Lewis Dot Structures

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Although drawing dots around elements sounds pretty straight forward, Lewis dots are a little more complicated. This lesson will explain how to draw Lewis dots for single, double, and triple bonds, as well as polyatomic ions.

What are Lewis Dots?

The world of chemistry can be a confusing place. There are periodic tables, elements, electrons, and Lewis dots. What are Lewis dots, you ask? Although they don't sound like something you'd be learning about in chemistry, Lewis dots are ways chemists represent valence electrons, which are the outermost electrons on an atom. Chemists are particularly interested in valence electrons because they are the electrons that are involved in chemical reactions.

Chemists represent each valence electron as a dot next to the element's symbol, and you can use the periodic table to determine how many dots the main group elements, or elements 1A-8A, contain.


A. The Lewis dot structure for francium
francium


For example, an element in group 1A will have one valence electron, and therefore one dot. An element in group 2A will have two valence electrons, and (you guessed it) two dots.... and so on and so forth. Lewis dots are named after Gilbert Lewis, a chemist who studied how elements bond, or attach together.


B. All elements in the vertical column (or group) 1 have 1 valence electron. All elements in group 2 have 2 valence electrons
periodic table


Atoms will gain or lose electrons in order to become stable in what is known as the octet rule. As you study more chemistry, you will learn more about this rule, but for now just know that an atom is stable if it has a full set of valence electrons, which in most cases is 8 (which is where the word 'octet' comes from). Your task when drawing Lewis dots is to make sure you don't have any unpaired dots after a chemical reaction. Don't worry, this will all make more sense momentarily, so grab a pencil and paper and let's practice!

Single Bonds and Lewis Dots

Now that you have a general idea of what Lewis dot structures are, let's see if we can draw the Lewis dots for elements that bond together and make molecules. Let's start with a simple example of a single bond, using table salt, or NaCl.

  • Step 1: Determine how many valence electrons each element brings to the molecule. If you look at the periodic table, Na (sodium) is in group 1A, which means it has 1 valence electron, and Cl (chlorine) is in group 7A, which means it has 7 valence electrons. We represent each valence electron as a dot next to the element. Start by placing one dot on each of the four sides and then repeat with any remaining dots. You should end up with something like we see in image C:


C. Lewis dots for Na and Cl
Lewis dot


  • Step 2: Place the bond between the atoms. Two atoms are bonded together if they have a set of unpaired valence electrons (or two unpaired dots) between them. In this case it's relatively simple because sodium has only one valence electron. Remember, you don't want any unpaired dots, so sodium's dot will join up with chlorine's unpaired dot. Chemists often depict a bond with a line, so sodium chloride can be written as Na-Cl.


D. The unpaired electron (dot) on Na pairs up with the unpaired electron on Cl
NaCl


Double and Triple Bonds

Our example of sodium chloride (NaCl) was pretty straight forward. But what happens when you have more than one set of unpaired dots on each atom? Excellent question! For this example, let's use something you're really familiar with: the oxygen that you're breathing, which occurs as O2.

  • Step 1: Use the periodic table to determine how many valence electrons oxygen has. Since oxygen is in group 6A, it has 6 valence electrons. Draw the dots around oxygen by placing one dot on each side and then repeat with any remaining dots. Just like you saw before with chlorine, some of your dots should form pairs. You should end up with something like image E:


E. Lewis dots for oxygen
lewis dot


  • Step 2: Use the unpaired dots to make bonds between the oxygen atoms. You may notice that you have more than one set of unpaired dots here. In this case, each set of dots pairs together so that you have two bonds between the oxygen atoms. This means you have a double bond, which is represented by two lines, or O=O.


F. Lewis dot pairing for O 2
Lewis


Next, let's use another gas found in the air, nitrogen, or N2.

  • Step 1: Use the periodic table to determine how many valence electrons nitrogen has. Nitrogen is in group 5A, so it has 5 valence electrons. Draw 5 dots around each nitrogen symbol.
  • Step 2: Pair up any unpaired dots between the two atoms to form bonds. You'll notice you again have more than one set of unpaired dots, but this time you have not just two bonds, but three - making a triple bond to form N2.


G. Lewis dot pairing for N 2
Lewis dots for nitrogen


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