Electronic Bands

Question 1

Describe how a band is formed in lithium

Answer 1

1. The two 2s orbitals combine to give a filled sigma orbital and empty sigma* orbital
2. Li2 has bond order of 1
3. In solid lithium there are many millions of atoms and each has a 2s orbital
4. These atomic orbitals all interact together to give many millions of molecular orbitals all with very similar energies
5. Average energy gap between energy levels decreases as more Li added
6. The energy gaps between these molecular orbitals are so small that the orbitals overlap to form a continuum of energy levels and give a band

Question 2

How many energy levels are in one mole of Li metal

Answer 2

1. 6.02* 10^23

2. N AOs= N MOs

Question 3

What is the band called in Lithium

Answer 3

1. Each 2s orbital is half filled so 2s band is half filled

2. Known as the valence band (containing valence shell electrons)

Question 4

What happens when a potential difference is applied across a sample of lithium

Answer 4

1. The electrons are able to move within the band
2. As the band is delocalised over all of the metal atoms
3. So the metal conducts electricity

Question 5

What bands do s,p and d orbitals form

Answer 5

1. s,p,d bands respectively

Question 6

What is the energy difference between bands called

Answer 6

1. Band gap

Question 7

What bands would Na form and which is the valence band

Answer 7

1. 1s, 2s, 2p, 3s bands

2. The valence band is 3s is the highest occupied band

Question 8

What do electronic property differences depend on

Answer 8

1. Band structure
2. Whether the valence bands are full or partly occupied
3. Magnitude of the energy band gap (between full and empty bands)

Question 9

What are the three different types of compounds

Answer 9

1. Metal
2. Semi-conductor
3. Insulator

Question 10

Describe the bands in a metal

Answer 10

1. Valence band is partly occupied- highest filled energy level- fermi level
2. Electrons delocalised over all metal atoms
3. Under an electric field, the electrons can move easily- leads to high electrical conductivity
4. Conductivity decreases (resistance increases) with increasing Temperature- due to thermal atomic vibrations and electronic scattering

Question 11

Describe the bands in a semiconductor

Answer 11

1. Valence band is full and band gap is small (0.2-2.0eV)
2. This allows electrons to be promoted over the band gap with enough thermal energy - jump into empty upper band (conduction band)
3. Conductivity increases with temperature with greater promotion of electronic charge carriers in upper band

Question 12

Describe the bands in an insulator

Answer 12

1. Valence band is full
2. Band gap is large
3. Forbidden band gap- leads to very poor electronic conductor

Question 13

As you go down group 14 what happens to the electronic properties

Answer 13

1. Carbon has the largest band gap and is an insulator
2. Silicon and Ge have decreasing band gaps and are semiconductors
3. Sn and Pb have no band gap- metals

Question 14

What is the conducting type and use of LiCl

Answer 14

1. Insulator- large band gap

2. Use unknown

Question 15

What is the conducting type and use of NaCl

Answer 15

1. Insulator- large band gap

2. Salt

Question 16

What is the conducting type and use of GaAs

Answer 16

1. Semiconductor

2. Solar cell

Question 17

What is the conducting type and use of CdTe

Answer 17

1. Semiconductor

2. Solar cell

Question 18

What is the conducting type and use of AgCl

Answer 18

1. Insulator

2. Photography

Question 19

What is the conducting type and use of TiO2

Answer 19

1. Semi-conductor

2. Solar and pigment