Molecular orbitals (Pike)

Question 1

compare and contrast atomic and molecular orbitals

Answer 1

atomic orbitals
- is a wavefunction describing behaviour of an electron in an atom
- electron influenced by the nucleus and other electrons present
- Aos are the solution to Schrodingers equation for the atom
- core electrons are low energy orbitals close to the nucleus
- valence electrons are in high energy orbitals

molecular orbitals
- is a wavefunction describing the behaviour of an electron in a molecule
- each electron is influenced by all the nuclei and other electrons
- MO are the solutions to Schrodinger equation for the molecule
- each atoms has its own core electrons that are not shared with other atoms
- valence electrons are shared amongst atoms and occupy molecular orbitals rather than atomic ones
- each molecular orbital extends over the whole molecule not just 2 atoms

Question 2

how are molecular orbitals formed

Answer 2

when bringing the 2 nuclei together the 1s orbitals begin to overlap, the electron can be shared by the 2 nuclei. the electron density is spread between the first nucleus, the second nucleus and the region in between, this sheilds the positive nuclei from each other. Tje overall energy is reduced this represents a bonding effect and the electron is now in a molecular orbital not an atomic one

Question 3

what happens if the electron is near the left or right nucleus

Answer 3

the influence of one nuclei is greater than the other
the behaviour of the electron would be very similar to that in a 1s atomic orbital of the nucleus

Question 4

what happens if the electron is between the two nuclei

Answer 4

it feels the influence of both nuclei

Question 5

what is LCAO

Answer 5

the linear combination of atomic orbitals
wavefunction describing the molecular orbitals
mathematically this means adding together the atomic orbitals with appropriate coefficents to create the molecular ones
the cannot add up to more than one as a square as probability cannot be more than 1

Question 6

when are interactions between atomic orbitals allowed

Answer 6

• if the symmetries of the atomic orbitals are comparable
• efficient if the region of overlap between the atomic orbitals is significant
• if the atomic orbitals are relatively close in energy
  the size of the overlap depends on how well these criteria are met

Question 7

what happens when two atoms overlap in terms of electron density

Answer 7

extra electron density built by constructive interference (as electrons are behaving like waves) of the two wavefunctions. the integral of this extra electron density is a measure of the bonding effect obtained by combining the two atomic orbitals

Question 8

what is a bonding molecular orbital

Answer 8

a molecular orbital with a positive overlap of the atomic orbitals
the energy of bonding molecular orbitals is lower than the energies of the atomic orbitals from which it is formes

Question 9

why are overlap integrals not always positive

Answer 9

the sign and value of an overlap integral depends on the size, shape, orientation and phase of atomic orbitals, the value of these can be positive, negative or zero

Question 10

what is positive overlap

Answer 10

atomic orbitals with lobes of the same phase come together, bonding contribution. the energy of the molecular orbital is lower than that of the atomic orbital

Question 11

what is negative overlap

Answer 11

atomic orbitals with lobes of different phase come together. A negative overlap integral represents an antibonding contribution. the energy of the molecular orbital formed is higher than that of atomic orbitals

Question 12

what is zero overlap

Answer 12

atomic orbitals give positive overlap in some regions and negative in others. these cancel each other out by symmetry
zero orbitals overlap means there is no bonding contribution.
atomic orbitals are remained unchanged in form and energy

Question 13

explain atomic orbitals of the same atoms in terms of orthogonality

Answer 13

orthogonal means they are perpendicular to each other, this implies they do not overlap with each other
- the overlap integral of any antomic orbital with itself is 1
- the overlap integral of any two different atomic orbitals on the same atom is 0

Question 14

explain the atomic orbitals from different atoms in terms of orthogonality

Answer 14

the overlap integral of two atomic orbitals on different atoms is a measure of their bonding interaction.
if this is zero there is no contribution to bonding from the pair of orbitals
(non-bonding)

Question 15

what is the number of molecular orbitals we get when we combine atomic orbitals

Answer 15

when combining atomic orbitals to form molecular orbitals the number of molecular orbitals will be equal to the number of atomic orbitals

Question 16

what are the properties of energy level diagrams

Answer 16

energy level diagrams show which molecular orbitals have a lower or higher energy
the number of electron in this molecule will be the sum of electrons of the atoms.

Question 17

what are the rules in energy level diagrams

Answer 17

Pauli exclusion principle each orbital can host a maxium of 2 electrons
Aufbau principle - orbitals are filled in order of increasing energy
Hunds rule - degenerate orbitals will be filled with the maximum number of spin paired electrons

Question 18

how do we know if a molecular orbital is energetically favourable

Answer 18

the total energy of the molecule is lower than the energy of the separate atoms/ ions. meaning the formation of the molecule is more favourable than having the individual atoms or ions

Question 19

how do we calculate bond order

Answer 19

1/2( number of bonding electrons - number of antibonding )

Question 20

what are the different bonds that p orbitals can produce

Answer 20

p atomic orbitals can interact in different ways depending on their orientation. P orbitals are degenerate but this is lost when two atoms form a bond.
2 pz overlap to give sigma and sigma star
2 px or 2 py overlap to give a pie bond
we start with 6p atomic orbitals 3 from each atom and we obtain 6 molecular orbitals

Question 21

why is sigma s* lower in energy than sigma p

Answer 21

some bonding MO are higher in energy than antibonding molecular orbitals
the term bonding an none bonding are relative to the contributing AOs and 2p orbitals have a higher energy than 2s

Question 22

what does a higher bond order mean

Answer 22

shorter and stronger bond

Question 23

what is the importance of unpaired electrons

Answer 23

unpaired electrons have their own spin and their own magnetic field. a substance which has unpaired electrons are called paramagnetic

Question 24

what is s-p mixed orbitals

Answer 24

so far we have assumed that the s and p atomic orbitals behave independently
for B2, C2 and N2 mixing becomes important, orbitals with the same symmetry interact for example 2s and 2pz both for sigma bonds will combine with each other. they will generate new orbitals
MIXING MOLECULAR ORBITALS NOT ATOMIC ONES
the sigma p bonding orbital has swapped with the pie bonding orbital

Question 25

what are the differences in the energy level diagrams for the following
[O2]+
O2
[O2]-
[O2]2

Answer 25

the energy level diagram is mostly the same
the exact energies of each level is different due to different number of electrons repelling each other
the number of antibonding electrons is different
bond strength decreases whereas bond length increases

Question 26

how do energy level diagrams change with heteronuclear diatomic molecules

Answer 26

atomic orbitals of the two atoms will have different energies, the energies of atomic orbitals a lower for the more electronegative atom. the energy level diagrams are not symmetrical

Question 27

what does isoelectronic mean

Answer 27

same/ similar amounts of electrons

Question 28

examples of heteronuclear diatomics

Answer 28

CO, NO, [CN]-, [NO]+

Question 29

what happens if the hetero atoms are far apart in energy

Answer 29

if AO1 and AO2 have very different energies there is essentially no overlap even though geometry allows it. The resulting MO is almost unaltered AO.
unequal coefficients when constructing the wave functions of MO

Question 30

what makes a molecule more stable

Answer 30

if the bonding and non-bonding MOs are filled while the antibonding is empty the molecule is often more stable

Question 31

what are the most important orbitals

Answer 31

HOMO - highest occupied molecular orbital
LUMO - lowest unoccupied molecular orbital

Question 32

describe features of the HOMO

Answer 32

the HOMO has got electrons which it can give away, donating electrons is therefore a lewis base

Question 33

describe features of the LUMO

Answer 33

the LUMO is empty there for it tends to act as a lewis acid accepting electron density

Question 34

why are NO+, CN- and CO know as sigma donor pie acceptors

Answer 34

donates electron density from the sigma 3
accepts electron density into pie*

Question 35

how does water being bend effect its energy level diagram

Answer 35

the most important change is that the px orbital can overlap with H 1s orbitals when the molecule is bent
the px orbital can contribute to the bonding/antibonding effect instead of being non-bonding
the overlap integral of the pz orbital becomes less good when the molecule is bent resulting in a change in energy of the MO.

Question 36

why is the energy level diagram of CO2 more complicates

Answer 36

CO2 is known to be a linear molecule and it has more valence orbitals on the outer atoms

Question 37

how are the antibonding/bonding MO with a sigma symmetry formed for CO2

Answer 37

2s and 2pz orbitals from the C atom
2s and 2pz orbitals from the O atoms

Question 38

how can we simplify the energy level diagram for CO2

Answer 38

oxygen is more electronegative than carbon so the AO of O are much lower in energy. To simplify we will use an approximation: O 2s orbitals dont combine with C2s or C2pz

Question 39

describe the energy level diagram of CO2

Answer 39

oxygens 2s AOs can form bonding and antibonding MO they do not combine with C 2s or 2Pz due to a difference in energy
- oxygens 2pz AOS form bonding and antibonding MOs with C 2s and 2pz AOS
- O 2px AOSs can form bonding and antibonding MOs with C 2px AOS the same thing happens to 2py
- one O 2px and one 2py remains none bonding
- there are 16 valence electrons

Question 40

how would a metal complex for an octahedral geometry be produced

Answer 40

if all ligands are symmetrical the complex has an octahedral symmetry, symmetry dictates which orbital from the metal and the ligands can overlap
only the AOs with an eg geometry will be able to overlap with the AOs of the ligands

Question 41

what symmetry labels are appropriate for octahedral symmetry

Answer 41

a1g, eg, t1u, t2g

Question 42

what are hybrid orbitals

Answer 42

linear combinations of the primary atomic orbitals of the same atom

Question 43

why do we use hybrid orbitals

Answer 43

they are a simple approximate way of showing which orbitals contribute effectively to the LCAO

Question 44

which of the d orbitals have eg geometry

Answer 44

dz^2 and dx^2-y^2

Question 45

which of the d orbitals have t2g geometry

Answer 45

dxy, dxz, dyz

Question 46

Draw the CO2 energy level diagram with mixing

Answer 46

See sheet

Question 47

Draw the HF energy level diagram

Answer 47

See sheet

Question 48

Draw the NO energy level diagram with mixing

Question 49

Draw the CN- energy level with mixing

Question 50

Draw the BeH2 energy diagram

Question 51

Draw the H2O energy diagram

Question 52

Draw the B2 energy level with mixing

Question 53

Draw the C2 energy level diagram with mixing

Question 54

Draw the N2 energy level diagram with mixing

Question 55

Draw the CO energy level diagram with mixing