Chapter 9 - Valence Bond Theory!!! <3 Flashcards
what is valence bond theory?
bond theory that treats covalent bonds as overlap of atomic orbital wavefunctions
in MO covalent bonds formed by…
constructive interference of AOs
3 approximations of VBT
- covalent bonds treated as independent interactions, only orbitals on 2 atoms taking part
BAD when resonance occurs, electrons delocalized (moved), in this case use some MO theory
- only considers overlap of 1 singly occupied atomic orbital - VBT bonds formed by half filled orbitals. inner core electrons ignored
- bonding/antibonding pair will only contain 2 electrons. in ground state these fill the bonding orbital, therefore ignore antibonding and each occupied bonding orbitals contributes 1 to the bond order
how to label orbitals in VBT
write names of each contributing atomic orbital when describing ex sigma symbol(1s-1s) or sigma symbol(2px-2px)
could sigma(2s-2px) be a possible VBT bonding type?
YES doesn’t create homonuclear bonds but when atoms are bonding the 2s orbital could be close to 2px
sigma vs pi orbital in VBT
sigmas just overlap forming sigma orbitals line up and symmetrical over internuclear axis rotate freely
pi overlap forming pi asymmetrical over internuclear axis, cannot rotate freely, more rigid
what would H2 bond be in VBT
sigma(1s-1s)
how to determine VBT for Li2 given MO and bond order
MO ground state config of Li2 is sigma1s2 sigma* 1s2 sigma 2s2
only orbital singly occupied (so no antibonding orbital) is 2s so we only consider that one in VBT therefore single sigma bond of sigma(2s-2s)
bond order = 1
internuclear axis
line formed from line passing through nucleus of two atoms creating a bond
what should you pick when results of VBT are ambiguous? sigma or pi?
sigma as sigma overlap is generally greater leading to greater energy stabilization
when is it worth energy cost
when extra bonds leading to greater stability could be formed from excited state atoms
hybrid orbitals
wavefunctions formed by the combination of atomic orbitals on the same atom
what are the 5 relevant hybridization combinations of orbitals
sp, sp^2, sp^3, sp^3d, sp^3d^2
when hybridizing atomic orbitals, where are hybridized and unhybridized orbitals relative to each other?
on the same atom!!!
why do electrons form hybrid orbitals
if participating in more than one bond ex. 2 electrons in a sigma bonding orbital, one excited to sigma * orbital, forms hybrid orbital
process of forming hybrid orbitals for carbon, to form CH4
first, it wants to form 4 hybrid orbitals, you know this because there are 4 areas of of electron density which matches a sp3 hybridization scheme. first, it must excite an electron from 2 to p, so its electron configuration is [He] 2s1 2p3
the s orbital would have lower energy, so they hybridize to create four orbitals with identical energy (degenerate) 1 electron in each, so room for 1 more
what determines the # of hybridized orbitals needed to form
of bonds
how many hybrid orbitals are formed for every atomic orbital hybridized?
one!
sp hybridization schemes
FORMED FROM ONE S and ONE P ORBITAL
two lobes, looks like a p orbital with one much larger dewdrop than the other
the large node is the same phase as the phase of the s orbital, constructively interfering with the lobe of the p orbital in the same phase
large lobe oriented 180 degrees to the other sp orbital
do you take the 2s orbital’s radial node into account when drawing sp orbital??
small size of inner lobe of 2s orbital, wont strongly affect the shape of the sp-hybrid orbital/ affect the bonding of the orbital
what energy does the sp orbital have relative to 2p and 2s?
in the middle, more energy than 2s, less than 2p
sp2 hybridization scheme
one s and 2 p orbitals
3 atomic orbitals hybridized, so 3 hybrid orbitals formed
based on the 2 p orbitals being hybridized, the 3 orbitals would be in that plane (ex 2px and 2py orbitals hybridized, 3 orbitals in the xy plane) 120 degrees away from each other
looks kinda like a 6 petal flower alternating big and small lobe, and phases
for p block atoms, would have both hybridized and unhybridized p orbitals, so hybridized 3 on one plane and unhybridized on other kinda going through
sp3 hybridization scheme
2s 2px 2py 2pz
4 atomic orbitals so results in 4 sp3 hybrid orbitals
- 2 orbitals lie in yz plane, 2 orbitals in xz plane
orbitals oriented toward vertices of a tetrahedron (triangular pyramid)
angle between each is 109.5 degrees
relative energy - sp3 energy is quite close to but slightly below 2p, more significantly higher than 2s
sp3d hybridation scheme
trigonal bpyramidal vsepr shape
only in atoms with n = 3 or higher, because only between orbitals of same shell (n)
sp3d2 hybridization scheme
octahedral vsepr shape
only in atoms valence shell in n = 3 or higher, hybridization only occurs between shells w same n value and d is n = 2
what is the shape of an orbital generally determined by?
the sigma bonding framework
steps of determining hybidization scheme for covalently bonded atom
- draw lewis structure
- determine parent shape using vsepr
linear parent shape - bond angles and hybridization scheme?
180 degrees
sp
trigonal planar - bond angles and hybridization scheme
120 degrees
sp2
tetrahedral parent shape - bond angles and hybridization scheme
109.5 degrees
sp3
trigonal bipyramidal parent shape - bond angles and hybridization scheme
120 and 90 degrees
sp3d
octahedral parent shape - bond angles and hybridization scheme
90 degrees
sp3d2
why dont elements with n = 1 or 2 form hypervalent molecules
3d orbitals are too high in energy to to be hybridized with 2s and 2p orbitals, must be in the same shell (= n value) to hybridize
