Slides11e

Lecture #11

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Lecture Outline

Molecular Structure

Lewis Structures

Formal charge and preferred structures

Exceptions to the Octet Rule

Incomplete octets

Odd electron numbers

Expanded octets (hypervalency)

Examples

Resonance

Equivalent preferred contributors

Bond order

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.

Note that Lewis dot structures are an accounting of where valence electrons are likely to be found. The structures are very useful for giving quick information about molecular structure, but do not represent what a rigorous theory tells us about valence electron distributions.

First exception to be discussed is what had previously been mentioned with respect to hydrogen; the lightest elements often do not complete their octets.