Chapter 9 - Valence Bond Theory!!! <3

Question 1

what is valence bond theory?

Answer 1

bond theory that treats covalent bonds as overlap of atomic orbital wavefunctions

Question 2

in MO covalent bonds formed by…

Answer 2

constructive interference of AOs

Question 3

3 approximations of VBT

Answer 3

1. 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

2. only considers overlap of 1 singly occupied atomic orbital - VBT bonds formed by half filled orbitals. inner core electrons ignored
3. 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

Question 4

how to label orbitals in VBT

Answer 4

write names of each contributing atomic orbital when describing ex sigma symbol(1s-1s) or sigma symbol(2px-2px)

Question 5

could sigma(2s-2px) be a possible VBT bonding type?

Answer 5

YES doesn’t create homonuclear bonds but when atoms are bonding the 2s orbital could be close to 2px

Question 6

sigma vs pi orbital in VBT

Answer 6

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

Question 7

what would H2 bond be in VBT

Answer 7

sigma(1s-1s)

Question 8

how to determine VBT for Li2 given MO and bond order

Answer 8

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

Question 9

internuclear axis

Answer 9

line formed from line passing through nucleus of two atoms creating a bond

Question 10

what should you pick when results of VBT are ambiguous? sigma or pi?

Answer 10

sigma as sigma overlap is generally greater leading to greater energy stabilization

Question 11

when is it worth energy cost

Answer 11

when extra bonds leading to greater stability could be formed from excited state atoms

Question 12

hybrid orbitals

Answer 12

wavefunctions formed by the combination of atomic orbitals on the same atom

Question 13

what are the 5 relevant hybridization combinations of orbitals

Answer 13

sp, sp^2, sp^3, sp^3d, sp^3d^2

Question 14

when hybridizing atomic orbitals, where are hybridized and unhybridized orbitals relative to each other?

Answer 14

on the same atom!!!

Question 15

why do electrons form hybrid orbitals

Answer 15

if participating in more than one bond ex. 2 electrons in a sigma bonding orbital, one excited to sigma * orbital, forms hybrid orbital

Question 16

process of forming hybrid orbitals for carbon, to form CH4

Answer 16

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

Question 17

what determines the # of hybridized orbitals needed to form

Answer 17

of bonds

Question 18

how many hybrid orbitals are formed for every atomic orbital hybridized?

Answer 18

one!

Question 19

sp hybridization schemes

Answer 19

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

Question 20

do you take the 2s orbital’s radial node into account when drawing sp orbital??

Answer 20

small size of inner lobe of 2s orbital, wont strongly affect the shape of the sp-hybrid orbital/ affect the bonding of the orbital

Question 21

what energy does the sp orbital have relative to 2p and 2s?

Answer 21

in the middle, more energy than 2s, less than 2p

Question 22

sp2 hybridization scheme

Answer 22

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

Question 23

sp3 hybridization scheme

Answer 23

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

Question 24

sp3d hybridation scheme

Answer 24

trigonal bpyramidal vsepr shape

only in atoms with n = 3 or higher, because only between orbitals of same shell (n)

Question 25

sp3d2 hybridization scheme

Answer 25

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

Question 26

what is the shape of an orbital generally determined by?

Answer 26

the sigma bonding framework

Question 27

steps of determining hybidization scheme for covalently bonded atom

Answer 27

1. draw lewis structure
2. determine parent shape using vsepr

Question 28

linear parent shape - bond angles and hybridization scheme?

Answer 28

180 degrees

sp

Question 29

trigonal planar - bond angles and hybridization scheme

Answer 29

120 degrees

sp2

Question 30

tetrahedral parent shape - bond angles and hybridization scheme

Answer 30

109.5 degrees
sp3

Question 31

trigonal bipyramidal parent shape - bond angles and hybridization scheme

Answer 31

120 and 90 degrees
sp3d

Question 32

octahedral parent shape - bond angles and hybridization scheme

Answer 32

90 degrees

sp3d2

Question 33

why dont elements with n = 1 or 2 form hypervalent molecules

Answer 33

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