 1sB

24. The Molecular Orbital Model
The molecular orbitals are centered on a line through the nuclei
MO1 the greatest probability is between the nuclei
MO2 it is on either side of the nuclei
this shape is called a sigma molecular orbital

25. The Molecular Orbital Model
In the molecule only the molecular orbitals exist, the atomic orbitals are gone
MO1 is lower in energy than the 1s orbitals they came from.
This favors molecule formation
Called an bonding orbital
MO2 is higher in energy
This goes against bonding
antibonding orbital

26. The Molecular Orbital Model
Energy 1s 1s
MO1

27. The Molecular Orbital Model
We use labels to indicate shapes, and whether the MO’s are bonding or antibonding.
MO1 = s1s
MO2 = s1s* (* indicates antibonding)
Can write them the same way as atomic orbitals
H2 = s1s2

28. The Molecular Orbital Model
Each MO can hold two electrons, but they must have opposite spins
Orbitals are conserved. The number of molecular orbitals must equal the number atomic orbitals that are used to make them.

29. H2-
s1s*
Energy 1s 1s
s1s

30. Bond Order
The difference between the number of bonding electrons and the number of antibonding electrons divided by two