Molecular Orbitals

Question 1

How is the maximum number of electrons a molecular orbital can hold

Answer 1

2

Question 2

The combination of two atomic orbitals results in the formation of a….. Where the bonding obrital….

Answer 2

Bonding molecular orbital and an anti-bonding orbital

Encompasses both nuclei

Question 3

In a non-polar covalent bond is (shape wise)

Answer 3

Symmetrical about the midpoint between two atoms

Question 4

In a polar covalent bond is (shape wise)

Answer 4

Asymmetrical about the midpoint between two atoms

Question 5

Sigma bonds form by

Answer 5

The end-on overlap of atomic orbitals along the axis of the covalent bond

Question 6

Pi Bonds are formed by

Answer 6

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

Question 7

What is hybridisation?

Answer 7

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

Question 8

Hybrid orbitals are (quality)

Answer 8

Degenerate

Question 9

Which molecular orbitals do electrons fill; High energy antibonding or bonding orbitals

Answer 9

Bonding Orbitals

Question 10

What is HOMO

Answer 10

HOMO is the highest bonding molecular orbital containing electrons

HOMO - Highest occupied Molecular Orbital

Question 11

What is LUMO

Answer 11

LUMO is the lowest antibonding molecular orbital

LUMO - Lowest Unoccupied Molecular Orbital

Question 12

Absorbtion of electromagnetic energy can result in….

Answer 12

electrons being promoted from HOMO to LUMO

Question 13

Why do most organic molecules appear colourless?

Answer 13

Because the energy difference between HOMO and LUMO is reletively large.

This results in absorption of light from the UV region of the spectrum

Question 14

What is a Chromophore?

Answer 14

• Present in some organic molecules
• A chromophore is a group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum
• light can be absorbed when electrons in a chromophore are promoted from the HOMO to the LUMO

Question 15

What type of molecules do Chromophores exist in?

Answer 15

Molecules containing a Conjugated System

Question 16

What is a conjugated System?

Answer 16

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

Question 17

Electron within a conjugated system are….

Answer 17

delocalised

Question 18

What types of molecules have conjugated systems?

Answer 18

Molecules with alternating single and double bonds and aromatic molecules

Question 19

How does the no. of atoms in a conjugated system relate to The colour exhibited by the compound?

Answer 19

• The more atoms in the conjugated system the smaller the energy gap between HOMO and LUMO
• A lower frequency of light (longer wavelength, lower energy) is absorbed by the compound.
• When the wavelength of light absorbed is in the visible region, the compound
  will exhibit the complementary colour.

Question 20

Describe the bonding in benzene and other aromatic systems in terms of sp2
hybridisation

Answer 20

• 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 π molecular system, perpendicular to the plane of the σ bonds.
• This π molecular system extends across all six carbon atoms. The electrons in this
  system are delocalised.

Question 21

Describe the bonding in alkynes in terms of sp hybridisation.

Answer 21

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 σ bonds.
• The remaining two 2p orbitals on each carbon atom lie perpendicular to each other and to the axis of the σ bond. The unhybridised p orbitals overlap side-on to form two π bonds.