Lecture Nine Flashcards
What is a homonuclear and heteronuclear bond?
Bonds formed between atoms of the same types. You can have molecules with both homo and heteronuclear bonds. E.g. C2H6
What is the molecular orbital theory?
Exploits the wave-like properties of electrons discussed earlier. A quantum mechanical treatment of molecules similar to that used for isolated atoms, based on the schrodinger wave equation. Describes regions of space that an electron might occupy over a whole molecule. Basic idea is that wave-functions are constructed by taking linear combinations of the atomic orbitals of the constituent atoms. MO’s can be constructed by mathematically combining the atomic orbitals of each atom that overlap in a suitable manner. Number of MO’s generated = Number of AO’s used. Combining two compatable AO’s results in an ‘in-phase’ and ‘out-phase’ combination.
What do constructive and destructive waves look like?
Show how atomic wave functions can constructivly or destructivly interfere with one another.
Constructive interference -
Wave functions reinforced.
Bonding MO is formed (region of high electron density exists between nuclei).
Destructive interference -
Wave functions cancel.
Anti-bonding MO (node of zero electron density between nuclei results in repulsion).
Notation:
A sigma refers to the symmetry (no phase change when orbital is rotated about the internuclear axis).
Sigma with an asterix refers to a nodal plane between the nuclei, perpendicular to the internuclear axis.
Why do bonds not form between noble gasses?
For He2, the bonding effect of 2 electrons in the sigma 1s orbital is cancelled by the anti-bonding effect of the 2 electrons in the sigma asterix 1s orbital.
No net decrease in energy.
No bond is formed.
He2 does not exist.
How are core and outer electrons relevant in molecular bond theory?
Bonding and ani bonding MO’s for core electrons cancel andso make no net contribution to bonding.
MO diagrams usually omit core orbitals, since only MO’s created combining valance electron AO’s are important for bonding.
How are P atomic orbitals different in molecular bonding theory?
Overlap of P atomic orbitals can form sigma bonds.
The 2pz AO’s that point directly at one antoher can form:
An anti bonding orbital = sigma * (2pz).
A bonding orbital = sigma (2pz).
Overlap of p atomic orbitals can also form pi bonds.
The 2px AO’s may overlap in a sideways manner to form:
An anti bonding orbital = pi * (2px).
A bonding orbital = Pi (2px).
End to end overlap is better than side to side overlap.
