Science, Tech, Math › Science Lewis Structure Example Problem Share Flipboard Email Print Daviewales / Wikimedia Commons / CC BY 4.0 Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated October 02, 2019 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 electronsHydrogen has 1 valence electronsOxygen has 6 valence electronsTotal 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 electronsHydrogen needs 2 valence electronsOxygen needs 8 valence electronsTotal 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)/2number of bonds = (20 - 12)/2number of bonds = 8/2number 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.