There's a bit of an art to "good" Lewis Structures... the things you should look for include:
- Hydrogen can only have 1 bond to it. It can only have 2 valence electrons. (1s-orbital) This is nearly always true, and you will not run into things that seem like exceptions unless you get deeper into Molecular Orbital Theory in a future class.
- Anything in the 2nd row of the Periodic Table cannot have more than 8 valence electrons ("Octet Rule"). Lithium, beryllium, and boron can have less than an octet, but nothing in the 2nd row can have more than an octet. (2s & 2p orbitals) Again, exceptions are probably beyond the scope of most general chemistry classes.
- Formal charge doesn't have to be zero, but you should try to minimize the formal charge distribution as much as possible. This is especially true if you have a positive formal charge and a negative formal charge in the same structure... you can usually minimize that formal charge distribution by making a double or a triple bond. NOTE: if you have two formal charge of -1 in a structure, that seems like distributed formal charge, which you'd like to minimize, but in practice it's usually difficult to do that without making a bit of a mess of the rest of the structure.
- If you do have residual formal charge, it is often considered a little more favorable to have negative formal charge on elements that are more electronegative. So if you have two options for a Lewis Structure and one of them has -1 formal charge on oxygen and the other has -1 formal charge on sulfur, the structure with -1 on oxygen is probably preferred because oxygen is more electronegative than sulfur. Remember, electronegativity is a measure of how strongly an element attracts the electrons in a bond, so if oxygen is attracting electrons more strongly than sulfur, it makes sense to have the negative formal charge on the oxygen.
I think those 4 points will get you through 95+% of the Lewis Structures you're likely to see in Gen Chem. Remember, Lewis Structures aren't magic... they can be used to help us understand bonding in a LOT of molecules and polyatomic ions, but there are always exceptions that can be better explained using different or more complex theories. When Lewis Structures work (which they often do), they're a great tool, so it's nice to have them in our toolbox.
As with everything else, use the simplest explanation that satisfactorily answers the question being asked. If you're in a physics class and someone asks "Why did that hammer fall on the floor?" it might be appropriate to start talking about how any two objects that have mass will be attracted to one another and since the Earth is a massive thing it attracts all other things with mass by a force called gravity. If you're in your garage and someone asks "Why did that hammer fall on the floor?" the answer might be "because I dropped it".
