Lecture #11
  Chapter 13 in text now. We will follow a different sequence than the text.

Text: Sections 13.2 and 13.10

  CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.
Lecture Outline Molecular Structure

Lewis Structures

Valence electrons (Main groups)

Octet Rule

The valence electrons can be represented by an "electron dot" around the symbol of each element. These are conventionally indicated either as separate single dots or as pairs, depending on the total number of valence electrons as shown in the figure.
In combining atoms to form molecules, Lewis' Octet Rule accounts for how valence electrons are distributed. Note this is not a theory to explain bonding, but merely a book-keeping scheme for tracking valence electrons consistent with what is observed in molecular structure.  
An example of sharing valence electrons to accommodate the Octet Rule.  
In symbolizing valence electrons, a single line can be used to represent a shared pair of electrons; that is, to represent one bonding pair of electrons.  
More than one pair of electrons can be shared to be consistent with the octet rule.  
Here, there are three shared pairs of electrons (six electrons involved) and also two pairs of electrons not involved in bonding at all, and called "lone pairs".  
These are a small selection of bond energies showing how the bond energy increases with increasing bond multiplicity
These values illustrate how bond lengths decrease with increasing bond order.
More than two atoms: When given a molecular formula for a polyatomic molecule, such as ethane, the very first step in constructing the complete Lewis dot structure is to arrive at a framework, a skeleton, of how the nuclei are linked.  
One of the first steps in constructing a Lewis dot structure is to decide on the "skeletal" arrangement of key atoms. Hydrogen and fluorine are (almost) always on the outside and thus not part of the skeleton.
Here are two different chemical species, each with exactly the same molecular formula. They are the fairly common substances ethanol and dimethyl ether. (b.p. abbreviates boiling point)
Without skeletal information, there can be more than one result in figuring out a Lewis electron dot structure.
Here we introduce the term "structural isomers" using the above illustration.
Here are eleven different skeletal structures for a molecule with the formula shown. Not all will prove to be relevant. Some will not work at all as frameworks to be completed. (Note added, one more linear possibility is missing: that with oxygen at each end.)
Here's a slightly more complicated situation in which the formula, C2H4O2, has a number of different skeletal structures possible. Additional information allows you to draw the framework shown in which the linkages have used 14 valence electrons.
Ten more valence electrons must be placed in the structure. They are placed in the atoms whose octets are not yet filled. Ten such electrons are placed here in blue. Other arrangements are possible, but not shown. The carbon in the C-O has only six electrons and violates the octet rule. A pair of electrons from neighboring oxygen can be moved over and shared with that carbon.
This is the correct Lewis structure for the compound identified. (Alternatively, a pair of electrons from the other oxygen would have satisfied the octet rule as well, but another consideration -- formal charge -- would reject that possibility as less favorable. We discuss formal charge later in the lecture.
A complete sequence of analysis would follow steps you are now at least moderately familiar with. A few topics relating to further details about structure need to be addressed next.
"Formal charge" for our purposes (in this course) are to be considered a required part of a complete Lewis structure.
Calculating formal charge using carbon monoxide as a simple example.
In deciding among various arrangements of valence electrons, formal charges serve to indicate which arrangement is probably closest to the best choice. Such an arrangement will be referred to as a "preferred structure."
An additional consideration for deciding upon a preferred Lewis structure in which there are formal charges.
Using the "azide" ion as another illustration of deciding upon a preferred Lewis structure.