Structures of molecules can be difficult to piece together at first when you are just starting in an organic chemistry class. Hopefully you retained some of this knowledge from general chemistry. If not, one of the tricks that can greatly help with this is to know the uncharged or “normal” state for atoms that are commonly found in organic molecules. Here is a table of the most common of those:
– C has four bonds and no lone pairs
– N has three bonds and one lone pair
– Halogens (F, Cl, Br, I) have one bond and three lone pairs.
– O has two bonds and two lone pairs
– H has one bond and no lone pairs
Three more rules:
– C, N, O are central atoms, meaning that they will always be in the middle of your molecule.
– H and halogens are terminal atoms, meaning that they will only have one bond and be at the ends of molecules.
– With the exception of H, atoms in group I & group II are only counterions (+1 or +2 and not involved in resonance).
Remember, these rules are for when the atom is uncharged; this does not apply to charged atoms. For example, a carbocation (a positively charged carbon atom) will have only three bonds with no lone pairs while a carbanion (a negatively charged carbon atom) wlll have three bonds with one lone pair, and a carbene will have two bonds with two lone pairs.
Notice that all of these carbons still follow the octet rule. However, beware of atoms that do not follow the octet rule, as phosphorus is an example of an atom that can have more than an octet of electrons. Shown below is triphenylphosphine oxide, a byproduct of the Wittig reaction.
Elements with open d-subshells, like phosphorous and sulfur, do not always follow the octet rule. More examples of this are SF6 and PCl5. However, carbon, nitrogen and oxygen will follow the octet rule.