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]
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