Atomic and Molecular Structure

Question 1

Metallic bonding

Answer 1

Atomic orbitals combine to form delocalized electron cloud shared by large number of atoms - non-directional

Question 2

Secondary bonds

Answer 2

• Van der Waals: weak bonding induced by fluctuating/permanent molecular dipoles
• H bonding: bonding between protons + available electron pair
• Directional

Question 3

Is covalent bonding directional?

Answer 3

Yuh

Question 4

Is ionic bonding directional?

Answer 4

No

Question 5

Comparison of bond energies for diff bonds

Answer 5

• Ionic: large
• Covalent: variable
• Metallic: variable
• Secondary: smallest

Question 6

Melting temperature bond energy relationship

Answer 6

Tm larger if Eo larger - proportional to depth of potential energy curve

Question 7

Elastic modulus bond energy relationship

Answer 7

E is larger if Eo larger - proportional to curvature at ro (unstretched length) energy

Question 8

Coefficient of thermal expansion bond energy relationship

Answer 8

Coeff is larger if Eo is smaller - proportional to symmetry at ro energy (low Eo = asymmetry)

Question 9

Hardness bond energy relationship

Answer 9

Proportional to Eo - hardness is resistance of surface to plastic deformation + influenced by height of total force curve so materials w/high Tm are harder

Question 10

Electrical conductivity from bond nature

Answer 10

• Ionic + covalent are poor conductors as electrons not free to leave atoms used in bonding
• Metallic are good as low ionization energy + electrons free within crystal lattice

Question 11

Ceramics material properties (they have ionic + covalent bonds)

Answer 11

Large bond energy, large Tm, large E + small coeff

Question 12

Metals material properties (metallic bonding)

Answer 12

Variable bond energy, moderate Tm, moderate E + moderate coeff

Question 13

Polymers material properties (covalent + secondary)

Answer 13

Directional properties, secondary bonding dominates, small T, small E + large coeff

Question 14

How does band splitting work?

Answer 14

• Occurs 1st on outer shells - effect smaller as we go towards nucleus + splitting of given level increases as distance decreases
• Discrete energy states w/small energy differences (continuum)

Question 15

Valence band

Answer 15

Band having highest energy electrons at 0K

Question 16

Conduction band

Answer 16

Next band at energy > valence band

Question 17

Fermi energy

Answer 17

At 0K all electrons have energy smaller or equal to Ef

Question 18

Conductors

Answer 18

Energy needed very small to move in conduction region + become free electrons

Question 19

Insulators

Answer 19

Large energy band gap exists between full valence band + conduction region

Question 20

Semi-conductors

Answer 20

Same as insulators but w/small band gap

Question 21

What is resistance of pure metals near 0 temperature?

Answer 21

Very small

Question 22

Does resistivity increase or decrease with temperature?

Answer 22

Increase

Question 23

Does resistivity increase at absolute 0 temperature?

Answer 23

Yuh

Question 24

Are good conductors pure or impure?

Answer 24

Pure

Question 25

What are bad conductors typically?

Answer 25

Alloys

Question 26

Do metals deformed by work hardening have higher or lower resistivity than same metal in stabilized state?

Answer 26

Higher

Question 27

Fermi energy

Answer 27

Energy where probability of occupancy is 50% for any T>0K

Question 28

Is electron mobility higher or lower than hole mobility?

Answer 28

Higher - more difficult to move a hole than an electron

Question 29

n-type extrinsic semiconductor

Answer 29

Surplus of 1 electron for each P atom added - electron easily goes to conduction band and delta E needed is very small

Question 30

p-type extrinsic semiconductor

Answer 30

Missing 1 electron for each B atom added creates hole - hole easily goes in conduction in valence band and delta E needed very small

Question 31

Do n or p types have higher fermi levels?

Answer 31

n

Question 32

Diode

Answer 32

p-n rectifier junction (current only flows in 1 direction)

Question 33

Transistor

Answer 33

Has 3 semiconductor sections - current only flows between source + drain if potential is applied at base

Question 34

Applications of transistor

Answer 34

- Amplifier: signal sent to base is amplified between emitter + collector - Electronic switches: current only flows between collector + emitter when voltage applied at base

Question 35

Microfabrication

Answer 35

Series of methods developed for fabrication of well defined structures on micro + nano scale - main app is integrated circuits

Question 36

Photolithography

Answer 36

Makes patterns on microscale - uses UV light to transfer a pattern in a metal mask to a photo reactive material (photoresist polymer) on a surface

Question 37

Peltier effect

Answer 37

If you force a current through a bi-metal junction then when electrons go from A to B they gain energy at interface so energy taken from material ... cooling effect + vice versa

Question 38

Peltier effect for p type n type semiconductors

Answer 38

Electrons flowing from p type semiconductor to n type results in energy transfer/ cooling depending on circuit