In our continued discussion of condensed formulas, now we take a look at parentheses with subscripts. In this case they indicate repeating units O. For example Chapter 2. If we take a look at the structural formula of this hydrocarbon, we have what Chapter 3, Chapter 2, Chapter 2, Chapter 3.
If we want to convert this to condensed, we learn that we could have done Chapter 3, and then we could do Chapter 2, Chapter 2, and then Chapter 3. But we can further condense this even more. We're going to say that these two Ch twos in the middle, we can combine them even more.
Putting them in parentheses and then putting a little subscript at the bottom tells me that there are two Ch twos in the middle of this hydrocarbon. So here this would be another version of a condensed formula where we have now repeating units.