Chemical bonding lecture 5 Flashcards
What are the two rules to be followed when constructing MOs for second-period elements?
- Two atomic orbitals contribute significantly to bond formation only if their atomic energies are close to one another ( 3ani no mixing between 1s with 2p and 2s, no mixing between 2s and 2p UNLESS THE 2S ORBTIAL APPROACHES THE 2P ORBITAL END ON)
- Two atomic orbitals on different atoms contribute significantly only if they over-lap significantly
What are the different values that could occur when two atoms overlap? How are such values calculated?
The net overlap may take positive, negative, or zero values depending on the amplitude and phases of the orbitals involved. We calculate it by taking the integral product of each atomic orbital all over space.
The overlap is positive when constructive interference occurs between the wave function of the AOs
The overlap is negative when deconstructive interference occurs between the wave functions of the AOs
The overlap is zero due to either having a very large distance between the orbitals or the positive overlap regions canceling out the negative overlap regions giving us a net zero such orbitals that behave in such a way are called non-binding orbitals
What are the Different MOs that occur in the second period?
297 - 299
note: when the constructively interfere the two atomic orbitals in LCAO approx are with different SIGNS on there wave function so that means when you get the density function of bonding or anti-bonding it’ll be the same
note 2: Pz has sigma notation, Py and Px have Pi notation due to symmetry
Personal note: These are very very important take your time with them understand the different questions provided!
Which 2P MO is stronger Sigma or Pi? What are the energies of bonding and anti-bonding MOs in comparison to there respective boning MOs?
It is sigma since the overlap between the pi orbital is smaller since it occurs across the intermolecular axis.
The anti-bonding and boning of a Mo have the same energy.
What are the different energy orderings for diatomic molecules made from period 2 elements? What is the graph that shows the energy ordering of the MOs? state the conclusions that can be made from it.
The two different orders are given by the first half of the period from Li to N (Z =< 7) and the second half of the period O to Ne (Z >= 8), wherein the first half it is expected that the MO of the 2pz has larger energy compared with the MO of 2px and 2py, and it is expected that for the second half the MO of 2px and 2py have larger energy than the 2pz and that could be explained by the fact that the energy of the 2pz is increased as it overlaps the 2s orbital, where when the Z increases the overlap becomes less significant and the energy of then therefore 2pz decreases where the energy of the 2px and 2py is constant since they have no overlapping potential so eventually (at Z=8) the energy os 2px and 2py take over the energy of 2pz
The graph is on page 300, the main conclusions are:
- B2 and O2 are paramagnetic due to the two unpaired electrons found in their valance MOs
- The high reactivity of the molecular oxygen in comparison to the N2 is due to the unpaired electron in its valence MO
What is the relation between bond order, bond length, and bond energy
as the bond order increases, there are more bonds between atoms so the bond length decreases and becomes stronger causing the bond energy to increase.
What main changes are made when we construct molecular orbital for heteronuclear diatomic molecules?
- For heteronuclear diatomic molecules the concept of symmetry that is found in the homonuclear diatomic molecules is no longer present since heteronuclear diatomic molecules lack inversion symmetry therefore no u or g labels in the MOs
- Due to heteronuclear having two different atoms which then correspond to AOs of different energies, the stoichiometric coefficients of bonding and anti-bonding MOs will not be equal to each other (Like how it was in homonuclear diatomic molecules) with the more electronegative atom contributing more the bonding MOs and the less electronegative atom contributing more to the antibonding MOs.
How do we draw molecular orbital correlation diagrams for heteronuclear diatomics? How can we show if such a molecule is paramagnetic? How can we determine which Heteronuclear diatomic molecule has the greatest bond energy?
Notes
To find if a molecule is paramagnetic find first its bond order and if it is not a full number then it’s paramagnetic(CHECK IM NOT SURE),
To find which Heteronuclear diatomic molecule has the greatest bond energy we find the bond order and compare, the Heteronuclear diatomic molecule with the largest bond number has the highest bond energy
What is the valance Bond Theory?
it is a theory that provides an explanation and justification for the Lewis electron-pair model in which a chemical bond is described as two electrons localized between two atoms. The VB theory constructs a wavefunction for each chemical bond by assuming that each atom in the bond holds one unpaired electron (the one that’s going to be used to make a bond) in its AO.
The VB wavefunction is given the product of the individual wave function of both the one-electron AOs.
If two objects are independent, what happens to their energies and wave functions?
Their total energy is the sum of their individual energies and the wave function is a product of the individual wave functions
How can we find the total electron density as a function of intermolecular distance for 2-electron functions>
When we square the given wave function we get the probability density of finding electron 1 at r1 and electron 2 at r2, but in order to find a better electron density we must find the individual electron density of each electron.
We do that by squaring the wave function and then averaging out all of the possible locations of one of the electrons, then we repeat the same process but this time average out all the possible locations of the other electron, and then we add both our results to give us the overall electron density.
what is the relation between both the electron density and the energy between nuclei and bond formation?
when the electron density is large between two nuclei and the energy between them is lower than that of two separated atoms, this shows stable bond formation, whereas vice versa would show anti-bond formation
how are sigma bonds represented in the VB model?
sigma bonds have their electron density cylindrically symmetric about the bond axis, and due to that they have no angular moment around the bond axis.
how are single and multiple bonds approximated using the VB theory?
single bonds - very simply by taking the valance AO with the lone electron that is ready to be paired with another lone electron from another valence AO and forming a model that takes the product of the wave function when electron 1 is in AO 1 and electron 2 is in AO 2 and sums it up with the product of the wave function when electron 1 is in AO2 and electron 2 is in AO1, and then multiply by a normalization constant C.
multiple bonds - each bond made between the molecules would have its own wave function expression, following the method stated in single bonds
What was the problem with predicting poly-molecule structures with VB? What is the solution?
The problem is that it does not give an accurate representation at all, so the solution proposed was that the electron would be promoted to a higher energy level and then hybridized for the VB model to be accurate. (view page 315 if confused)
