Can you determine bond pairs just by looking at Lewis Structure?

Question

I'm trying to figure the number of bonded and single pairs in H2O2, CH2O, Cl2, CH4O, and C2H4. I drew a Lewis Structure for each but can I get the number of pairs from the lewis structure? If not, how is the number of pairs determined?

Answer 1

In the Lewis structure, dots left alone want to pair up with those of another atom. If there are 5-6-7 dots all around, the atom will accept electrons to fill it last layer. If there are 1-2-3 dots, the atom will want to give those electrons away.

So you look at your Lewis structures, and try to pair up alone dots from one giving atom to a receiving atom.

For your peroxyde, you would get H. and :.O.: twice so one link between and H and a O, another link between the other O and the other H, and then the two O's link together.

For you chlorine, there are 7 dots all around each atom, so they will share their un-matched electron in a covalent bond.

For the CH4O, their are 4 dots all around the carbon, so you would think that one H will bond to each (that's 4 H then) but to add the oxygen, you have to insert it between a H and the C (that the hydroxyde radical). It is written CH3H usually to show the -OH polyatomic ion.

For the C2H4, the carbon has 4 dots all around it, and they will bond to each other. Each will link to two hydrogen (to make up the four in the molecule), leaving two dots on each C, so this means that there will be one double bond between the two carbon atoms.

Finally, the CH2O os trickyer. That carbon, with its four dots, will have one H bond to each one dot, so there are two free electrons left. That oxygen has six dots, two pairs and two free. This means that each free electron of the O will link up to one of the free electrons on the C, which means that there is one double bond between the carbon and the oxygen. You'll have H-C-H for the first part and C=O for the second part.

It's kinda hard to type these :-) Hope this helps anyway

Answer 2

Of course there is a rule. At a simple level, atoms are trying to get a "noble gas" shell of 8 electrons. Each nitrogen starts with 5 outer shell electrons, but wants 8. Two nitrogens can't can't steal 3 from each other, so they share them. This means 6 electrons are shared altogether. Both nitrogen atoms think they can see 8 electrons, but only as long as they stay together. 6 electrons is 3 bonds. Each nitrogen then has 2 electrons "left over" - non-bonding electrons. As a simple rule for covalent (shared-electron) bonding, group 7 (F, Cl, Br, I) make one bond group 6 (O,S) make two bonds Group 5 (N and P) make 3 bonds group 4 (C, usually) makes 4 bonds.