Time to put those atom models to use and make molecules.
Textbook Reading: Chapter 10, pages 325-349.
Lewis Dot Diagrams and Structures
Last week we learned that the outer electrons, called valence electrons, of an atom are the ones involved in bonding and chemical reactions. Lewis structures consist of the element’s symbol surrounded by dots to indicate the valence electrons.
There are a few rules for creating Lewis structures:
1. Find the number of valence electrons for a given element using the group numbers of the periodic table. There will never be more than 8.
2. Represent the valence electrons by placing dots on four sides around the symbol for the element.
3. Start filling with single dots. If there are 5 or more valence electrons, pair them after the four single dots have been placed. Exception: Helium has a single pair of two dots because that is its stable configuration (see illustration above).
4. Exact location of dots can vary (which side placed on doesn’t matter).
Bozeman Science has an in depth explanation about Lewis structures. (Those who like their chemistry to “pop” will enjoy the first part of this video. The last part about Lewis himself is sad.)
When drawing molecules, the sharing of two electrons is represented by a line connecting the symbols. If the two atoms share two pairs of electrons, then the resulting double bond is shown as two lines.
Be aware that as powerful as it is, there are exceptions to the octet rule!
Valence Shell Electron Pair Repulsion (VSEPR)
Remember how the magnets of the same pole repelled each other, pushing away? The VSEPR model uses that concept to predict the shape of molecules.
Some of the structures we will investigate are:
- trigonal planar
- trigonal pyramidal
(See the Sidebar about VSEPR post to see these shapes)
Mr. Isaacs at IsaacsTeach has an introduction to molecular geometry that explains why water molecules are bent.
Bozeman Science pulls the Lewis Structures and VSEPR together for a comprehensive overall of drawing molecules. He goes into a bit more depth than is covered in your text.
Electronegativity and Polarity
Another aspect of molecules that determines their shape and chemical properties is the ability of the atoms of an element within a bond to attract electrons, or its electronegativity.
You can read the electronegativity of an atom from a specially designed periodic chart like this one:
(Illustration from UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.)
Non-polar bonds form when the difference in electronegativity between the two atoms is between 0 and 0.4, polar bonds form when the difference in electronegativity between the two atoms is between 0.4 and 2.0, and ionic bonds form when the difference in electronegativity between the two atoms is greater than 2.0.
Why does this matter? Knowing whether a molecule is polar helps predict its characteristics, such as whether it will be soluble in a given solute.
That’s it for this week. Let me know if you have any questions!