CHEM Midterm #1 Flashcards
What is a planar node?
horizontal node
What is a radial node?
vertical node
Define degenerate orbitals
orbitals that have the same energy
What are the rules for filling MOs and AOs
- start by filling the lowest energy orbital first
- only two electrons can occupy an orbital
- apply Hund’s rule for filling degenerate MOs
What is Hund’s rule?
every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied
What is the bond order formula?
1/2 (number of electrons in bonding orbitals - number of electrons in antibonding orbitals)
How do you calculate orbital energy?
electron kinetic energy + electron-nuclear attraction + electron-electron repulsion
How is a sigma bonding orbital created?
Adding orbitals in the same phase
How is a sigma antibonding orbital created?
Subtracting orbitals (adding orbitals in 2 different phases)
Define constructive interference
when two waves superimpose and the resulting wave has a higher amplitude than the previous waves
Define destructive interference
when two waves superimpose and cancel each other out, leading to a lower amplitude
Define superimpose
The ability for an object to be placed over another object
In an MO diagram, which orbitals are bonding, antibonding, and non-bonding?
Bonding = lower in energy than the atomic orbitals
Antibonding = higher in energy than the AOs
Non-bonding = same energy as the AOs
How many MOs will exist per AO?
There are the same number of MOs as AOs
Which bond order causes the bond to break?
0
What does out phase and in phase mean?
out phase = 2 orbitals that are in different phases
in phase = 2 orbitals that are in the same phase
Define orthogonal
perpendicular to
What does a pi-bond look like?
Has a nodal plane through the bond axis
When can 2s orbitals combine with 2p orbitals?
when the orbitals are close enough in energy (within +/- 1 Ry = 2.179x10^-18 J = RH)
What is a parallel spin?
Multiple electrons in same direction
What is a paired spin?
Two electrons in the same direction
Define paramagnetic
multiple half-filled orbitals
Define diamagnetic
all electrons are paired
What orbitals can mix to form MOs?
atomic orbitals of the same symmetry (with respect to the bond axis)
What are the four general guidelines for constructing MOs from AOs?
- Sort the AOs into sigma symmetry vs. pi symmetry
- within each category, group together AOs with energies = 1 Ry
- mix each group of AOs to make a set of MOs
- Label and number the MOs from lowest energy to highest. You need one set of numbers for sigma and one set for pi (remember that degenerate pi orbitals would have the same number)
Define heteronuclear
requires that they be comprised of one atom of two different elements
Define homonuclear
they contain two atoms of the same element
Define isoelectronic
having the same numbers of electrons or the same electronic structure.
Molecules will only absorb infrared radiation if…
doing so changes the dipole movement of the molecule
- means that no homonuclear molecules
- for diatomic molecules, this means that only polar ones will be IR active
What is the photoelectron effect?
phenomenon in which electrically charged particles are released from or within a material when it absorbs electromagnetic radiation. The effect is often defined as the ejection of electrons from a metal plate when light falls on it.
What is photoelectron spectroscopy (PES)?
If a molecule is excited by a high-energy photon in the ultraviolet region of the spectrum that has sufficient energy to ionize the molecule, the excited species will eject electrons.
What is the chemical formula for benzene?
C6H6
What kind of MO diagram can you make from a frost circle?
Pi MO
How do you calculate bond order from Lewis structure?
- Draw the Lewis structure.
- Count the total number of bonds.
- Count the number of bond groups between individual atoms.
- Divide the number of bonds between atoms by the total number of bond groups in the molecule.
Define Fermi level
the highest energy state when no such excitation has occurred
Define conduction band
a delocalized band of energy partly filled with electrons in a crystalline solid. These electrons have great mobility and are responsible for electrical conductivity.
Define valence band
the range of permissible energy values that are the highest energies an electron can have and still be associated with a particular atom of a solid material
Define overlap
The interaction of orbitals on different atoms in the same region of space.
Define band gap
the minimum amount of energy required for an electron to break free of its bound state
Define absorption spectroscopy
Absorbance is a measure of the quantity of light absorbed by a sample
What is a conductor?
no energy gap between the conduction and valence bands
What is a semiconductor?
small energy gap between the conduction and valence bands
What is an insulator
large energy gap between the conduction and valence bands
What does the band gap correspond to?
the lowest frequency (longest wavelength) absorbed by a solid
What is the rule of thumb for kBT?
- Band gap»_space; kBT —> insulator
- Band gap is smaller than or close to kBT —-> conductor
- Band gap is approximately 10X kBT —> semiconductor
Intrinsic semiconductors
- Naturally have a moderate band gap. A small fraction of the electrons in the valence band can be excited into the conduction band. They can carry current
- The “holes” these electrons leave in the valence band can also carry current as other electrons in the valence band can be excited into them
Extrinsic semiconductors
-have had impurities added in order to increase the amount of current they can conduct (impurities called dopants; process called doping)
- the dopants can either provide extra electrons or provide extra “holes”
- a semiconductor doped to have extra electrons is an n-type semiconductor (‘n’ is for ‘negative’)
- a semiconductor doped to have extra holes is a p-type semiconductor (‘p’ is for ‘positive’)
Define donor band
A band that provides an additional band full of electrons that is higher in energy than the other molecule and closer to the conduction band
-electrons in this band are more easily excited into the conduction band
Diodes
devices in which an n-type and p-type semiconductor are connected (usually in order to allow electric current to pass in one direction)
- the acceptor band in the p-type semiconductor gets filled with the extra electrons from the n-type semiconductor. The extra holes from the p-type semiconductor thus “move” to the n-type semiconductor
- with negative charge moving one way and positive charge the other, charge separation builds up and stops both electrons and holes from moving unless the diode is connected to a circuit
How do you make an MO diagram?
- Determine # of valence electrons
- Figure out the electron configuration
- Draw the orbitals according to which atom is more electronegative (more should be higher, lower is less)
- Sort into pi-symmetric and sigma-symmetric
