2.3 molecular orbitals

Question 1

when do molecular orbitals form?

Answer 1

what atomic orbitals combine

Question 2

the number of molecular orbitals formed…

Answer 2

is equal to the number of atomic orbitals that combine

Question 3

the combination of two atomic orbitals results in the formation of…

Answer 3

a bonding molecular orbital (HOMO) and an antibonding molecular orbital (LUMO)

Question 4

the bonding molecular orbital encompasses…

Answer 4

both nuclei

Question 5

describe the attraction that is the basis of bonding between atoms

Answer 5

positively charged nuclei and negatively charged electrons in the bonding molecular orbital

Question 6

how many electrons can each molecular orbital hold?

Answer 6

a maximum of 2

Question 7

describe the bonding molecular orbital in a non-polar covalent bond

Answer 7

symmetrical about the midpoint between 2 atoms

Question 8

describe the bonding molecular orbital in a polar covalent bond

Answer 8

asymmetric about the midpoint between two atoms

Question 9

in a polar covalent bond, which atom has a greater share for bonding electrons?

Answer 9

the atom with the highest electronegativity

Question 10

where are bonding molecular orbitals located in an ionic bond?

Answer 10

almost entirely around just one atom

Question 11

describe sigma (σ ) bonds

Answer 11

bonds that form when molecular orbitals form by end-on overlap of atomic orbitals

Question 12

describe pi (π ) bonds

Answer 12

molecular orbitals that form by side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond

Question 13

how can the bonding and shape of orbitals be explained?

Answer 13

hybridisation

Question 14

what is hybridisation?

Answer 14

the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals

Question 15

hybrid orbitals are…

Answer 15

degenerate

Question 16

which arrangement do the four sp3 orbitals adopt in alkanes?

Answer 16

tetrahedral

Question 17

how many sigma bonds are in ethane?

Answer 17

7

Question 18

which arrangement do the three sp2 orbitals adopt in alkenes?

Answer 18

trigonal planar

Question 19

how do electrons fill MOs?

Answer 19

they fill the lower energy bonding ones first and leave the higher energy antibonding ones unfilled

Question 20

where does end-on overlap occur?

Answer 20

along the axis of the bond that forms

Question 21

why can sp3 hybridisation occur?

Answer 21

as 2s and 2p orbitals are close in energy, allowing one 2s orbital to be promoted to the 2p orbital

Question 22

how many electrons occupy each sp3 orbital?

Answer 22

1

Question 23

which is weaker, pi or sigma bonds?

Answer 23

pi

Question 24

what is different about hybridisation in alkenes?

Answer 24

only two of the p orbitals mix with the 2s orbital

Question 25

where does the unhybridised p orbital lie in alkenes?

Answer 25

perpendicular to the sp2 orbitals

Question 26

what does a C=C bond consist of?

Answer 26

one sigma bond and one pi bond

Question 27

how is a pi bond formed in alkenes?

Answer 27

sideways overlap occurs between the two unhybridised p orbitals

Question 28

which type of hybridisation do carbon atoms in benzene undergo?

Answer 28

sp2

Question 29

what occurs between the unhybridised p orbitals in aromatics?

Answer 29

a doughnut-shaped pi-bonding system above and below the carbon ring

Question 30

how do the 6 electrons behave in a benzene pi system?

Answer 30

they are delocalised over the system

Question 31

which type of hybridisation occurs in alkynes?

Answer 31

sp

Question 32

how does sp hybridisation occur?

Answer 32

an electron from the 2s is promoted to the empty 2p. the 2s orbital and one of the 2p orbitals hybridize to form two degenerate sp hybrid orbitals

Question 33

which shape do sp orbitals form around the carbon?

Answer 33

linear

Question 34

where do the unhybridised p orbitals lie around the sp orbitals?

Answer 34

perpendicular to the hybrid and each other

Question 35

which bonds does a carbon-carbon triple bond contain?

Answer 35

1 sigma and 2 pi bonds

Question 36

the absorption of energy (corresponding to the energy gap) can cause...

Answer 36

electrons to be promoted from the HOMO to the LUMO

Question 37

why does light absorbed in most organic molecules lie outwith the visible spectrum?

Answer 37

due to large energy gap

Question 38

what is a chromophore?

Answer 38

a group of atoms within a molecule that is responsible for the absorption of light in the visible region of the spectrum

Question 39

where do chromophores exist?

Answer 39

in molecules with a conjugated system

Question 40

what is a conjugated system?

Answer 40

a system of adjacent unhybridised p orbitals that overlap sideways to form a molecular orbital across a number of carbon atoms where the electrons are delocalised

Question 41

what does a conjugated system often consist of?

Answer 41

alternating single and double bonds (or multiple connected benzene rings)

Question 42

a chromophore/conjugated system always...

Answer 42

starts and ends with a double bond

Question 43

as the conjugated system gets larger...

Answer 43

the energy gap between the HOMO and the LUMO decreases

Question 44

what does a decreased energy gap mean?

Answer 44

that lower energy/higher wavelength of light is required to promote an electron. light will be absorbed in the visible spectrum

Question 45

different colours of organic compounds are the result of...

Answer 45

differing sizes of conjugated system