VBT Polyatomics
BCHEM 402 - W21 – M3 L8
Polyatomics
• Larger atoms have more than one valence atomic orbital to consider
• As before, consider the overlapping, the addition of wavefunctions to be the
source of bonding
This is water…what is wrong
with this picture?
• Another issue: according to VBT carbon should only form 2 bonds
Promotion
• This does not involve having a YouTube channel and posting a
dozen selfies a day.
• Allows paired electrons to unpair. Allows electrons in a full
atomic orbital to occupy an unfilled atomic orbital
• Carbon nominally 1s22s22p2 becomes 1s22s12p3
• now have 4 unpaired electrons available for bonding with unpaired
electrons like the 1s electrons in H. Do this four times and you get CH 4
(methane).
• Takes energy, but the energy is returned upon bond formation
Hybridization
• Promotion does not address the poor bond angles of
VBT…need hydridization for that.
• A wavefunction is just that, a mathematical function
and in spherical coordinates it is a function of (r, θ, φ)
• A hybrid wavefunction combines the wavefunctions of
two or more electrons in their orbitals upon squaring,
this changes the expected electron density for the
resulting hybrid orbitals
• The number of base wavefunctions used is the
number of hybrid wavefunctions obtained
sp - orbital
• A combination of a single s and single p atomic orbital
Ethyne: C2H2
Do the Lewis
structure
sp2 - orbital
• Add an additional p-type orbital so the hybridization
includes 1s and 2p orbitals yielding 3 sp2 orbitals
Unhybridized pz-orbital
• What atomic orbital is missing from this
hybridization?
Ethylene bonding (C2H4)
sp3 - orbital
• What orbitals are used?
• How many hybrid orbitals does one obtain?