Lecture 6

Question 1

Why can only certain AOs overlap to form MOs?

Answer 1

• only AOs with the same symmetry with respect to the internuclear axis can overlap to form MOs (e.g. pair of s AOs)
• An s orbital is cylindrically symmetric about the internuclear axis; so is a pz orbital.
  Overlap possible.
• A px (or py) orbital is not cylindrically symmetric about the internuclear axis. Overlap
  not possible between s and px.
• AOs close together in energy overlap best

Question 2

Give similarities between homonuclear and heteronuclear diatomics

Answer 2

1. Only orbitals of the same symmetry can overlap to form bonds.
2. Pairs of orbitals will only have significant overlap if they are relatively
   close in energy.

Question 3

Give differences between homonuclear and heteronuclear diatomics

Answer 3

1. Different types of orbitals on the two sides of molecule.
2. Because AOs are at different Es, each bonding or antibonding MO is
   unequally shared between the two atoms.

Question 4

Give the key points for building an MO diagram for a heteronuclear diatomic

Answer 4

1. List the occupied AOs on both atoms. Label as core and valence.
2. Identify the atom with the greatest Zeff. The AOs for this atom will lie lower in energy.
3. Identify pairs of AOs that can overlap. They should be close in energy. They must be of
   appropriate symmetry.
4. Sketch the MO diagram for the heteronuclear diatomic. AOs of diatomics on left and right hand side. Both ordered in increasing energy.
5. MOs of diatomic in the middle. In an interacting pair of AOs, the bonding MO lies below the
   lower AO, the antibonding MO above the higher AO.
6. Put in the right number of electrons.

Question 5

Describe some properties of LiH

Answer 5

• has a high H content, used for H storage
• colourless solid
• very reactive- violent reaction with water forming H2 and LiOH

Question 6

What is a non-bonding orbital?

Answer 6

• A non-bonding orbital has same E as one of the AOs on an atom in the molecule
• It is an MO
• It can be occupied by electrons
• It neither stabilizes or destabilizes a molecule compared to the atoms that form it
• Electrons in a non-bonding orbital don’t contribute to bond order

Question 7

Draw the bonding, antibonding and nonbonding arrangement of orbitals

Answer 7

answer on page 48

Question 8

What is the HF electronic structure?

Answer 8

• Bond order= 1
• highly polar because of the AO mismatch in energy
• there are 4 high energy non-bonding valence electrons in HF

Question 9

What is HFs chemistry?

Answer 9

• highly reactive
• highly corrosive

Question 10

Compare the MO bonds of LiH, HF and LiF

Answer 10

• Each possesses a bond order=1.
• Bonding MO is a sigma orbital=
  Bonding MO in LiH formed from Ѱ
  Bonding MO in HF formed from Ѱ
  Bonding MO in LiF formed from Ѱ
• The different AOs that contribute to the MOs lead to big differences in the % contributions
  that the AOs make to the MOs
  In LiH, the bonding MO contains
  In HF, the bonding MO contains
  In LiF, the bonding MO contains