CH301H - Principles of Chemistry I: Honors
Fall 2012, Unique 51390

Lecture Summary, 13 September 2012

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 senario.  There are a number of exceptions to these rules, the most interesting being expanded octets.