Online Introductory Chemistry
Dr. Walt Volland, Bellevue Community College
Drawing Lewis Structures

After you have studied this lesson you should be able to:

  • state a general method for drawing Lewis structures
  • identify the central atoms in a molecular formula
  • tell the number of valence electrons on a main-group element
  • explain what a central atom is in a Lewis structure
  • tell how many total valence electrons are in a Lewis structure
  • tell how to account for electrical charges in the electron count in a Lewis structure
  • explain how electron pairs are represented by "lines"
  • identify unshared pairs in a Lewis structure
  • draw Lewis structures for molecules when given the formula

Drawing Lewis structures is a trial and error process. There are some basic steps that can make the process go better. You are not going to be able to draw Lewis structures for everything that exists, but you can successfully draw Lewis structures for many common molecules.

Step 1: Look up the group numbers for all of the atoms in the formula to determine the number of valence electrons on each.

Step 2: Calculate the total number of valence electrons contributed by the atoms. This requires multiplying the valence electron count by the number of atoms of the element.

Step 3: Check to see if there is a charge on the formula unit. Adjust the total in Step 2 by the number of electrons added or removed because of any electrical charge. This gives you the final total of valence electrons that are available to hold the particle together.

Step 4: Identify the central atom/atoms in the formula unit.

Step 5: Think of your structure as a target. Write down the symbol for the central atom in the "bullseye". Then arrange the symbols for the oxygen atoms evenly in the next ring around the center atom.


Step 6: Divide the total number of valence electrons for the formula unit (Step 3) by 2 so you know the number of "electron pairs" for the particle.


Step 7: Connect the center atom with a single bond (1 pair of electrons) to each neighboring atom in the second ring. Then, using a single bond, connect each atom in the second ring to its neighbor in the third ring. Count the number of electron pairs used.

Step 8: Subtract the number of electron pairs used in Step 7 from the total number of electron pairs for the particle (Step 6). Use these remaining electron pairs to fill out the octets for atoms in the second ring.

Step 9: The almost last step is verification that the atoms each have an octet or duet. If they do you are finished.

If the central atom lacks an octet when only single bonds are used you need to move a lone electron pair to make one or more multiple bonds. This may mean double or triple bonds as appropriate.

NOTE: Any lone pair can be used here.