LEWIS STRUCTURES
OBJECTIVE
To learn how to draw Lewis structures and  build 3-D molecular (ball-and-stick) models of various compounds.  Determine the  shape of each molecule based on the Lewis structure and 3-D  molecular model .

INTRODUCTION
Different types of chemical formulas
One way of representing chemical compounds is to use their molecular formulas. The only information provided by molecular formulas is the types of atoms involved in a molecule of a compound and their relative numbers.
Structural formulas provide additional information concerning the relative position of atoms in a molecule of a given compound, or the spacial arrangement of atoms.
In addition to this, Lewis structures indicate the number and placement of unshared valence electrons.  The actual 3-D shapes of  molecules may be represented by the   ball-and-stick models in which the balls of various colors represent atoms and the sticks represent the bonds.

Drawing Lewis structures
Remember that the number of valence electrons in a main group  atom is equal to the number of periodic table group in which this atom is located. When drawing Lewis structures, keep in mind that  the duet rule applies for hydrogen atoms and the octet is valid for the majority of other atoms. However, there are exceptions to the octet rule, particularly when third and lower row elements are considered. Also remember that some atoms may form multiple bonds ( double or tripple)
The procedure for drawing Lewis structures is outlined in Chapter 5, page 108,  of  Fundamentals of General, Organic, and Biological Chemistry (McMurry and Castellion).
You may  also visit this site for more information.

Ball-and-stick Models.
To build a ball-and-stick model based on the Lewis structure drawn, use the information organized in Table1.

Molecular shapes.
Molecular shapes may be predicted based on the number of covalent bonds and the number of  lone electron pairs involving  the molecule’s central atom. Please refer to Table 2 to find the relationship between the number of electron pairs and the molecular shape. Refer to the lecture text for more information.

Example:
Write a Lewis structure and determine molecular shape of phosgene (COCl₂).
1.  Calculate the total number of valence electrons in COCl₂  :   1x 4 (C) + 1 x 6 (O) + 2 x 7 (Cl) = 24
Click here to  see the  pictorial  representation of  step 2, 3 and  8 (to leave this picture, click Back)
2. Carbon is the central atom in this molecule, therefore  other atomic cores are placed as symetrically as possible around it.
    Draw the lines connecting the central carbon atom with the two  chlorine atoms and one  oxygen atom  (lines represent             bonding pairs of electrons, single bonds)
3. Add lone pairs of electrons ( two dots each) to each peripheral  atom  until the octet is satisfied for each Cl  and O atom.
4. Count electrons surrounding each peripheral atoms:  O  3x2+2= 8,  Cl  2 (6 +2)= 16
5. Determine the number of remaining electrons: 24-24= 0. There are no more electrons to be added to atomic cores.
6. Check if all atoms have octet: Cl - does, O - does , C- has only 6 electrons.
7. Need  one more pair of electrons between C and O to satisfy octet rule for both;  use the lone pair on O to create another bond ( line) between C and O
8. To determine the  molecule shape, count the electron pairs surrounding the central atom ,carbon (C). It is 3, beacuse double bond C=O counts as one bonding electron pair  for the purpose of molecular shape determination.
9.  There are no lone pairs present on the C atom, therefore, based on the info in  Table 2, the phosgene molecule will have the trigonal planar molecular shape ( 3 total electron pairs with 3  bonding electron pairs), and the bond angle of 120^o

Table 1                                      Table 2

PROCEDURE
1. Obtain a jar  with balls and sticks. Transfer the jar  contents to a large beaker or large evaporating dish. Take care not to spill and loose any balls or sticks!
2. Have your data sheet (see next page)  ready.
3. For each compound,  calculate the total  number of valence electrons (record in data table), draw  Lewis structure (in table), build a ball-and-stick model and draw it in the data table. Then determine the molecular shape and record  it in the  data table.

PRELAB QUIZ
Quiz must be taken before the lab. Late quizzes will not be accepted. You can only take this quiz once.
Instructions:
In the Quiz window, type your  instructor's name in the  "Name" box  your name and  e-mail address in the "e-mail "box.
For the first four questions  click on circle by the correct  answer.
Question 5 requires a written response.
After answering all questions, click Submit button.
You will see "Thank you" in the screen. You quiz has been sent to the lab coordinator's e-mail address and will be forwarded to your instructor.
Click here to take the quiz
DATA  TABLE