Lewis Structure Example Problem

Lewis dot structures are used to predict the geometry of a molecule. You will be able to draw a Lewis structure of the formaldehyde molecule after using this equation.

Question

Formaldehyde is a toxic organic molecule with molecular formula CH₂O. Draw the Lewis structure of formaldehyde.

Step 1

Find the total number of valence electrons.

Carbon has 4 valence electrons
Hydrogen has 1 valence electrons
Oxygen has 6 valence electrons
Total valence electrons = 1 carbon (4) + 2 hydrogen (2 x 1) + 1 oxygen (6)
Total valence electrons = 12

Step 2

Find the number of electrons needed to make the atoms "happy"
Carbon needs 8 valence electrons
Hydrogen needs 2 valence electrons
Oxygen needs 8 valence electrons
Total valence electrons to be "happy" = 1 carbon (8) + 2 hydrogen (2 x 2) + 1 oxygen (8)
Total valence electrons to be "happy" = 20

Step 3

Determine the number of bonds in the molecule.
number of bonds = (Step 2 - Step 1)/2
number of bonds = (20 - 12)/2
number of bonds = 8/2
number of bonds = 4

Step 4​

Choose a central atom.
Hydrogen is the least electronegative of the elements, but hydrogen is rarely the central atom in a molecule. The next lowest electronegative atom is carbon.

Step 5: ​

Draw a skeletal structure.

Connect the other three atoms to the central carbon atoms. Since there are 4 bonds in the molecule, one of the three atoms will bond with a double bond. Oxygen is the only choice in this case since hydrogen only has one electron to share.

Step 6: ​

Place electrons around outside atoms.
There are 12 valence atoms total. Eight of these electrons are tied up in bonds. The remaining four complete the octet around the oxygen atom.
Each atom in the molecule has a complete outer shell full of electrons. There are no electrons left over and the structure is complete. The finished structure appears in the picture at the beginning of the example.