CH301H - Principles of Chemistry I: Honors
Fall 2016, Unique 50015

Lecture Summary, 8 September 2016

Polar Covalent Bonds:  Today we combined all of the ideas about atoms and diatomic molecules that we have discussed so far to come up with a way of describing real bonds and molecules. Most bonds are neither completely ionic nor completely covalent, but something in between.  When two atoms with different electronegativities form a bond, the atom with the higher electronegativity will tend to sequester the electrons in the bond.  This creates a partial negative charge on the higher electronegativity atom and a partial positive charge on the lower electronegativity atom.  This distribution of partial charge in turn creates a dipole moment, a vector that points from the negative end of the atom to the positive end.  Molecular dipole moments can be measured experimentally.  For molecules larger than a diatomic, individual bond dipole moments can be inferred from knowing molecular geometry.

Lewis Dot Structures:  Today we did a brief review of the rules for drawing Lewis dot structures.  Lewis dot structures are a formalism for determining atomic connectivity in a molecule, which in turn will help us figure out molecular geometry. To draw an accurate Lewis dot structure, make a series of covalent bonds, each containing 2 electrons, in such a way that all atoms have 8 electrons in their filled valence shell (except for H, which will have 2 electrons in its valence shell).  Here are my general rules for Lewis dot structures:

1.  Hydrogen and halides can only form one bond and are always terminal atoms on a molecule.

2.  Write out each atom with its own valence electrons and make an initial guess about the structure.  In general, atoms with the fewest valence electrons will be the central atom.  Chemists very often write the molecular formula with the central atom listed first, although this is not always true.

3.  Start making bonds, either between single electrons on two different atoms, or with both electrons from a single atom.  Remember to either add or remove electrons as needed to achieve the appropriate molecular charge.

4.  Assign formal charges.  Add up the formal charges to make sure it equals the known molecular charge.

5.  If you have multiple reasonable structures, in general, the "correct" structure is the one with the fewest nonzero formal charges, and where formal charges are the lowest (+/-1).  

6.  Then draw resonance structures.  

These rules are useful, but will not guide you through any possible scenario.  There are a number of exceptions to these rules, the most interesting being expanded octets. 

VSEPR:  Valence shell electron pair repulsion (VSEPR) theory is a way to describe molecular structure based only on the way atoms are connected in a bond.  Provided we are able to identify a correct Lewis dot structure (i.e. correct number of bonds connecting correct atoms and correct number of lone pairs on correct atoms identified), then we can use VSEPR to figure out how atoms and lone pairs will arrange themselves around a central bond.  

Steps for VSEPR:

   1.  Write a correct Lewis dot structure
   2.  Find the steric number of the central atom.  SN = # of bonded atoms + # of lone pairs.
   3.  Identify the shape associated with that SN:

                 SN = 2    linear
                          3     trigonal planar
                          4     tetrahedral
                          5     trigonal bipyramidal
                          6     octahedral

The shape determines the angle between adjacent bonds around the central atom.

   4.  If there are lone pairs, figure out where they need to go, and how that will distort the shape of the molecule.  

VSEPR does a very good job of predicting the structure around central atoms, particular for molecules whose central atom is C, N, and O, and for trigonal bipyramidal and octahedral molecules.  Like Lewis structures, however, VSEPR is only a formulism, and it isn't too difficult to find molecules whose experimentally determined shape is quite different from what would be predicted by VSEPR.