Band theory

Question 1

Limitations of the free electron model of metals

Answer 1

Cannot explain insulators and semiconductors and doesn’t account for the fact that many non-metals have metallic properties

Question 2

How do the ‘bands’ arise

Answer 2

A solid is considered as a molecule with an infinite number of atoms. The corresponding infinite number of MOs are so close in energy that ‘bands’ of electronic states form

Question 3

How do the bonding and anti-bonding energies change with increasing number of atoms

Answer 3

The addition of an atom to molecule size increases the energy of the anti-bonding orbital and decreases the energy of the bonding orbital

Question 4

Fermi-level definition

Answer 4

Energy of the highest occupied molecular orbital

Question 5

Reasons for metallic properties even in species with filled orbitals

Answer 5

Overlap between the filled and LUMO orbitals gives rise to bands and eliminates the band gap

Question 6

Band gap size of a semi-conductor

Answer 6

< 3 eV