Topic 5: Molecular orbitals in solids, spectroscopy and ionic bonding Flashcards
What is network solids and how are their molecular orbitals and associated energy levels different from those in small molecules?
- A really large number of small molecules come together producing a network solids
–> large number of atomic orbitals mean large number of molecular orbitals
–> number of molecular orbitals and associated energy levels are so high that they form electronic bands - Valence band: orbitals fully occupied
- Conductance band: orbitals unoccupied
- Band gap:
+ the energy difference between conductance and valence band
+ the minimum energy required to promote electrons from valence to conductance band
Explain conductivity in terms of valence and conductance band
Conductivity is the ability of a substance to conduct electricity or the ability to promote electrons from valence to conductance band. There will be holes left behind in valence band.
Explain the differences between insulators, intrinsic and extrinsic semiconductors and conductors in terms of band gap.
- Insulators: the band gap is too large for electrons to be promoted from valence to conductance band
- Conductors: the band gap is so small that it is negligable and the bands are considered overlap
–> very easy for electrons to move from valence to conductance band - Intrinsic semiconductors: materials that can become conductors when heat is being applied to provide enough energy to pump electrons from valence to conductance band
- Extrinsic semiconductors: some materials cannot rely on temperature and need to be more stable –> rely on doping to become conductors
Describe the band gap, valence and conductance band in metals
Metals are composed of extensive and rigid networks of atoms.
The band gap is so smal that it is negligable, so the conductance and valence band can be considered to overlap.
Describe doping in general, n-doping and p-doping
- Doping: adding impurities in the material substance to make it an extrinsic conductor
- n-doping: adding elements with higher number of electrons
+ extra negative charge carriers
+ extra electrons stay at a donor level carrying electrons and sitting right below the conductance band
+ electrons can be easily pumped to the valence band
+ most of current conductors are due to free electrons - p-doping: adding elements with fewer number of electrons
+ extra positive charge carriers
+ add a receiver level sitting right above the valence band, ready to receive the electrons from valence band
+ most of the current conductors are due to the holes leaving behind
Explain why the conductivity of semiconductors increases with temperatures
Intrinsic semiconductors rely their conductivity on thermal energy. As temperature rises, the material receives more energy needed to promote more electrons from valence to conductance band. More electrons having access to valence band means the material has higher conductivity.
Describe the relationship between absorption and observed color
A material has an ability to absorp a certain range of radiation wavelength. Which wavelength it doesn’t absorb will get reflected as the color we see.
Explain terms: transparent, transclucent and opaque
Describe materials being i) opaque and colorless and ii) transparent and colorless
- Transparent: a transparent material allows all light to pass through (not totally true as materials allowing all ultraviolet light may not be similar in infrared light)
- Translucent: allow some light to go through but the light is scattered and deflected
- Opaque: not let any light going through; either reflected or absorbed
- Being opaque and colorless: white
- Being transparent and colorless: clear
How is absorbance related to the sample concentration and path length through which the light has to travel?
What law is this?
Mention the equation
A = εcl
A or absorbance is directly proportional to concentration of the sample and path length.
Beer-Lambert Law
The higher the absorbance, the more concentrated the solution and the solution may appear darker.
Explain the mechanisms of atomic spectroscopy (details for how hollow cathode lamp works)
- A sample is exposed to flame to atomized the sample or to yield free ground atoms that can be excited.
- A hollow cathode lamp is used to eject a beam of light going through the sample.
+ the lamp containing the atoms of sample as well as inert gas
+ when energy is applied, the atoms are hit with inert gas and then be excited to higher energy states
+ when the excited atoms return to ground state, they emit a beam of light with characteristic wavelength to that sample atoms - The light will go through the sample and the amount beind absorbed will depend on how concentrated the sample is.
- Monochromator ensures only light with the characteristic wavelength is measured and the detector reads the absorbance.
Identify the main difference between atomic and molecular spectroscopy
- Atomic spectroscopy: measures the electromagnetic radiation absorbed by atoms
- Molecular spectroscopy: measures the electromagnetic radiation absorbed by molecules
Describe and explain what electronegativity is and its periodic trends
- Electronegativity is the ability of an element to attarct bonding electrons
- Periodic trend:
+ increases across period
+ decreased down group
Why do atoms want to lose or gain electrons? What are some good electron donors and acceptors; their position in periodic table?
- Atoms gain or lose electrons to become isoelectronic with their nearest noble gases with full valence shell.
- Good electrons donors are more to the left of periodic table.
- Good electron acceptors are more to the right of periodic table.
Explain the origin of ionic bonding and its purpose in terms of potential energy
Ionic bonding occurs due to long-range attaction between cations and anions and short range repulsion between electrons in adjacent ions.
Ionic bonding occurs to minimize the potential energy level of new molecules.
Explain electrostatic interactions and its contribution to the crystal structure rather than small molecules
Electrostatic interactions refer to attractive and repulsive forces between charged atoms.
+ isotropic or non-directional
+ long-ranged
- A shell of oppositely charged ions surround a central ion.
- Another shell of ions is attracted and this keeps going –> extensive structuce gives rise to ionic crystals
