Molecular Orbitals

Question 1

What can VSEPR not explain

Answer 1

VSEPR cannot explain the bonding in all compounds.

Question 2

What can provide an explanation for more complex molecules

Answer 2

Molecular orbital theory can provide an explanation for more complex molecules

Question 3

When do molecular orbitals form

Answer 3

Molecular orbitals form when atomic orbitals combine

Question 4

What is the number of molecular orbitals formed equal to

Answer 4

The number of molecular orbitals formed is equal to the number of atomic orbitals that combine

Question 5

What does the combination of two atomic orbitals result in

Answer 5

The combination of two atomic orbitals results in the formation of a bonding molecular orbital (HOMO) and an anti bonding orbital (LUMO)

Question 6

What can each molecular orbital hold

Answer 6

Each molecular orbital can hold a maximum of two electrons

Question 7

What does the bonding molecular orbital encompass

Answer 7

The bonding molecular encompasses both nuclei

Question 8

Which orbital bonding or anti bonding has the lowest energy

Answer 8

The bonding orbital always has the lowest energy

Question 9

What is the basis of bonding between atoms

Answer 9

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

Question 10

What is the arrangement of the bonding molecular orbital in a non - polar covalent bond

Answer 10

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

Question 11

What do polar covalent bonds result from

Answer 11

Polar covalent bonds result from bonding molecular orbitals that are asymmetric about the midpoint between two atoms.

Question 12

Which atom has greater share of the bonding electrons

Answer 12

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

Question 13

What are ionic compounds

Answer 13

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 14

What are sigma bonds

Answer 14

Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond are called sigma (σ) molecular orbitals or sigma bonds

Question 15

What are pi bonds

Answer 15

Molecular orbitals that form by side - on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond are called pi (π) molecular orbitals or pi bonds

Question 16

What can the electronic configuration of an isolated carbon not explain

Answer 16

The electronic configuration of an isolated carbon atom cannot explain the number of bonds formed by carbon atoms in molecules

Question 17

How can the bonding and shape of molecules be explained

Answer 17

The bonding and shape of molecules of carbon can be explained by hybridisation

Question 18

What is hybridisation

Answer 18

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 19

What Do the 2s and 2p orbitals hybridise to form in alkanes

Answer 19

In alkanes, the 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals.

Question 20

What arrangement do the orbitals in alkanes have

Answer 20

These adopt a tetrahedral arrangement

Question 21

What bonds do the sp3 in alkanes form

Answer 21

The sp3 hybrid orbitals overlap end-on with other atomic orbitals to form σ bonds

Question 22

What does hybridisation of alkanes produce

Answer 22

Hybridisation produces four identical orbitals which are a mixture of one s orbital and three p orbitals forming four new sp3 orbitals

Question 23

In What terms can bonding in alkenes be explained

Answer 23

The bonding in alkenes can be described in terms of sp2 hybridisation

Question 24

What do alkenes hybridise to form

Answer 24

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

Question 25

What arrangement do the orbitals in alkenes have

Answer 25

These adopt a trigonal planar arrangement

Question 26

What bonds do the Sp2 orbitals and remaining 2p orbitals form

Answer 26

The hybrid sp2 orbitals overlap end -on to form sigma bonds. The remaining 2p orbital on each carbon atom of the double bond is un hybridised and lies perpendicular to the x-axis of the sigma bond. The un hybridised p orbitals overlap side - on to form pi bonds

Question 27

Alkynes

Answer 27

In the excited state one of the s electrons is promoted to the p shell, the s orbital and one of the p orbitals then hybridise to sp hybrid orbitals

Question 28

Which bond do alkenes have

Answer 28

This means that the c-c double bond one sigma and one pi bond

Question 29

What bonds do alkynes form

Answer 29

This molecule contains a triple bond formed from two pi bonds and one sigma bond

Question 30

When is energy absorbed

Answer 30

Energy (and therefore light) is absorbed when electrons are promoted from the HOMO to the LUMO

Question 31

Why are most organic compounds colourless

Answer 31

Most organic compounds are colourless as the energy gap between the HOMO and the LUMO molecular orbitals is relatively large. This results in absorption of light from the ultraviolet region of the spectrum

Question 32

What do some organic compounds contain

Answer 32

Some organic compounds contain chromophores

Question 33

What is a chromophore

Answer 33

A chromophore is a group of atoms within a molecule responsible for absorption of light in the visible region of the spectrum and therefore responsible for its colour

Question 34

Which molecules do chromophores exists in

Answer 34

Chromophores exist in molecules containing a conjugated system

Question 35

What is a conjugated system

Answer 35

A system of connected p -orbitals containing delocalised electrons over several atoms, which lowers the overall energy of the molecule and increases stability

Question 36

Which molecules have conjugates systems

Answer 36

Molecules with alternating c=c bonds and c-c and aromatic molecules

Question 37

What does the conjugated system begin and end with

Answer 37

As the conjugated system is a system of connected p orbitals it begins and ends with a c-c bond

Question 38

What do the conjugated double bonds allow

Answer 38

The conjugated double bonds reduce the energy gap between the HOMO and the LUMO molecular orbitals allowing the compound to absorb in the visible part of the spectrum

Question 39

What does the wavelength of light absorbed depend on

Answer 39

The wavelength of light absorbed depends on the length of the conjugated chain

Question 40

The larger the conjugated system…

Answer 40

The larger the conjugated system the lower the energy of light absorbed and therefore the longer the wavelength of light absorbed

Question 41

Where is the wavelength of molecules which contain chromophores

Answer 41

In molecules which contain chromophores that wavelength is in the visible region of the spectrum

Question 42

When does the colour arise

Answer 42

The colour arises as the electrons absorb energy and are promoted from the HOMO to the LUMO. The colour of the compound is the colour complementary to that absorbed