Chemistry Ch 9
bond angles for hybrid orbitals are related to what
% s character and % p character they contain
what is the calculation for bond order
(bonding electrons/antibonding electrons)/2
what are the main points of the VB approach
- orbitals overlap to form a bond between two atoms - overlapping orbitals hold two electrons of opposite spin. usually one electron is supplied by each of the two bonded atoms - the bonding electrons are localized with a higher probability of being found within a region of space between the bonding nuclei. both electrons are simultaneously associated with both nuclei
what are the advantages of MO theory
- provides quantitative information on energies of orbitals, charge distribution - describes bonding molecules in both their ground and excited state - goes a good job of predicting diamagnetism and paramagnetism - predicts bond order of a bond but doesnt need resonance structures to describe molecules
how do you determine hybridization? 4 steps
1. draw lewis structure 2. use VSEPR to determine e pair geometry 3. choose hybrid orbitals that match e pair geometry 4. multiple bonds consist of a sigma bond and 1 or 2 pi bonds
what are the steps to predicting hybridization
1. draw lewis structure, double check by checking formal charges 2. use VSEPR theory to determine the electron group geometry about the central atom 3. relate the central atom e group geometry to corresponding hybrid 4. identify and label the orbital overlap in each bond 5. label the bonds with sigma and pi bonds
because MO theory also treats molecules with an odd number of electrons, what other bond orders are possible
1/2, 3/2, and 5/2
the three 2p orbitals on each side combine to form six new MOs that can accommodate up to ___ electrons
12
represent the 1s wavefunctions from the two separated hydrogen atoms
1sa and 1sb
the three 2p MO configuration is seen for which elements
F and O
slide 75 ??
Leads to the formation of sigma bonding MO's and "sigma-star" anti-bonding MO's.
describes bonding between atoms using a different approach than valence bond theoru
MO ghdofy
closely tied to the lewis idea of bonding electron pairs between atoms and lone pairs localized on others
VB theory
when bonding occurs above and below a bond axis, it is referred to as..
a pi bond
what is a double bond usually made up of
a sigma and a pi bond
since there are four sp3 hybrid orbitals, they must spread out to form...
a tetrahedron around a central atom to minimize repulsion
in a pi bond, which space do the electrons occupy
above and below the nuclei
higher in energy than the parent AOs
anti-bonding MOs
in order to attain the needed geometry, what happens to form new valence bond orbitals
atomic orbitals mix or hybridize
electrons in bonds in the valence bond theory are localized where
between atoms or as lone pairs isolated on an individual atom
lower in energy than the parent AOs
bonding MO
what does MO give an excellent description of
bonding for molecules in their ground and excited states
what is the premise of valence bond theory
bonding involves valence electrons through orbital overlap
how are sets of hybrid orbitals constructed
by combining an s orbital with as many as p orbitals and d orbitals if necessary to have enough hybrids to accommodate the bonding and lone pair electrons on the central atom
atom of concern in a molecule
central atom
when two waves add and the amplitude increases
constructive interference
in MO theory, electrons become ___, or distributed over the entire molecule as a whole
delocalized
valence bond theory predicts that aromatic molecules which display resonance structures exhibit what over the entire molecule
delocalized pi bonding
when two waves subtract and the amplitude decreases
destructive interference
one sigma bond and one pi bond
double bond
compounds containing _____ usually exhibit sp2 hybridization
double bonds (sigma + pi)
when two of the orbitals overlap along the bond axis
end on overlap
what can sp orbitals do? what about the unhybridized p orbitals?
form sigma bonds or hold lone pairs may form two pi bonds in a triple bond
each ___ orbital is capable of forming a covalent bond
half filled
electrons are assigned to orbitals of successively ___ energy
higher
determines electron pair geometry for the central atom
hybdridization
bonding orbitals that arise from the mixing of atomic orbitals
hybrid orbital
linear combination of atomic orbitals
hybridization
these older approaches in VB that invoke d orbital hybrid to describe bonding in trigonal bipyramidal and octahedral based molecules are now considered to be...
incorrect
in MO theory, the stability of a covalent bond is related to what? explain/give examples
its bond order bond order 1 is a single bond bond order 2 double bond bond order 3 triple bond
sp hybrid orbitals direct away from one another to form a ____, leaving the other two p orbitals perpendicular to the molecular axis
linear geometry
who developed valence bond theory
linus pauling
as with electron configurations of atoms, electrons fill from the _______, only pairing when forced to.
lowest energy MO first
can be constructed by placing the sigma and sigma* MOs at their energetic positions relative to those for the uncombined AOs
molecular orbital diagrams
when atomic orbitals combine to yield molecular orbitals delocalized over the entire molecule
molecular orbital theory
what does the valence bond theory enable the prediction of and supplement
molecular shape VSEPR theory
must mix in order to form three new sp2 hybrid orbitals what does this provide the opportunity for?
one s and two p orbitals three sp2 orbitals can now form sigma bonds with each half filled p orbital
two orbitals existing in the same region of space
overlap
the remaining ___ orbital is perpendicular to the three sp2 valence bond orbitals that spread out in a plane
p
what can restricted rotation around a c=c bond lead to
the formation of ci/trans-isomers
what happens when wavefunctions are added (what is a bonding MO)
the matching phases produce the constructive interference, which gives enhanced electron probability between the nuclei
best approach for determining which AOs on the central atom mix to form bonds in trigonal-pyramid and octahedral- based molecular geometries
the molecular orbital
what happens to a H2 molecules if one of the sigma 1s electrons is excited to the sigma* 1s orbital
the molecule falls apart, photodissociation
lewis structures and VSEPR do not tell us anything about the nature of _____ or their energies
the nature of molecular orbitals
what does the number of hybrid orbitals equal
the number of atomic orbitals that are mixed to create the hybrids
in a sigma bond, what space do the electrons occupy
the space between the nuclei
what are the consequences of multiple bonding
there is restricted rotation around a double or triple bond
what happens when wavefunctions are subtracted? (what is an antibonding MO)
they are out of phaser and destructive interference occurs between the opposite phase, leading to a node between the nuclei
what mixes in order to attain a trigonal planar geometry
three AOs mix or hybridize to form three new valence bond orbitals
for elements beyond the second period, we found several examples where the central atom in a lewis structure had 8+ electrons in the valence shell. what EDG do these molecular structures adopt?
trigonal bipyramidal octahedral
one sigma bond and two pi bonds
triple bond
how many electrons can each MO hold
two electrons of opposite spin
what are the ways the sigma bond can form
two s orbitals overlap overlap of s&p two p orbitals overlap
what are two common approaches to rationalizing chemical bonding based on orbitals
valence bond theory and molecular orbital theory
in MO theory, the atomic orbitals are treated like what?
waves that constructively or destructively add to form new molecular orbitals
when are bonds formed in the valence bond theory
when half-filled orbitals on two adjacent atoms overlap in the same region of space
when is VB theory the method of choice
when one wants to provide a qualitative visual picture of molecule structure and bonding
when is Mo theory the method of choice
when quantitative information on energies, charge distribution, etc is needed
when do AOs combine to form MOs most effectively
when they are of similar energy
the total number of molecular orbitals equals the total number of...
AO that have combined to create the MO
sideways overlap results in a
pi bond
overlap above and below the internuclear axis, two names
pi bond, "sideways overlap"
in sp hybridization, what can the unhybridized p orbitals form
pi bonds
what was the older approach in VB theory when elements have 8+ electrons in the outer shell
place more electrons in the valence shell by bringing d orbitals into the mix
what is a more accurate depiction of trigonal pipyramidal and octahedral bonding is not thought to consist of what
resonance mixing of several different VB structures, some of which are ionic in charcater
MO theory makes use of the _______ to figure out the bonding
schrodinger
how does a pi bond form
sideways overlap of two p orbitals
when orbitals subtract, a higher energy ______ forms
sigma * antibonding MO
always lower in energy, as they result in bonding between atoms
sigma MOs
what does MO theory predict when p orbitals interact with p orbitals on another atom
sigma MOs and pi MOs will form
a bond in which electron density lies along the axis of the bond
sigma bond
when bonding occurs along a bond axis, it is referred to as a...
sigma bond
arises from the overlap of two half filled s orbitals, one from each atom
sigma bond in h2
overlap along the internuclear axis, two names
sigma bond, "end-on overlap"
when orbitals add, a lower energy ___ forms
sigma bonding MO
always higher in energy, as they are responsible for cancelling out the bonding between atoms
sigma* MOs
one sigma bond
single bond
compounds containing triple bonds (sigma + 2pi) or adjacent double bonds have what type of hybridization
sp
when a central atom has a linear EDG with no lone pairs, what is it most likely to bond through
sp hybridization
separated by 180*, linear
sp hybrids
each of the three ____ orbitals can form a sigma bond with another atom, " end on " overlap
sp2
if a central atom has a trigonal planar edg, it is most likely to bond through what
sp2 hybridization
separated by 120*, trigonal planar
sp2 hybrids
when the central atom in a molecule has a combination of four total sigma (single) bonds and lone pairs, what is the hybridization at the central atom
sp3
any central atom with a tetrahedral electron domain geometry must have what
sp3 hybdridization
separated by 109.5*, tetrahedral
sp3 hybrids
what are the new orbitals called
sp3 valence bond orbitals
electrons in sigma bonding molecular orbitals add _____, and electrons in sigma* antibonding molecular orbitals _____
stability, reduce stability
they hybrid orbitals are directed toward ____ atoms. what does this lead to?
terminal. it leads to better orbital overlap and stronger bonds with the terminal atoms