Here we need to draw the Lewis dot structure for the formaldehyde molecule, which is CH2O. We're going to start out with step 1, which says to determine the total number of valence electrons of the structure. So, hydrogen, carbon, oxygen. Carbon is in group 4A, there's one of it, so that's 4 valence electrons. We have two hydrogens, each one's in group 1A, so that's 2 valence electrons. And then we have oxygen, which is in group 6A, and there's one of it, so that's 6 valence electrons. That comes out as a total of 12 valence electrons. Because remember, the group number equals the number of valence electrons.
Next, we're going to place the least electronegative element in the center and connect all elements with single bonds. To do this, we're going to follow our bonding preferences guide to determine atom connectivity. Now remember, hydrogen cannot go in the center, so next up would be carbon. Here, to deal with symmetry, I'm just doing single bonds with the hydrogens on both sides, and then oxygen up here.
Next, we're going to add electrons to all surrounding elements until they have 8 electrons because we're talking about the octet rule, except for hydrogen which follows the duet rule. It only wants 2 valence electrons around it. Okay. So let's do that. We're going to add electrons, so 1, 2, 3, 4, 5, 6. And remember, in a covalent bond, we're sharing electrons, so that's 8. Alright. How many electrons have we used? That's 8 there, 10, 12. We used all 12 of our electrons, so we have none left.
Now place any remaining electrons on the central atom, which we cannot because there are no more. Now if any elements don't have 8 octet electrons, add double or triple bonds between them. Alright. So this carbon here only has 2, 4, 6 electrons around it. Remember the bonding preferences of carbon is that carbon wants to make 4 bonds. Oxygen ideally wants to make 2 bonds. So for them to do that, what I'm going to do is I'm going to take one of these lone pairs here, and then just bring it down to help make a double bond. And by doing that, I'm still using the 12 electrons that I have total, but carbon is being satisfied by making 4 bonds and so is oxygen. So this would be the formula for the formaldehyde molecule. Carbon is single bonded to both hydrogens, double bonded to the oxygen. Oxygen itself has 2 lone pairs. So this would be the correct structure for formaldehyde.
