chapter 10 p2 Flashcards

1
Q

what does molecular orbital theory apply to?

A

Applies quantum mechanics to molecules, similar to

how it was applied to atoms

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

how does VB picture molecules?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what does MO assume about orbitals?

A

MO assumes that orbitals are delocalized over the

whole molecule and occupied by electrons

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

how are MOS generated

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what are MOs similarities as AO?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what happens when two waves align and are in phase?

A
When two waves align
and are in phase, the
resulting wave is sum
of the two waves
!Constructive
interference
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what happens when two waves align out of phase?

A
• When two waves align
out of phase, the
resulting wave is the
difference of the two
wave
!Destructive
interference
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

how many phases do s orbitals have?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

how many phases do p oribtals have?

A
p-orbitals have two
phases
!(+) and (-)
!Each lobe is a
different phase
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what do we use to approximate molecular orbitals?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

when is a bonding oribtal formed? energy?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what happens when wave functions overlap destructively?

A

• 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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

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

A

The number of AOs combined always equals the

number of MOs formed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what is bond order?

A

Bond Order = difference between number of

electrons in bonding and antibonding orbitals

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

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

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

formula for bond order

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

BO of He2? does it exist?

18
Q

how many p orbitals can overlap end on end?

A

one, the other two cant

19
Q

interactions of p orbitals: constructive end to end

A

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

in the formation of a bonding σ2p MO

20
Q

interactions of p orbitals: destructive end to end

A

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

21
Q

interactions of p orbitals: constructive side by side

A

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

22
Q

interactions of p orbitals: destructive side by side

A

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

23
Q

energy of MOs

A
• MOs formed from 2s orbitals
are lower in energy than MOs
from from 2p orbitals
!s orbitals are lower in
energy than p orbitals
24
Q

which orbitals overlap more extensively

A

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

25
theta vs pi energy
``` !σ2p MO is usually lower in energy than π2p !Destabilizing effect of σ2p* MO is greater than π2p * ```
26
look at diagrams for energy
ok
27
mixing assumption
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
28
which elements have mixing? | leads to?
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
29
diagram with heteronuclear molecules?
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
30
is oxygen magnetic? how do we know? know diamagnetic and paramagnetic
• 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
31
similarities between vb and mo
• 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
32
ex benzene
``` • 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 ```
33
chart for bond angles and shapes
``` 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 ```
34
what combo gives us linear electron shape? molecular geometry? angles?
electron group 2, bonding groups 2, lone pairs 0 - linear - 180
35
what combo gives us trigonal planar electron shape? molecular geometry? angles?
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
36
3 combos for tetrahedral electron shape
440, 431, 422 -440 is tetrahedral, 109.5 -431 is trigonal pyramidal, <109.5 422 is bent, <109.5
37
4 combos for trigonal bipyramidal electron shape
550, 541, 532, 523 - 550 trigonal bipyramidal 120 equatorial, 90 axial - 541 seesaw <120 (equatorial) <90 axial - 532 t shaped <90 - 523 linear 180
38
3 combos for octahedral electron group
- 660 octahedral 90 - 651 square pyramidal <90 - 642 square planar 90
39
describe MO diagram with mixing
sigma star pi star sigma pi
40
describe mo diagram without mixing
sigma star pi star pi sigma