Chapter 8 - Molecular Orbital Theory!

Question 1

MO theory general description

Answer 1

treats a covalent bond as a pair of electrons w/ bonding orbital derived from the overlap of orbitals of adjacent atoms

Question 2

paramagnetic

Answer 2

unpaired electrons, interacts with magnetic fields

Question 3

diamagnetic

Answer 3

no unpaired electrons, weakly influenced by magnetic field

Question 4

valence bond theory

Answer 4

localized application of MO theory to specific bonds

Question 5

superposition of waves

Answer 5

combined to produce wave with amplitude equivalent to sum of other two - or changing electron density

Question 6

covalent bond forms when

Answer 6

waves superimpose and increased electron density between nuclei

Question 7

opposite vs same phases and what time of orbitals are formed

Answer 7

2 wavefunctions w/ same phase (++ or –), constructive intereference forms a BONDING ORBITAL with increased electron density between the nuclei

wavefunctions w opposite phases, destructive interference forms ANTIBONDING ORBITAL, reduces electron density so works against bonding. nodal planes could form between nuclei

Question 8

bonding orbital

Answer 8

wavefunctions w same phase from constructive interference, increased electron density

when drawing, combine so pretty much two same phase circles slowly combine to form one bigger circle

lower energy

Question 9

antibonding orbital

Answer 9

wavefunctions w different phases destructively interfere to reduce electron density

nodal planes can form between nuclei

when drawing they dont really fully combine, like two diff coloured circles approaching, remain different just change shape, more electron density away from the other

higher energy

Question 10

where does optimal bond length occur?

Answer 10

at the distance with lowest possible energy, depends on identity of bonding atoms

Question 11

what does greater overlap in orbitals mean?

Answer 11

greater difference in energy between bonding and antibonding orbitals

Question 12

increases and decreases in energy w distance, antibonding and bonding

Answer 12

bonding orbitals - energy decreases as internuclear distance decreases

antibonding - energy increases as internuclear distance decreases

Question 13

affect repulsion between nuclei on PE

Answer 13

repulsion increases PE at short internuclear distances

Question 14

what would a representation of MOS using cross sections of 90% probability surface look like?

Answer 14

draw two atoms with 90% prob surface, ex for 1s in Hydrogen it would be two circles, with phase noted + or -, draw a + sign between and an arrow pointing to result, if same phase they would connect to form a kind of oblong orbital with two dots for the two nuclei, and indicate if + or -, if opposite phases, would end up funky and asymetrical, part of the 90% prob surface would be super close to the nuclei, and space in between

Question 15

rules for forming MOs

Answer 15

1. total MOs formed is equal to # atomic orbitals interaction
2. when 2 AOs combine, resulting bonding MO fewer nodes than antibonding MO
3. the more the AOs overlap the larger the energy gap bonding and antibonding orbitals
4. only AOs of similar energy interact significantly (ex 1s & 1s)
5. prob of finding an electron @ x = wavefunction squared
6. phase changes at each node
7. prob of finding electron @ node = 0

Question 16

how do bonding and antibonding orbitals differ from atomic orbitals?

Answer 16

bonding MO is lower in energy comp to AO, antibonding has greater energy by pretty much the same amount

Question 17

MO with more nodes higher or lower energy?

Answer 17

higher energy than those with few nodes

Question 18

construction of an orbital interaction diagram

Answer 18

atomic orbitals of seperate atoms shown on right and left side, the orbital name written under a line.

bonding and antibonding orbitals are drawn between them at relative energy levels, * rep antibonding, bonding below AO, antibonding above

dotted lines connect ao to bonding and antibonding, showing where electrons come from

Question 19

labelling of orbital diagrams

Answer 19

1. symmetry of orbital, the o with a line out the right top (sigma) for symmetric w respect to rotation about internuclear axis, pi if asymmetrical (one nodal plane containing internuclear axis)
2. whether bonding or antibonding (* for antibonding)
3. AO which MO originates from in subscript ex pi1s, 1s in subscript

Question 20

how to write electronic configuration of a molecule

Answer 20

ex for hydrogen it would be (sigma symbol)1s in subscript^2

Question 21

how does MO theory and orbital interaction diagrams show why H2 exists instead of H

Answer 21

energy level of bonded electrons in H2 is lower than the energy of the electrons unbonded, therefore energetically favourable

Question 22

equation for bond order of diatomic molecule w particular molecular configuration

Answer 22

Bond order = (e bonding - e antibonding)/2

where e bonding is # of electrons in all bonding orbitals

e antibonding is # of electrons in all antibonding orbitals

Question 23

steps for filling orbital interaction diagram w electrons

Answer 23

1. calculate total # electrons in AOs
2. Fill MOs w/ number of electrons calculated in step 1
   - fill lowest energy orbitals first (aufbau)
   - place max of 2 spin paired electrons each MO (pauli)
   -electrons in degenerate (same energy) MOs singly, w/
   same spin before pairing (hund’s rule)

Question 24

why doesn’t He2 exist?

Answer 24

antibonding = 2, # bonding = 2, therefore bond order = 0 so no bonds form and the element doesn’t exist

Question 25

homonuclear diatomic species

Answer 25

molecules made of exactly 2 identical atoms

Question 26

second row homonuclear diatomic species energy level diagrams

Answer 26

same sort of set up as fist row, first set up energy levels of seperate atoms, big space between 1s and 2s, do the dotted line thing, write proper amount of electrons into original AOs then transfer to MOs

Question 27

molecular orbitals that include p block

Answer 27

s block set up the same as normal, space between 1s 2s etc

in p block, start the same, set up ao configuration diagrams for the seperate atoms starting with 2p

assume x is the internuclear axis, therefore px would be symmetrical and sigma, py and pz would be asymmetrical and therefore pi

signma px would have the lowest energy of the ps, then pi pz and py, then pi star pz and py then sigma star px

Question 28

recap - what leads to greater energy difference between bonding and antibonding?

Answer 28

more overlap of AOs, orbitals with higher n value would have more overlap and therefore a greater energy difference

Question 29

HOMO

Answer 29

highest occupied molecular orbital - highest energy MO that contains any electrons

Question 30

LUMO

Answer 30

lowest unoccupied molecular orbital - molecular orbital next higher energy after highest occupied MO

Question 31

order that MOs fill up for F2

Answer 31

sigma 2s then sigma star 2s then sigma 2p then pi 2p then pi star 2p then sigma star 2p

Question 32

order that MOs fill up for O2

Answer 32

sigma 2s then sigma star 2s then sigma 2p then pi 2p then pi star 2p then sigma star 2p

Question 33

order that MOs fill up for N2

Answer 33

sigma 2s then sigma star 2s then pi 2p then sigma 2p then pi star 2p then sigma star 2p

Question 34

order that MOs fill up for C2

Answer 34

sigma 2s then sigma star 2s then pi 2p then sigma 2p then pi star 2p then sigma star 2p

Question 35

order that MOs fill up for B2

Answer 35

sigma 2s then sigma star 2s then pi 2p then sigma 2p then pi star 2p then sigma star 2p

Question 36

MO energy levels change across a period

Answer 36

energy levels decrease from left to right across a period bc increase in Z and Zeff

Question 37

rationale for change in relative energies of sigma 2p and pi 2p orbitals between N2 and O2

Answer 37

explained partially from looking at relative energies of 2s and 2p orbitals. when energies of 2s and 2p orbitals similarly mixing could happing, meaning sigma orbitals not derived from “pure” 2s-2s and 2px-2px but some 2px-2s

unsettles energy of all sigma orbitals therefore energy of sigma 2p is higher than pi 2p when 2s and 2p are similar in energy