Lecture 4

Question 1

Describe the energy trends across the row

Answer 1

• energy of 2s and 2p orbitals goes down across row
• gap between 2s and 2p increases across row
• energy of 1s goes down very sharply after He

Question 2

How are AO energies measured?

Answer 2

come from photoelectron spectroscopy or from computational chemistry

Question 3

How does the bond length and BDE of Li2 differ to H2?

Answer 3

• strongly electropositive
• longer at r= 267pm
• weaker at BDE= 101 kJmol-1

Question 4

How can MO theory explain the fact that Li2 has a much weaker BDE than H2?

Answer 4

Li2 has a much weaker bond than H2 because overlap of 2 2s is less effective than 2 1s
draw diagram on pg 34 of handout

Question 5

Which shells are required for MO formation?

Answer 5

• Valence outer shells must be considered for MO formation
• Core inner shells can be ignored for MO formation

Question 6

Why can core orbitals be ignored for MO formation?

Answer 6

• overlap of core orbitals cancels out
• overlap of core orbital is small- weak bonding/antibonding interaction