Molecular Orbital Diagrams

Question 1

What is the layout of an MO diagram?

Answer 1

AOs on outside
MOs on the inside
Energy is shown going up on the y axis

Question 2

What is the rule for numbers of AO and number of MOs

Answer 2

The number of valence atomic orbitals= the number of MOs formed

Question 3

What does a bonding sigma orbital look like?

Answer 3

Spherically symmetrical about the inter nuclear axis (z axis)
The boundary surface contains 95% of the electron density

Question 4

What does the antibonding sigma orbital look like?

Answer 4

There is a nodal plane between the two lobes of the orbital- reduction of electron densities between the nucleus

Question 5

What is the aufbau principle?

Answer 5

This states that the lowest energy orbitals are occupied first

Question 6

What is the Pauli principle?

Answer 6

This states that there can be a maximum of two electrons with opposite spins occupying one orbital

Question 7

What is hunds rule?

Answer 7

This states that parallel spins are favoured ( orbitals are filled singly) when the subshell orbitals have the same energy

Question 8

What is bond order?

Answer 8

It is a measure of the stabilisation resulting from the formation of the molecules

Question 9

How are molecular orbitals formed?

Answer 9

The MO orbitals are formed through an approach called the linear combination

Question 10

How do you work out bond order?

Answer 10

Bond order:

Number of electrons in bonding MOs- number of electrons in antibonding MOs/ 2

Question 11

Describe what happens to the bonds as bond order increases

Answer 11

As the bond order increases, the bond dissociation enthalpy increases and the bond length decreases

Question 12

What bond order does a molecule require to exist ?

Answer 12

Generally, a molecule can only exist if the bond order between the atom is greater than zero

Question 13

What is required for interactions between two AOs to be allowed?

Answer 13

• correct shape symmetry
• large overlap
• similar energy

Question 14

How do you build up an MO diagram?

Answer 14

1) identify valence AOs
2) show how they combine to give MOs
3) add electrons

Question 15

Describe the trend in energy of the 2s and 2p atomic orbitals

Answer 15

The energy of the 2s and 2p orbitals goes down as you go across the group as zeff increases
The energy gap between the 2s and 2p increases as you go across as a electrons and more penetrating than p electrons

Question 16

Why do big atomic orbitals give weak bonds?

Answer 16

The orbital overlap is poor

Question 17

What is a valence orbital?

Answer 17

Atomic orbitals with the highest value of n

Question 18

Why do 1s and 2s orbitals not interact?

Answer 18

They are too far apart in energy and so do not overlap

Question 19

What is a core orbital?

Answer 19

Any atomic orbital with lower values of n than the highest value of n within the atom are known as core orbitals

Question 20

Why can core orbitals be ignored in MO diagrams?

Answer 20

This is because the overlap cancels out and is small

This means they only provide weak sigma bonding and antibonding interactions

Question 21

Why will 1s orbitals not interact very much?

Answer 21

The smaller 1s orbitals will not interact very much as the vast majority of their wavefunction lies closer to the nucleus