A Lewis structure is a structural representation of a molecule where dots are used to show electron position around the atoms.
Things you will need:
Step 1: Find the Total Number of Valence Electrons (refer to the Instructions Below the Pictures)
Valence electrons are the electrons that live in the outer shell of an atom. They are the electrons that form chemical bonds and interact with the environment.
We need to find the valence electrons as they are the total number of dots we will have in the drawing.
To find the total number of valance electrons:
a) Label the vertical columns on your periodic table 1-18 (these are also known as group numbers)
b) Find the element on the periodic table and look for the last digit of it's group number. (refer to first picture)
c) The last digit corresponds to the number of electrons the atom has. (refer to first picture)
d) Repeat steps b and c for each of your elements
e) For atoms that show up more than once, make sure to multiply the number of electrons by the number of times an element appears. (refer to second picture)
f) Add up all of the valence electrons for each atom. (refer to third picture)
Step 2: Placing the Elements in the Drawing
a) Place the element with the least number of particles in the center of your drawing.
In the example, O only has 1 particle, so it is placed in the center.
b) Place the elements that you have more particles of, symmetrically around your center element.
In the example, H shows up twice so it is placed symmetrically around O.
Step 3: Draw Two Electrons Between Atoms to Form the Chemical Bonds
The number of electrons that you calculated in step 1 are the total amount of dots that you will have in your drawing.
a) place two dots between each element to indicate the bond that forms between them (each bond uses two electrons or "dots").
b) Count the number of dots you used and subtract it from the total amount that you originally calculated in step 1
c) This new number will give you the number of dots you have left to complete your drawing.
we have 4 dots left
Step 4: Use the Rest of the Dots to Finish Your Lewis Structure
We have 4 dots left to use.
There are two rules to follow when placing the remaining dots.
a) Make sure each H atom only as two dots surrounding it.
b) Make sure all other elements have 8 dots surrounding them.
Place the dots in pairs.
In the example (refer to the picture)
H --> already has 2 dots.
O--> needs 4 more dots, so they are placed in pairs on either side of the O.
Step 5: If You Have Fewer Dots Than Needed to Complete 8 Dots Around Each Element Then....
Since the previous example does not need to share electrons, I will be using a different molecule in this example.
If you use up all of your dots before each element has 8 dots around it, then you must share some of the dots.
To share dots:
a) Draw the lewis structure by repeating steps 1-4.
(refer to the first picture)
b) find the element(s) that has more dots than the rest. If there are multiple elements that have more dots than the rest, then just pick one at random among the higher dotted elements.
(refer to the second picture)
in the example, the oxygen on the left has more dots around it
c) Transfer the chosen dots to go in between two elements in such a way that every element still has 8 dots surrounding it.
(refer to the third picture)
It may take some trial and error.
Congratulations, you are now done!