week 3 Flashcards

1
Q

what are molecular orbitals made of

A

atomic orbitals

adding wavefunctions of AOs across the molecule

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

number of AOs = …

A

number of MOs

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

what 2 things must AOs fulfil to form MOs

A

AOs must match interns of symmetry

AOs must be close enough in energy to overlap
- the closer in energy, the larger the interaction

AOs must be of similar energies

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

in which two ways can AOs overlap

A

constructively/in-phase

  • peaks and troughs of wave line up perfectly
  • leads to a wave that is the sum of both waves

destructively/out-of-phase

  • waves are oppositely aligned
  • positive peaks of one coincide with negative peaks of the other
  • waves cancel out, leads to a node

see w3 vid13 for examples

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

what does a sigma orbital suggest about an orbital

A

sigma means the orbital is symmetric about the internuclear axis

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

what is used to denote an anti-bonding orbital

A

star - *

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

describe the orbital formed wen two in-phase 1s AOs overlap

A

a sigma gerade orbital

  • sigma bc symmetric about internuclear axis
  • gerade because symmetric wrt inversion centre
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what is a nodal plane in an orbital

A

region where there is 0 probability of finding electron density

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

what is more energetically stable, bonding or anti-bonding MOs

A

bonding MOs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what does bond order indicate

how is it calculated

A

overall strength of bonding interaction

= 1/2(no. of bonding e⁻- no. of anti-bonding e⁻)

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

describe the in phase and out of phase overlap of the 2pz orbital

A

2pz orbitals will overlap in a horizontal manner with the heads of the lobes overlapping into each other ∞↔︎∞like that

in phase
- sigma gerade orbital σg

out of phase
- sigma ungerade σu*

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

describe the in-phase and out of phase overlap of the 2px/y orbitals

why are they π orbitals

A

px and py form π orbitals because

  • the resulting MOs change sign when rotated 180° across internuclear axis
  • pi-orbitals are anti-symmetric wrt rotation around internuclear axis

they overlap like this = 8↔︎8 with the lobes vertical

in phase

  • lobes at the top and at the bottom are the same sign
  • forms πu orbital

out of phase
- pi gerade anti bonding orbital πg*

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

what does increased overlap between AOs mean for the energy of MOs

A

more overlap between AOs = bigger energy separation between bonding MO and anti-bonding MO

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

which have more overlap, pz or px/py

A

pz have greater overlap

so larger energy separation between bonding and anti-bonding MOs

see w3 vid14 for examples fo MO diagrams

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

what is sp mixing

A

when MOs of the same symmetry label interact with each other

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

describe the sp mixing in the 2nd energy level

what effect does this have on orbital energy?

A

σg MO from 2s orbitals and σg MO from 2pz orbitals will interact
just like two AOs of similar symmetry can combine, MOs of similar symmetry can combines

results in one MO of higher energy and one of lower energy

  • σg (2pz) will become destabilised and increase in energy, past the πu MOs of px and py
  • σg (2s) becomes more stable and moves down in energy

the same is observed for the two σu* orbitals from 2s and 2pz

17
Q

what is needed for sp mixing to happen

in which period 2 elements is this observed and in which ones is it not

A

2s and 2p energy gap must be small enough

in B, N and C, it is observed, energy gap is small enough

not seen in O and F because energy gap is too big

18
Q

what is the relationship between bond order and:

  • bond distance
  • bond dissociation enthalpy
A
  • higher bond order = greater bond distance

- higher bond order = higher bond dissociation

19
Q

how do the following vary as you go down a group:

  • bond order
  • bond distance
  • bond dissociation
A
  • bond order is the same
  • bond distance increases
  • bond dissociation generally decreases
20
Q

when constructing MO diagrams for heteronuclear diatomic, what is not necessary

A

gerade and ungerade because there is no centre of inversion in heteronuclear molecules

21
Q

MO diagrams for heteronuclear diatomics:

which element will the bonding MO have more character of

A

will have more character of the more electronegative element

see w3 vid16 fo examples of heteronuclear diatomic MO diagram

22
Q

MO diagrams for heteronuclear diatomics:

which element will the anti-bonding MO have more character of

A

will have more character of the less electronegative element

23
Q

define electronegativity

give the trends of it

A

the ability of an atom in a molecule to attract electrons to itself = greek latter kai ϗ

decreases down groups
increases left to right

24
Q

how does electronegativity affect stability of orbitals

A

more electronegative = more stable orbital

electrons are held more ‘tightly’ towards highly attractive atom and so more stable

25
Q

how do energies of AOs differ with electronegativity

A

AOs of more electronegative element are always lower in energy

see w3 vid17 for examples