Atomic and Molecular Structure Flashcards
Metallic bonding
Atomic orbitals combine to form delocalized electron cloud shared by large number of atoms - non-directional
Secondary bonds
- Van der Waals: weak bonding induced by fluctuating/permanent molecular dipoles
- H bonding: bonding between protons + available electron pair
- Directional
Is covalent bonding directional?
Yuh
Is ionic bonding directional?
No
Comparison of bond energies for diff bonds
- Ionic: large
- Covalent: variable
- Metallic: variable
- Secondary: smallest
Melting temperature bond energy relationship
Tm larger if Eo larger - proportional to depth of potential energy curve
Elastic modulus bond energy relationship
E is larger if Eo larger - proportional to curvature at ro (unstretched length) energy
Coefficient of thermal expansion bond energy relationship
Coeff is larger if Eo is smaller - proportional to symmetry at ro energy (low Eo = asymmetry)
Hardness bond energy relationship
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
Electrical conductivity from bond nature
- 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
Ceramics material properties (they have ionic + covalent bonds)
Large bond energy, large Tm, large E + small coeff
Metals material properties (metallic bonding)
Variable bond energy, moderate Tm, moderate E + moderate coeff
Polymers material properties (covalent + secondary)
Directional properties, secondary bonding dominates, small T, small E + large coeff
How does band splitting work?
- 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)
Valence band
Band having highest energy electrons at 0K
Conduction band
Next band at energy > valence band
Fermi energy
At 0K all electrons have energy smaller or equal to Ef
Conductors
Energy needed very small to move in conduction region + become free electrons
Insulators
Large energy band gap exists between full valence band + conduction region
Semi-conductors
Same as insulators but w/small band gap
What is resistance of pure metals near 0 temperature?
Very small
Does resistivity increase or decrease with temperature?
Increase
Does resistivity increase at absolute 0 temperature?
Yuh
Are good conductors pure or impure?
Pure
What are bad conductors typically?
Alloys
Do metals deformed by work hardening have higher or lower resistivity than same metal in stabilized state?
Higher
Fermi energy
Energy where probability of occupancy is 50% for any T>0K
Is electron mobility higher or lower than hole mobility?
Higher - more difficult to move a hole than an electron
n-type extrinsic semiconductor
Surplus of 1 electron for each P atom added - electron easily goes to conduction band and delta E needed is very small
p-type extrinsic semiconductor
Missing 1 electron for each B atom added creates hole - hole easily goes in conduction in valence band and delta E needed very small
Do n or p types have higher fermi levels?
n
Diode
p-n rectifier junction (current only flows in 1 direction)
Transistor
Has 3 semiconductor sections - current only flows between source + drain if potential is applied at base
Applications of transistor
- Amplifier: signal sent to base is amplified between emitter + collector
- Electronic switches: current only flows between collector + emitter when voltage applied at base
Microfabrication
Series of methods developed for fabrication of well defined structures on micro + nano scale - main app is integrated circuits
Photolithography
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
Peltier effect
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
Peltier effect for p type n type semiconductors
Electrons flowing from p type semiconductor to n type results in energy transfer/ cooling depending on circuit