Chemistry 121 (Amar) -- Fall 2007
Drawing Lewis Structures
(©2007, François G. Amar, All rights reserved)

• Count the number of valence electrons in the molecule

CO₂ has 4+ 6 +6 =16 valence electrons

• Connect the atoms together in the correct arrangement with single lines

  (each representing 2 electrons)

  

• Fill in pairs of electrons according to the octet rule, starting with the more electronegative atoms

  

• Verify octet rule for all atoms and share electrons if necessary to make octets

    <-->   <-->  

  Note that these three Lewis structures are called resonance structures

• Calculate formal charges for each structure and select "best".

	<-->		<-->
6 4 6 -2 -0 -6 -3 -4 -1	val e- -unshared -1/2shared	6 4 6 -4 -0 -4 -2 -4 -2		6 4 6 -6 -0 -2 -1 -4 -3
+1 0 -1	formal ch.	0 0 0		-1 0 +1

The middle structure with 2 double bonds has the lowest (zero) formal charge

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