Valence Bond Theory Flashcards
hybrid orbitals
when atomic orbitals are combined eg. s orbital combines with a p orbital to form sp hybrids
- explains bonding in molecules
hybridisation
the process of forming hybrid orbitals (mixing). energy level becomes the weighted average
sp hybrid orbital
one s and one px orbital mix to produce two sp orbitals
sp2 hybrid orbital
one s orbital, one px and one py orbital
three sp2 hybrid orbitals
120 degrees between each orbital
sp3 hybrid orbital
s, px, py and pz
4 sp3 orbitals form
109.5 degrees apart (tetrahedral)
sigma bond
- type of covalent bond
- if you rotate the axis between two nuclei the electron distribution looks exactly the same (cylindrically symmetrical)
- head on overlap
pi bond
- 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
what is a nodal plane?
where finding an electron is 0
how to determine types of orbitals
- 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
do orbitals disappear in hybridisation?
no
- if px and py are hybridised, pz does not disappear etc.
drawing hybrid orbitals
- 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
molecular orbital theory
- 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
how many molecular orbitals?
MO = the number of atomic orbitals used
In phase
peaks and troughs have same amplitude
- sum of waves has 2x amplitude
- reinforce, constructive
out of phase
peaks and troughs have opposite amplitudes and cancel
- cancel to zero amplitude
- deconstructive
hydrogen example
- 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
addition
- 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
subtraction
- 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
rules of MO
- aufbau
- pauli exclusion
- Hund’s rule
- total number of electrons from each AO = electrons in MO
diamagnetic
all MOs are occupied and paired up in molecules and do no exhibit magnetic properties
paramagnetic
partially filled MOs in molecules means that they do exhibit magnetic properties
electron spin
- spin gives rise to magnetic properties
- when electrons in MOS are paired up, spin cancels, giving spin of zero
bond order
how strongly two nuclei interact. higher the bond order, the stronger the bond
bond order for MO equation
bond order = 1/2 (number of bonding electrons - no of anti bonding electrons)
bond orders
1 - single bond
2 - double bond
3 - triple bond
(fractions are possible)
trends in bond order
higher bond order:
- stronger bond
- larger bond enthalpy (energy stored in bonds)
- shorter bond length
2pz MO
- head on overlap (sigma)
- destructive with node
- constructive with overlap
2py MO
- overlap sideways to form pi bond (constructive)
- have a node (anti bonding pi)
do px form pi orbitals?
yes
drawing MO diagrams
- 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
sigma - pi crossover
- 2s and 2pz could mix if close enough in energy
- this is why B2 is paramagnetic
- oxygen and onwards cannot do this
heteronuclear diatomic molecules
- 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
