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AH Chemistry > 3.1 molecular orbitals > Flashcards

3.1 molecular orbitals Flashcards

1
Q

can VSEPR explain the bonding in all compounds

A

no

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2
Q

what can molecular theory provide an explanation for

A

more complex molecules

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3
Q

when do molecular orbitals form

A

when atomic orbitals combine

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4
Q

what is the number of molecular orbitals formed equal to

A

the number of atomic orbitals that combine

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5
Q

what does the combination of two atomic orbitals result in

A

the formation of a bonding molecular orbital and an antibonding orbital

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6
Q

what encompasses both nuclei in a molecular orbital

A

the bonding molecular orbital

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7
Q

what forms the basis of bonding between atoms

A

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

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8
Q

how many electrons max can each molecular orbital hold

A

2

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9
Q

how are bonding molecular orbitals for non polar covalent bonds described

A

symmetrical about the mid point between two atoms

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10
Q

what do polar covalent bonds result from

A

bonding molecular orbitals that are asymmetric about the midpoint between the two atoms

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11
Q

which atom in an orbital has the greater share of bonding electrons

A

the one with the greater value for electronegativity

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12
Q

how are ionic compounds described in relation to molecular orbitals

A

an extreme case of asymmetry, with the bonding molecular orbital being almost entirely located around just one atom, resulting in the formation of ions

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13
Q

what are sigma bonds

A

the molecular orbitals that form by end on overlap of atomic orbitals along the axis of the covalent bonds

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14
Q

what are pi bonds

A

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

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15
Q

what can electronic configuration of an isolated carbon atom not explain

A

the number of bonds formed by carbon atoms in molecules

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16
Q

how can the bonding and shape of molecules of carbon be explained

A

by hybridisation

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17
Q

what can hybridisation explain

A

the bonding and shape of molecules of carbon

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18
Q

what is hybridisation

A

the processs of mixing atomic orbitals within an atom to generate a set of new atomic orbitals

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19
Q

what are hybrid orbitals

A

the degenerate orbitals produced by hybridisation

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20
Q

in alkanes, the 2s orbital and the three 2p orbitals of carbon hybridise to form…

A

four degenerate sp3 hybrid orbitals

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21
Q

what do degenerate sp3 hybrid orbitals arrange in

A

a tetrahedral arrangement

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22
Q

how do sp3 hybrid orbitals form sigma bonds

A

by overlapping end on with other atomic orbitals

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23
Q

how can bonding in alkenes be explained

A

in terms of sp2 hybridisation

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24
Q

how does hybridisation occur in alkenes

A

the 2s orbital and two of the 2p orbitals hybridise to form three degenerate sp2 hybrid orbitals

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25
Q

what arrangement is adopted by sp2 hybrid orbitals

A

trigonal planar

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26
Q

which bond type is formed by hybrid sp2 orbitals

A

sigma

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27
Q

which bond type is formed by hybrid sp3 orbitals

A

sigma

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28
Q

what happens to the remaining 2p orbital on each carbon atom of the double bond in alkenes when hybridisation occurs

A

remains unhybridised and lies perpendicular to the axis of the sigma bond

29
Q

which bond type is formed by the unhybridised p orbitals

A

pi

30
Q

how can the bonding in benzene and other aromatic systems be described in terms of

A

sp2 hybridisation

31
Q

how are the carbon atoms in benzene arranged

A

a cyclic structure with sigma bonds between the carbon atoms

32
Q

which bonds hold the carbon atoms together in a benzene ring

A

sigma

33
Q

what do the unhybridised p orbitals on each carbon atom in a benzene ring do

A

they overlap side on to form a pi molecular system, perpendicular to the plane of the sigma bonds

34
Q

how can the electrons in a benzene ring be described

A

delocalised

35
Q

how can the bonding in alkynes be explained

A

in terms of sp hybridisation

36
Q

what arrangement do the 2s and 2p orbitals of carbon adopt when they hydridise to form two degenerate hybrid orbitals

A

linear

37
Q

which bonds are formed when the hybrid sp orbitals overlap

A

sigma

38
Q

which bonds are formed when the unhybridised p orbitals overlap

A

pi

39
Q

what can molecular orbital theory be used to explain

A

why organic molecules are colourless or coloured

40
Q

what is the HOMO

A

the highest occupied molecular orbital

41
Q

what is the LUMO

A

the lowest unoccupied molecular orbital

42
Q

what is the highest occupied molecular orbital

A

the highest bonding molecular orbital containing electrons

43
Q

what do electrons leave when they fill bonding molecular orbitals

A

they leave higher energy antibonding orbitals unfilled

44
Q

what can cause electrons to be promoted from HOMO to LUMO

A

absorption of electromagnetic energy

45
Q

why do most organic molecules appear colourless

A

because the energy difference between

46
Q

why do most organic molecules appear colourless

A

because the energy difference between HOMO and LUMO is relatively large, resutling in absorption of light from the UV region of the spectrum

47
Q

what are chromophores

A

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

48
Q

when can light be absorbed

A

when electrons in a chromophore are promoted from the HOMO to the LUMO

49
Q

in which molecules do chromophores exist

A

those containing a conjugated system

50
Q

what is a conjugated system

A

a system of adjacent unhybridised p orbitals that overlap side on to form a molecular orbital across a number of carbon atoms

51
Q

what state are electrons in when they are in a conjugated system

A

delocalised

52
Q

what are examples of molecules with conjugated systems

A

molecules with alternating single and double bonds
aromatic systems

53
Q

what is the relationship between the number of atoms in the conjugated system and the energy gap between HOMO and LUMO

A

the more atoms in the conjugated system, the smaller the energy gap between HOMO and LUMO.

53
Q

what is the relationship between the number of atoms in the conjugated system and the energy gap between HOMO and LUMO

A

the more atoms in the conjugated system, the smaller the energy gap between HOMO and LUMO.

54
Q

what happens when a wavelength of light is absorbed in the visible region

A

the compound will exhibit the complementary colour

55
Q

what happens to the bonds in reactant molecules when an organic reaction takes place

A

they are broken down and the bonds in the product are made

56
Q

what is bond fission

A

the process of bond breaking

57
Q

what are the different types of bond fission

A

homolytic and heterolytic

58
Q

what does homolytic fission result in

A

the formation of two neutral radical

59
Q

when does homolytic fission occur

A

when each atom retains one electron from the sigma covalent bond and the bond breaks evenly

60
Q

when does homolytic fission normally occur (in relation to bond types)

A

when non polar covalent bonds are broken

61
Q

what do reactions involving homolytic fission tend to result in the formation of

A

a very complex mixture of products, making them unsuitable for organic synthesis

62
Q

what does heterolytic fission result in

A

the formation of two oppositely charged ions

63
Q

when does heterolytic fission occur

A

when one atom retains both electrons from the sigma covalent bond and the bond breaks unevenly.

64
Q

when does heterolytic fission usually occur in relation to bonds

A

when polar covalent bonds are broken

65
Q

why are reactions involving heterolytic fission better suited for organic synthesis

A

they tend to result in far few products than reactions involving homolytic fission

66
Q

how can the movement of electrons during bond fission and bond making be represented

A

using curly arrow notation

67
Q

what is a conjugated system

A

molecules containing delocalised electrons spread over a number of atoms

68
Q

what must be present for the bonds to be conjugated in long carbon chains

A

alternating single and double bonds

AH Chemistry (35 decks)

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