How to Draw Lewis Structure

# How to Draw Lewis Structure - How to Draw Lewis Structures...

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How to Draw Lewis Structures Wayne Wesolowski, PhD.and Althea Jones,BS (UA2006) tested by CHEM103A class Fall 2006 revised by Andrew J. Grall, Ph.D. Spring 2007 Step 1: Determine the number of valence electrons by adding the periodic table group numbers of the elements in the compound. Remember if you have an ion add electrons if the ion is negative and subtract electrons if the ion is positive. Example: NF 3 valence e- = N (5 valence e - ) + F (3 x 7 valence e - ) = 26 valence e - Ion: SiO 4 4- valence e- = Si (4 valence e - ) + O (4 x 6 valence e - ) + 4 e - = 32 valence e - Step 2 : Draw reasonable skeleton structures by joining atoms with single bonds. Some suggestions for reasonable structures: 1) H is always a terminal atom (on the end with one bond because it has only the 1s orbital available. 2) C can bond to itself easily but not always while other atoms usually do not. So in ClO 3 1- I would not string the O’s in a line but put the Cl in the middle and make the O’s terminal atoms. In I 3 1- we have no choice but to string them in a line. 3) Elements seldom make rings of less than five or six atoms. The angles are too small to bond p orbitals that like to be at 90 degrees to each other. For example O 3 (ozone) is in a bent line not a triangle. 4) Put the more electropositive element (further to the left and down on the periodic table) in the middle because being positive it has less attraction for electrons and can make bonds (share) with several atoms at the same time. An element like F never makes multiple bonds or is in the middle because it is so electronegative it can’t share beyond one bond with any other element. Example: NF 3 : N F F F Step 3 : For each bond in your skeleton remember to subtract two electrons from your valence electron total.

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Example: NF 3 : N F F F 26 e - - 6 e - = 20 e - remaining This is the only reasonable skeleton because we know fluorine only wants to make one bond so it would not be the central atom or in the middle of a chain (we will reject this by step 7 – formal charge). Step 4:
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How to Draw Lewis Structure - How to Draw Lewis Structures...

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