Chem 109 - General Chemistry - Spring 2015
Lecture Notes 32: 15 April
Hybrid Atomic Orbitals, cont.
Remember, that with Hybrid Orbital Theory we are looking at individual atoms, not molecules. It is a Localized theory, all of our calculations and predictions are for the atoms. We make molecules by overlapping the new hybrid orbitals with other hybrid orbitals or with atomic orbitals of other atoms to make molecules.
Let's look now at some examples and illustrations in your text, noting single and multiple bonds etc.
Tetrahedral Electronic Geometry = sp3.
- Four orbitals (s + 3 p's) combined. (Note the sum of "exponents" = number of orbitals) [text Fig 9.3]
- Note in the following example text figures the side Hs should be behind the hybrid orbitals.
- Methane (CH4) - tetrahedral molecule. [text Fig 9.6]
- Ammonia (NH3) - trigonal bipyramidal molecule. [text Fig 9.7]
Trigonal Planar Electronic Geometry = sp2.
- Three orbitals (s + 2 p's) combined, one p orbital left as is. [text Fig 9.8, 9.9]
- Ethylene (H2CCH2) - each carbon has trigonal planar geometry. [text Fig 9.10, 9.12, 9.13]
Linear Electronic Geometry = sp1, or sp.
- Two orbitals (s + p) combined, two p orbitals left as is. [text Fig 9.14, 9.15, 9.16]
- Carbon dioxide (CO2) [text Fig 9.17, 9.18, 9.19]
- Nitrogen (N2) [text Fig 9.20]
Note we get two basic bond types when we overlap orbitals:
- Sigma (sigma) bonds: These are cylindrically symmetrical around the axis connecting the bonded atoms. Single bonds are always sigma bonds, and in a multiply bonded system the "first" or "central" bond is a sigma bond. [text Fig 9.20b]
- Pi (pi) bonds: these are made up of two lobes with planar symmetry round a plane though the nuclei of the two bonded atoms. The "second" and "third" bond of multiply bonded atoms are pi bonds. For systems with two pi bonds the bond panes are perpendicular to each other. [text Fig 9.20c]
To reiterate, the hybrid atomic orbital model is a localized electron model - the quantum calculations are looking at the atoms individually.
The hybrid orbital model is particularly useful to us at this time because it gives nice pictures of two aspects of bonding:
- Molecular shape - look at sp, sp2, sp3, dsp3, and d2sp3 [text Fig 9.24] Note that # of effective pairs corresponds to Steric Number in VSEPR Theory.
- Example: PCl5 [text Fig 9.21] dsp3
- Example: XeF4 [text p 425] d2sp3
- Multiple bond formation - sigma and pi bonds.
© R A Paselk
Last modified 15 April 2015