3.1 Molecular Orbitals

Question 1

When do molecular orbitals form?

Answer 1

when atomic orbitals combine

Question 2

What is the number of molecular orbital formed equal to?

Answer 2

the number of atomic orbitals that combine

Question 3

What does the combination of two atomic orbitals result in?

Answer 3

the formation of a bonding molecular orbital and an antibonding orbital

Question 4

What does the bonding molecular orbital encompass?

Answer 4

both nuclei

Question 5

What is the basis of bonding between atoms?

Answer 5

the attraction of positively charged nuclei and the negatively charges electrons in the bonding molecular orbital is the basis of bonding between atoms.

Question 6

How many electrons can each molecular orbital hold?

Answer 6

2

Question 7

Explain bonding molecular orbitals in non-polar covalent bonds?

Answer 7

the bonding molecular orbitals is symmetrical about the midpoint between two atoms

Question 8

Explain bonding molecular orbitals in polar covalent bonds?

Answer 8

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

Question 9

explain differences between electronegitivity and bonding electrons

Answer 9

The atom with the greater value for electronegativity has the greater share of the bonding electrons.

Question 10

Explain bonding molecular orbitals in ionic bonds?

Answer 10

Ionic compounds
are an extreme case of asymmetry,
with the bonding molecular orbitals being almost
entirely located around just one atom, resulting in the formation of ions.

Question 11

How are sigma bonds formed?

Answer 11

end-on overlap of molecular orbitals along the axis of the covalent bond

Question 12

How are pi bonds formed?

Answer 12

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

Question 13

How can the electronic configuration of an isolated carbon atom be explained?

Answer 13

• cannot explain the number of bonds formed by carbon atoms in molecules.
• The bonding and shape of molecules of carbon can be explained by 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.
• These hybrid orbitals are degenerate.

Question 15

Explain bonding in alkanes

Answer 15

• the 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals.
• These adopt a tetrahedral arrangement.
• The sp3 hybrid
  orbitals overlap end-on with other atomic orbitals to form sigma
  bonds.

Question 16

Explain bonding in alkenes

Answer 16

• can be described in terms of sp2 hybridisation.
• The 2s orbital and
  two of the 2p orbitals hybridise to form three degenerate sp2 hybrid orbitals.
• These adopt a
  trigonal planar arrangement.
• The hybrid sp2 orbitals overlap end-on to form
  sigma bonds.
• The remaining 2p orbital on each carbon atom of the double bond is unhybridised and lies perpendicular to the axis of the
  sigma bond.
• The unhybridised p orbitals overlap side-on to form pi bonds.

Question 17

Explain bonding in benzene and other aromatics

Answer 17

• can be described in terms of sp2
  hybridisation.
• The six carbon atoms in benzene are arranged in a cyclic structure with
  sigma bonds between the carbon atoms.
• The unhybridised p orbitals on each carbon atom overlap side-on to form a pi molecular system, perpendicular to the plane of the sigma bonds.
• This pi molecular system extends across all six carbon atoms.
• The electrons in this
  system are delocalised.

Question 18

Explain bonding in alkynes

Answer 18

• can be described in terms of sp hybridisation.
• The 2s orbital and
  one 2p orbital of carbon hybridise to form two degenerate hybrid orbitals.
• These adopt a
  linear arrangement.
• The hybrid sp orbitals overlap end-on to form
  sigma bonds.
• The remaining
  two 2p orbitals on each carbon atom lie perpendicular to each other and to the axis of the sigma bond.
• The unhybridised p orbitals overlap side-on to form two pi bonds.

Question 19

Why can molecular orbitals be used to explain weather organic molecules are coloured or colourless?

Answer 19

Electrons fill bonding molecular orbitals, leaving higher energy antibonding orbitals unfilled.

Question 20

What does HOMO mean?

Answer 20

The highest bonding molecular orbital containing electrons is called the highest occupied molecular orbital

Question 21

What does LUMO mean?

Answer 21

The lowest antibonding molecular orbital is
called the lowest unoccupied molecular orbital (LUMO).

Question 22

What could cause electrons to be promoted from HOMO to LUMO

Answer 22

absorption of electromagnetic energy

Question 23

What causes most organic molecules to appear colourless and what does it result in?

Answer 23

• the energy difference between HOMO and LUMO is relatively large
• results in absorption of light from the ultraviolet region of the spectrum

Question 24

What is a chromophore?

Answer 24

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

Question 25

How can light be absorbed in a chromophore?

Answer 25

Light can be absorbed when electrons in a chromophore are promoted from the HOMO to the LUMO.

Question 26

When do chromophores exist?

Answer 26

in molecules containing a conjugated system.

Question 27

What is a conjugated system?

Answer 27

• a system of adjacent
  unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of
  carbon atoms.
• Electrons within this conjugated system are delocalised.

Question 28

What type of molecules contain a conjugated system?

Answer 28

• alternating single and double bonds
• aromatic molecules

Question 29

As the number of atoms in the conjugated system increases what happens to the energy difference between HOMO and LUMO? and what doe this mean?

Answer 29

• it decreases, there is a smaller energy gap
• lower frequency of light - longer wavelength, lower energy - is absorbed

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Question 30

what colour is transmitted in a conjugated system?

Answer 30

• when the wavelength of light is absorbed is in the visible region, the compound will exhibit the complementary colour