Condensed Lewis structures are a “shortcut” from the complete line Lewis structures. They only show the bonds between the main groups of atoms but skip the bonds within each group. For instance:
The condensed Lewis structure shows the “backbone” of the molecules while “condensing” all other peripheral atoms around the backbone structure. This is a quick way to show all atoms in the molecule but still do it relatively quickly without having to draw out every single bond.
Condensed Lewis structures dos and don’ts
The molecular representation that you choose to use for your molecule must adequately represent the bonding (connectivity) in your molecule. Thus, when you’re drawing your condensed structures try to always make a connection via the core atoms and not the peripheral ones.
While it it not strictly incorrect to do it either way, it is a pet peeve for many organic chemists and the last thing you want to do is irritate your instructor when they’re grading your test. So, be mindful when drawing your end groups.
Same applies to the up and down branches. When you’re showing a branch going up or down from the molecule stem, make sure you’re showing a connection to the appropriate atom:
Just like you probably wouldn’t want your arm to be attached to your body at the elbow, you don’t want to show your branch “hanging” in the air from the middle of it.
The use of parentheses in condensed Lewis structures
For molecules containing repeating group or multiples of the same group, we often use the parentheses. This is a shortcut in molecule drawing, however, it is important to correctly interpret it. For instance:
Parentheses in condensed Lewis structures can also be used to indicate a branch going off the parent chain:
Finally, parentheses are used to indicate the repeating chain link or a double-bonded oxygen:
Why so much emphasis on the condensed Lewis structures?
The simple answer is for historic and low-tech reasons. Structure drawing software is a recent development and for a long time representing organic molecules in a printed media was problematic. Thus, everything that was possible to show as a condensed Lewis structure, was done so. Many textbooks and instructors stick to this type of molecular representation more often than it is reasonable for whatever reason. So, it is important that you develop a solid skill of deciphering those into something meaningful and not just a scary string of symbols.