Caroline's weblog
Monday July 17, 2006
Molecular Geometry lab
Up to this point we have been talking about ionic compounds - compounds with two ions (cation- girl, and anion- boy). Now in this lab we start investigating covalent compounds. Covalent compounds will share electrons, and they are usually made up of two or more nonmetals, for example CO2.
A Lewis structure is a way to depict a covalent compound on a two-dimensional piece of paper. Here is a website that describes what I describe below. (Maybe a different way will help you learn this material.)
The steps to take when drawing a Lewis structure are:
1st : Count number of Valence electrons (VE) the molecule has. How do you find this out? By looking at where the atoms that make up the molecule are in the periodic table. Noble gases have 8 valence electrons (except for He, only has 2). The rest of the nonmetals valence electron count is determined by its group number. All the elements in the Fluorine column have 7, all the elements in the Oxygen column have 6, all the elements in the Nitrogen column have 5, and so forth.
2nd: Count the number of required electrons (ER) that should be used to make up the molecule. How do you find this? Just remember that all the atoms want to fulfill the octet rule (have 8 electrons), except for hydrogen (wants to have just 2). 3rd: Find the number of shared pairs (SP) that the molecule has. This is found by taking ½ * (Required electrons - Valence electrons). 4th: Find the central atom - usually the one with the lowest electronegativity, but they can NEVER be hydrogen. 5th: Draw it in the center and the surrounding atoms around the central atom. Place the number of shared pairs found in step 3 between the central atom and the surrounding atoms. (You can have double and triple bonds as well). 6th: Place the rest of the valence electrons around all the atoms until their octet is filled. 7th: Calculate formal charge for each atom. Formal charge for an atom = Valence electrons - (nonbonding electrons in the lewis structure + ½ number of bonding electrons in the lewis structure). When you have your Lewis structure drawn you can determine shape and geometry by applying VSEPR's theory, and you can determine the hybrid orbitals by determining the number of electron regions around the central atom. In this lab you will be drawing Lewis stuctures and then looking at their three-dimensional structure. Here is a website about 3-dimensional structure of molecules. You should be asking yourself what are the differences between the picture you drew and the three-dimensional shape. You will also be investigating bond lengths and bond angles. I want to warn you that the pre-lab for this lab is very long. I would start it early!
Posted at 01:08PM Jul 17, 2006 by CHRISTIAN, CAROLINE in General | Comments[4]
This is a personal weblog, I do not speak for my employer.
most helpful
Posted by Kyle on July 18, 2006 at 09:18 PM EDT #
Thank you for posting this! It makes so much more sense once you break it down and really explain each step. Unfortunately, I didn't look at this until after the pre-lab, but it makes more sense now.
Posted by Mandy on July 19, 2006 at 09:13 PM EDT #
I am glad that people found my instructions for how to draw a Lewis structure helpful. I usually try to post things about a lab the day before the lab. So if you have any time the day before or the day of - you can always check it out.
Posted by Caroline on July 20, 2006 at 09:08 AM EDT #
Yeah, I definately had to guess at a most of the structures in the pre-lab. After seeing this though, I went back and actually understood it. This will be good for future reference and tests.
Posted by Whitney Salter on February 18, 2008 at 07:29 PM EST #