Drawing Lewis Structures

What is a Lewis dot structure and why are you making me draw one?  Those are probably questions that have gone through your head during lecture.  This post will help answer those questions and teach you how to draw a Lewis dot structure so that you can ace your next exam.

A Lewis structure, named after a prominent chemist, Gilbert Lewis, is sort of a shorthand method for depicting elements and their valence electrons.  Lewis structures can be just a singular element with its valence electrons surrounding the element’s symbol or they can be much more involved and actual depict a molecular compound.  Today we are going to focus on drawing molecular compounds.

The first step in drawing a Lewis structure is the count the total number of valence electrons in the compound.  How do we find the valence electrons?  Simply identify which group (i.e. column) your element falls in on the periodic table and the number at the top (i.e. 1A, 2A, up to 8A) will tell you the number of valence electrons.  Take for example H_{2}0, how many valence electrons are in this molecular compound?  H is in column 1A and  therefore has 1 valence electron.  Since there are 2 hydrogens, (as noted by the subscript), there are 2 total valence electrons.  O is in group 6A and so therefore has 6 valence electrons for a total of 8 valence electrons in a water molecule.

Now if your compound happens to be associated with a charge, say for example, NO_{3}^{-}, then we add an additional electron to our total to account for the negative charge on the compound.  If the charge reads 2^{-}, or 3^{-}, we add 2 or 3 electrons respectively.  Same if the compound is positively charged, except in this case, we subtract electrons from the total.

Our next step is to draw bonds connecting the elements to each other.  Each bond you draw equals 2 electrons.  So keep track of how many electrons you are adding to your drawing.

Once you have connected each element, you must make sure that each element has its “happy” number of electrons surrounding it.  In most cases, this “happy” number is eight (also known as the octet rule).  But in some cases it might be 6 (as with boron) or 2 (as with helium or hydrogen).  Add dots to each element to make them all happy.

Now that your drawing is complete, you can double check your work.  Count up all your electrons as dots and bonds.  That number should equal the total you originally calculated.  Next make sure each element in the compound has its “happy” number of electrons surrounding it.  Once you have satisfied both of these rules, you have a valid Lewis dot structure.

Don’t worry if you have multiple valid structures, your compound might be an example of resonance.  Also as the old saying goes, if you find your diagram doesn’t satisfy both of your checks, try, try again.  Drawing Lewis structures really is a trial and error process.

Let’s review the steps:

1. Count the total number of valence electrons, adding electrons if the polyatomic ion is negative, or subtracting electrons from the total if positive.

2. Draw bonds between each element, making sure to count 2 electrons for each bond.

3. Place the remaining electrons around the elements as dots, making sure each element has an octet (or 6-tet, or duet if you are working with one of the few exceptions).

 

To double check your work:

A. Count up all your electrons and bonds in your drawing.  The total number should equal the total in step 1.

B. Make sure each element has its “happy” number of electrons surrounding it (that’s either 8,6, or 2.)

 

Stay tuned for some example problems with drawing Lewis Structures!

This entry was posted in Chemistry, General. Bookmark the permalink.

Leave a Reply