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