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CHM1051 > Valence Bond Theory > Flashcards

Valence Bond Theory Flashcards

1
Q

hybrid orbitals

A

when atomic orbitals are combined eg. s orbital combines with a p orbital to form sp hybrids
- explains bonding in molecules

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2
Q

hybridisation

A

the process of forming hybrid orbitals (mixing). energy level becomes the weighted average

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3
Q

sp hybrid orbital

A

one s and one px orbital mix to produce two sp orbitals

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4
Q

sp2 hybrid orbital

A

one s orbital, one px and one py orbital
three sp2 hybrid orbitals
120 degrees between each orbital

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5
Q

sp3 hybrid orbital

A

s, px, py and pz
4 sp3 orbitals form
109.5 degrees apart (tetrahedral)

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6
Q

sigma bond

A
  • type of covalent bond
  • if you rotate the axis between two nuclei the electron distribution looks exactly the same (cylindrically symmetrical)
  • head on overlap
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7
Q

pi bond

A
  • type of covalent bond
  • side by side overlap of unhybridised p orbitals
  • rotation is restricted (double bond would need to break)
  • in a double bond one bond is pi and the other is sigma
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8
Q

what is a nodal plane?

A

where finding an electron is 0

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9
Q

how to determine types of orbitals

A
  • draw lewis structure
  • apply VSEPR to determine the number of electron domains
  • identify geometry which will give us the hybridisation of the atomic orbitals (eg. if tetrahedral then it is sp3)
  • trigonal planar is sp2
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10
Q

do orbitals disappear in hybridisation?

A

no

- if px and py are hybridised, pz does not disappear etc.

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11
Q

drawing hybrid orbitals

A
  • after determining types of bonds and parent geometry
  • draw each atom separately and then combine them
  • two regions of pi result in one pi bond
  • label all bonds
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12
Q

molecular orbital theory

A
  • based on schrodinger wave equation
  • describes regions where electrons might occupy
  • wavefunctions are constructed
  • constructive and deconstructive interferences of waves need to be considered
  • useful for predicting if molecules will exist given the constituent atoms electron config, strength of covalent bonds and magnetic properties of molecules
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13
Q

how many molecular orbitals?

A

MO = the number of atomic orbitals used

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14
Q

In phase

A

peaks and troughs have same amplitude

  • sum of waves has 2x amplitude
  • reinforce, constructive
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15
Q

out of phase

A

peaks and troughs have opposite amplitudes and cancel

  • cancel to zero amplitude
  • deconstructive
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16
Q

hydrogen example

A
  • valence orbital is 1s and has one electron
  • 1s from each hydrogen bonding (2) so 2AOs
  • combine wave functions to form 2 MOs
  • linear combination of AOs
17
Q

addition

A
  • results in constructive interference
  • large electron density between two nuclei
  • favourable charge interaction (electrons act as glue and MO has lower energy than isolated atoms)
  • bonding MO is called sigma1s (for hydrogen)
  • sigma because of symmetry
  • 1s because of where the orbitals were derived from
18
Q

subtraction

A
  • results in destructive interference
  • little electron density between nuclei
  • a node is at the centre of the internuclear space
  • non-favourable charge interaction (nuclei repel, unstable, higher energy than AOs)
  • anti-bonding MO, sigma star
19
Q

rules of MO

A
  • aufbau
  • pauli exclusion
  • Hund’s rule
  • total number of electrons from each AO = electrons in MO
20
Q

diamagnetic

A

all MOs are occupied and paired up in molecules and do no exhibit magnetic properties

21
Q

paramagnetic

A

partially filled MOs in molecules means that they do exhibit magnetic properties

22
Q

electron spin

A
  • spin gives rise to magnetic properties

- when electrons in MOS are paired up, spin cancels, giving spin of zero

23
Q

bond order

A

how strongly two nuclei interact. higher the bond order, the stronger the bond

24
Q

bond order for MO equation

A

bond order = 1/2 (number of bonding electrons - no of anti bonding electrons)

25
Q

bond orders

A

1 - single bond
2 - double bond
3 - triple bond
(fractions are possible)

26
Q

trends in bond order

A

higher bond order:

  • stronger bond
  • larger bond enthalpy (energy stored in bonds)
  • shorter bond length
27
Q

2pz MO

A
  • head on overlap (sigma)
  • destructive with node
  • constructive with overlap
28
Q

2py MO

A
  • overlap sideways to form pi bond (constructive)

- have a node (anti bonding pi)

29
Q

do px form pi orbitals?

A

yes

30
Q

drawing MO diagrams

A
  • set up boxes for the electrons (omitting core) of both individual molecules and the MO
  • label AOs and MOs at bottom
  • s electrons form sigma bonds
  • p electrons form sigma and pi orbitals
  • two electrons per box
  • dotted lines form bonds
31
Q

sigma - pi crossover

A
  • 2s and 2pz could mix if close enough in energy
  • this is why B2 is paramagnetic
  • oxygen and onwards cannot do this
32
Q

heteronuclear diatomic molecules

A
  • can use to MO model
  • must consider how similar/different energy states of orbitals are
  • look at symmetry and energy to see if atoms can bond

CHM1051 (9 decks)

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