Supplemental Guide: How to Draw Lewis Structures for Covalent Compounds

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1.  Count the total number of valence electrons from all the atoms in the compound. (If the compound is charged, be sure to add or subtract the appropriate number of electrons from your total.)

EX:      C₂H₄

Carbon has 4 valence e^- and Hydrogen has 1 valence e^-, so…

4(2 atoms C) + 1(4 atoms H) = Total valence e^-

or         8 + 4 = 12 total valence e^-

 

 

 

2.  Draw a skeleton structure showing how the atoms are connected. Put single bonds between each atom you want to connect.    

EX: C₂H₄                          

 

 

 

3.  Subtract the valence electrons used in the skeleton structure from the total valence electrons to see how many you have left to distribute. (Remember each bond is a shared PAIR of electrons.  That’s 2 electrons for every bond.)

 

EX: C₂H₄                    12 total valence e^- – 5 bonds(2 e^-) = Total remaining valence e^-

                                                            12 – 10 = 2 remaining valence e^-

 

 

 

 

4.  Distribute the remaining electrons IN PAIRS on elements that don’t already have 8 valence e^-. (Remember Hydrogen, H, can only ever have 2 valence e^-).

EX: C₂H₄                            

                                                       

 

 

5.  If any atom doesn’t have 8 valence electrons, share electrons from another atom by making another bond. If this is not possible (as in the above example) or does not fix the problem, try a different arrangement of atoms and/or electrons.

 

 

Let’s try something else.

 

 

 

5- Part B.  We can rearrange the atoms placing 2 hydrogens on each carbon.

EX: C₂H₄

 

 

So the Lewis structure becomes the following:

ROCK ON!