chemistry - 4.1

Question 1

When atomic orbitals combine, what are formed?

Answer 1

Molecular orbitals.

Question 2

What is the number of molecular orbitals 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 the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital.

Question 6

What can each molecular orbital hold?

Answer 6

A maximum of two electrons.

Question 7

In a non-polar covalent bond, where is the bonding molecular orbital?

Answer 7

Symmetrical about the midpoint between two atoms.

Question 8

What do polar covalent bonds result from?

Answer 8

Bonding molecular orbitals that are asymmetric about the midpoint between two atoms.

Question 9

In a polar covalent bond, which atom has the greater share of the bonding electrons?

Answer 9

The atom with the greater value for electronegativity.

Question 10

What are an extreme case of asymmetry?

Answer 10

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

Question 11

What are sigma molecular orbitals or sigma bonds?

Answer 11

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

Question 12

What are pi molecular orbitals or pi 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 molecules of carbon be explained?

Answer 13

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.

Question 15

What are these hybrid orbitals?

Answer 15

Degenerate.

Question 16

In alkanes, what occurs during hybridisation?

Answer 16

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 17

How can the bonding in alkenes be described?

Answer 17

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 18

How can the bonding in benzene and other aromatic systems be described?

Answer 18

In terms of sp^2 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 19

How can the bonding in alkynes be described?

Answer 19

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 20

What can molecular orbital theory be used to explain?

Answer 20

Why organic molecules are colourless or coloured.

Question 21

What do electrons fill?

Answer 21

Bonding molecular orbitals, leaving higher energy anti bonding orbitals unfilled.

Question 22

What is the highest bonding molecular orbital containing electrons called?

Answer 22

The highest occupied molecular orbital (HOMO).

Question 23

What is the lowest anti bonding molecular orbital called?

Answer 23

Lowest unoccupied molecular orbital (LUMO).

Question 24

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

Answer 24

Absorption of electromagnetic energy.

Question 25

Why do most organic molecular appear colourless?

Answer 25

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

Question 26

What do some organic molecules contain?

Answer 26

Chromophores.

Question 27

What is a chromophore?

Answer 27

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

Question 28

When can light be absorbed?

Answer 28

When electrons in a chromophore are promoted from the HOMO to the LUMO.

Question 29

Which molecules do chromophores exist in?

Answer 29

Molecules containing a conjugated system — 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 30

What have conjugated systems?

Answer 30

Molecules with alternating single and double bonds, and aromatic molecules.

Question 31

The more atoms in the conjugated system…

Answer 31

…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 32

What is absorbed by the compound (with the conjugated system)?

Answer 32

A lower frequency of light (longer wavelength, lower energy).

Question 33

When the wavelength of light absorbed is in the visible region, what will the compound exhibit?

Answer 33

The complementary colour.