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May 19, 2020
That sounds like a Chicago song, but …

… in various places on this site, you will see references in the AP sections to “17 TOPICS”, “6 UNITS” and “9 UNITS”. This can be confusing, so here’s the background to those distinctions.

  • 17 TOPICS (up to the 2013 exam, and now deprecated on the site, and unavailable)

Up to, and through the 2013 AP Chemistry exam, teachers had to refer to what was commonly known as “The Acorn Book” in order to decipher what to teach in an AP chemistry course. I say ‘decipher’, since the old Acorn book (one example here) offered a horribly non-detailed breakdown of content, to the point where I felt that I had to make my own, detailed list of TOPICS and content. My guide to AP Chemistry content was adopted by many AP chemistry teachers at the time. I broke down the course into 17 chunks of material that I called TOPICS, hence the use of “17 TOPICS” that you see in some places on this site. I used my 17 TOPICS as the drivers of content for the first decade+ of my AP teaching. Then, after me suggesting this for years …

  • 6 UNITS (2014 – 2019 exam)

… in 2013 the College Board conducted a major overhaul of the AP Chemistry course. This was reflected in an updated Course & Exam Description (CED) that was published at the time. The first examination of the new, and newly organized material was in May of 2014. As part of that organization the College Board adopted the ‘edubabble‘ term, “Big Idea”. The content was organized into six “Big Ideas”, which were really no more than content areas. In order to appease my many subscribers, and because some new content was added to the AP Chemistry course (PES, semi-conductors, alloys etc.), I converted the original 17 TOPICS of content that made up my AP chemistry course prior to 2013-14, into 6, new UNITS, hence the use of the term “6 UNITS”. My 6 UNITS aligned with the College Board’s 6 Big Ideas, but had simpler names that made more sense to teachers, and that made less sense to edubabble enthusiasts.

  • 9 UNITS (2020 exam forward)

The organization of my course into 6 UNITS lasted until May of 2019. Following the 2019 AP Chemistry exam, the CED was updated once more. In this rendition, the 6 Big Ideas became 9 UNITS, hence the use of the term “9 UNITS”. With the exception of a few tweaks, the May 2019 re-arrangement changed almost none of the chemistry content that was to be examined in the 2020 exam (at least before COVID-19 gave us a new perspective), but it did relegate Big Ideas – so central to the edubabble a few years earlier – to almost a no more than a footnote! [Imagine that, an edubabble meaningless term coming in and out of fashion with no regard to any kind of real world relevance. The schools of education really are pointless, theoretical nobodies when compared to us and the actual work that we do]. Because the chemistry content hardly changed at all in this new document, in the academic year 2019-20 it was entirely possible to still teach the course via the 6 UNIT organization, with almost no tweaks, and have a course that in terms of content would be ultimately 99.99% identical to the 9 UNIT organization. In the 2019-20 academic year some AP chemistry teachers decided to keep delivering the chemistry material in the 6 UNIT organization, but I went over to the 9 UNIT organization, mainly so I would have experience of both. Since, in terms of chemistry content, these two organizations are basically identical, for now, the 6 UNIT and the 9 UNIT will continue to co-exist on the site.


  1. Kene

    Thank you for helping to explain these confusing changes! What are the best textbooks/review books available that cover content in the 6-unit and the 9-unit order?

    • Adrian

      A couple of points.

      Essentially, the textbooks typically recommended by the College Board as companions for the AP course are often useless. They cover a HUGE amount of irrelevant material, and are not targeted at AP specifically. That is why I wrote my own ‘textbook’ in form of the AP notes. It should also be noted that the order than anyone either chooses to teach (or study) the material is also mostly irrelevant. As long as all of the material is covered prior to the exam, the order is not really that important (of course, there are some topics that make sense to study before others, but that’s chemistry driven, not College Board driven).

      As for review books there’s only one that I recommend, the one that I wrote!


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