Question

when drawing lewis structure how do you know which atoms bond with which atoms?

Answer 1

There is no general fool-proof procedure, and occasionally the answer can only be determined by experiment or sophisticated calculation.

That said, there are number of useful rules of thumb that will get you the answer for the simplest molecules, viz.:

(1) First and second period atoms will usually form a specific number of bonds, and often there is only one, or a few ways to put together the atoms following these rules. For example, H and F will only form one bond each, so for a molecule like C2H4 you *know* the two carbons must be bonded to each other with a double bond, because there is no other way to build the molecule and only give each H a single bond.

(2) In small molecules, it's often the case that the best structure minimizes the formal charge, so you can assign formal charges and look for the structure that makes them as small as possible, and puts any negative charges on the more electronegative atoms. That will allow you to realize that in CO2 the carbon is in the middle, not one of the oxygens.

(3) Experience. In fact molecules are better thought of as being made up of chemical groups that are easy to form, and once you learn these typical groups, it becomes easier to guess what the molecule looks like. For example, CH3CO2H has the characteristic signature of a carboxylic acid (CO2H), which looks like this:

The rest of it then looks like a methyl group (CH3). So for the experienced chemist, you just have to hook up one mehtyl group to one carboxylic acid group, and there is only one way to do that. Similarly something like (CH3)2O looks like two methyl groups with an ether linkage. And so on. This is most useful for organic molecules, but of course those are just the ones that have the most variation.

Generally in a beginning chemistry course you should find using the valence of the atoms (H and halogens tend to form one bond, N and P three, O and S two, C and Si four, and so on) and using formal charge is sufficient, and sometimes you can speed things along by recognizing typical organic groups, like methyl and methylene groups, carbonyl, aldehyde and carboxylic acid groups, amines and amides and ethers, and occsionally an ester.

Beyond that...as I said, often enough the structure of a molecule must be determined by experiment, so even highly trained chemists won't necessarily be able to tell just by looking at the (unfamiliar) formula.