Molecular Orbitals

Question 1

When do molecular orbitals form?

Answer 1

When atomic orbitals combine

Question 2

What is the number of molecular orbitals formed equal to?

Answer 2

The number of atomic orbitals that combine

Question 3

The combination of two atomic orbitals results in the formation of what two types of orbital?

Answer 3

Bonding and antibonding. The bonding molecular orbital encompasses both nuclei.

Question 4

What is the basis of bonding between atoms?

Answer 4

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

Question 5

In a non-polar covalent bond, the bonding molecular orbital is …what…about the midpoint between two atoms.

Answer 5

Symmetrical

Question 6

Polar covalent bonds result from bonding molecular orbitals that are…what…about the midpoint between two atoms?

Answer 6

Asymmetric

Question 7

Which atom has the greater share of bonding electrons?

Answer 7

The atom with the greater electronegativity value

Question 8

What causes the formation of ions?

Answer 8

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 9

What is a sigma bond?

Answer 9

Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond

Question 10

What is a pi bond?

Answer 10

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

Question 11

How can the shape of molecules of carbon be explained?

Answer 11

Hybridisation

Question 12

What is hybridisation?

Answer 12

Hybridisation is 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 13

Describe bonding in alkanes.

Answer 13

In alkanes, 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 14

Describe bonding in alkenes.

Answer 14

The bonding in alkenes 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 15

Describe bonding in benzene.

Answer 15

The bonding in benzene and other aromatic systems 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 16

Describe bonding in alkynes.

Answer 16

The bonding in alkynes 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 17

What can molecular orbital theory be used to explain?

Answer 17

Why molecules are coloured or colourless

Question 18

What is the highest bonding molecular orbital containing electrons called?

Answer 18

Highest occupied molecular orbital (HOMO)

Question 19

What is the lowest antibonding
molecular orbital called?

Answer 19

Lowest unoccupied molecular orbital
(LUMO)

Question 20

What can cause electrons to be promoted from HOMO to LUMO?

Answer 20

Absorption of electromagnetic energy

Question 21

Why do most organic molecules appear colourless?

Answer 21

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

Question 22

What is a chromophore?

Answer 22

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

Question 23

In what molecules can chromophores exist?

Answer 23

Molecules containing a conjugated system

Question 24

What is a conjugated system?

Answer 24

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

Question 25

How do electrons behave within a conjugated system?

Answer 25

They are delocalised

Question 26

What molecules have conjugated systems?

Answer 26

Molecules with alternating single and double bonds, and aromatic molecules

Question 27

How does the number of atoms in a conjugated system correspond the energy gap between HOMO and LUMO?

Answer 27

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

Question 28

When the wavelength of light absorbed by a conjugated system is from the visible region what is observed?

Answer 28

The complementary colour