chapter 10 p2

Question 1

what does molecular orbital theory apply to?

Answer 1

Applies quantum mechanics to molecules, similar to

how it was applied to atoms

Question 2

how does VB picture molecules?

Answer 2

VB pictures a molecule as a group of atoms bonded
through localized overlap of valence or hybrid
atomic orbitals

Question 3

what does MO assume about orbitals?

Answer 3

MO assumes that orbitals are delocalized over the

whole molecule and occupied by electrons

Question 4

how are MOS generated

Answer 4

You cannot solve Schroedinger’s equation exactly for
molecular orbitals
• Through a series of trial and errors we start with an
educated guess (trial function) and a computer program
tries to find the function that minimizes the energy of the
molecules orbitals
• MO’s can be mathematically approximated by the linear
combination of atomic orbitals (LCAO)

Question 5

what are MOs similarities as AO?

Answer 5

Similar characteristics as AO
!Each can hold two electrons
!The square of the wavefunction gives the probability
density (shape)

Question 6

what happens when two waves align and are in phase?

Answer 6

When two waves align
and are in phase, the
resulting wave is sum
of the two waves
!Constructive
interference

Question 7

what happens when two waves align out of phase?

Answer 7

• When two waves align
out of phase, the
resulting wave is the
difference of the two
wave
!Destructive
interference

Question 8

how many phases do s orbitals have?

Answer 8

• s-orbitals only have
one phase
!Either (+) or (-)

Question 9

how many phases do p oribtals have?

Answer 9

p-orbitals have two
phases
!(+) and (-)
!Each lobe is a
different phase

Question 10

what do we use to approximate molecular orbitals?

Answer 10

We use a linear combination of atomic orbitals (LCAO) to
approximate molecular orbitals - combine atomic
orbitals of similar energy

Question 11

when is a bonding oribtal formed? energy?

Answer 11

When the wave functions overlap constructively they
form a bonding orbital
• They are lower in energy than the original atomic orbitals
constructive interference

Question 12

what happens when wave functions overlap destructively?

Answer 12

• When wave functions overlap destructively they form
anti-bonding orbitals
! A node is created between two atoms (no electrons)
! Energy of the orbital is higher than that of the original
atomic orbitals

Question 13

look at mo diagram. what do the number of AOs equal?

Answer 13

The number of AOs combined always equals the

number of MOs formed

Question 14

what is bond order?

Answer 14

Bond Order = difference between number of

electrons in bonding and antibonding orbitals

Question 15

which electrons are considered for bond order? what does a higher bond order mean? bond order of 0?

Answer 15

• only need to consider valence electrons
• higher bond order = stronger and shorter bonds
• if bond order = 0, then bond is unstable compared to
  individual atoms and no bond will form

Question 16

formula for bond order

Answer 16

Bond Order = (# bonding electrons) - (# antibonding electrons) divided by
2

Question 17

BO of He2? does it exist?

Answer 17

= 0

Question 18

how many p orbitals can overlap end on end?

Answer 18

one, the other two cant

Question 19

interactions of p orbitals: constructive end to end

Answer 19

The constructive end-to-end overlap of p-orbitals results

in the formation of a bonding σ2p MO

Question 20

interactions of p orbitals: destructive end to end

Answer 20

The destructive end-to-end overlap of p-orbitals results in
the formation of an antibonding σ*
2p MO

Question 21

interactions of p orbitals: constructive side by side

Answer 21

Constructive side-by-side
overlap of p-orbitals results in
the formation of π2p bonding
orbitals

Question 22

interactions of p orbitals: destructive side by side

Answer 22

• Destructive side-by-side overlap
of p-orbitals results in the
formation of π*2p bonding
orbitals

Question 23

energy of MOs

Answer 23

• MOs formed from 2s orbitals
are lower in energy than MOs
from from 2p orbitals
!s orbitals are lower in
energy than p orbitals

Question 24

which orbitals overlap more extensively

Answer 24

Atomic p orbitals overlap
more extensively end to end
than side by side

Question 25

theta vs pi energy

Answer 25

!σ2p MO is usually lower in
energy than π2p
!Destabilizing effect of σ2p*
MO is greater than π2p
*

Question 26

look at diagrams for energy

Answer 26

ok

Question 27

mixing assumption

Answer 27

We might assume that the 2s and 2p do not mix when
we combine AO to form MO
• True LCAO treatment would combine all AO that are
relatively close in energy

Question 28

which elements have mixing?

leads to?

Answer 28

It occurs in some elements (B2,C2,N2), mixing between
the 2s and 2p (σ bond)
• Larger, less electronegative atoms have σ2s & σ2p
interaction
• Leads to reversal in the σ2p and π2p orbitals

Question 29

diagram with heteronuclear molecules?

Answer 29

Asymmetric MO diagram due to different
electronegativities
• Element with higher electronegativity is lower in energy
• If the molecule contains O or F, no s-p mixing occurs

Question 30

is oxygen magnetic? how do we know? know diamagnetic and paramagnetic

Answer 30

• Molecular oxygen is magnetic (O2)
Based on VB theory we have paired up all of the
electrons
• But if we look at the MO diagram we see there are two
unpaired electrons

Question 31

similarities between vb and mo

Answer 31

• In most cases involving σ bonding, MO theory produces
results very similar to the predictions in VB theory
• VB theory is simpler to apply to large molecules
• The failures of the valence bond theory usually involve π
bonding
!If you need to use resonance when drawing the Lewis
structure, valence bond theory will fail
• It is common to see MO theory applied to only π bonds
in qualitative descriptions of bonding in many molecules

Question 32

ex benzene

Answer 32

• Valence bond theory
does a good job of
describing the sigma
bonding in a molecule
of benzene
• MO theory is needed
to properly describe
the π bonds
!All of the C-C
bonds in benzene
are equivalent

Question 33

chart for bond angles and shapes

Answer 33

Electron
groups	Molecular
geometries	Angle
(degrees)	Hybrid orbitals
2	linear	180	sp
3	trigonal planar,
bent	120	sp2
4	tetrahedral,
trigonal pyramidal,
bent	109.5	sp3
5	trigonal bipyramidal,
seesaw,
T-shaped	90, 120, 180	sp3d
6	octahedral,
square pyramidal,
square planar	90, 180	sp3d2

Question 34

what combo gives us linear electron shape? molecular geometry? angles?

Answer 34

electron group 2, bonding groups 2, lone pairs 0

• linear
• 180

Question 35

what combo gives us trigonal planar electron shape? molecular geometry? angles?

Answer 35

either EG 3, BG 3, LP O or EG 3, BG 2, LP 1
330 is trigonal planar and 120
321 is bent and <120

Question 36

3 combos for tetrahedral electron shape

Answer 36

440, 431, 422
-440 is tetrahedral, 109.5
-431 is trigonal pyramidal, <109.5
422 is bent, <109.5

Question 37

4 combos for trigonal bipyramidal electron shape

Answer 37

550, 541, 532, 523

• 550 trigonal bipyramidal 120 equatorial, 90 axial
• 541 seesaw <120 (equatorial) <90 axial
• 532 t shaped <90
• 523 linear 180

Question 38

3 combos for octahedral electron group

Answer 38

• 660 octahedral 90
• 651 square pyramidal <90
• 642 square planar 90

Question 39

describe MO diagram with mixing

Answer 39

sigma star
pi star
sigma
pi

Question 40

describe mo diagram without mixing

Answer 40

sigma star
pi star
pi
sigma