I took this class during the COVID-19 pandemic in an online format.
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As you will see from other reviews, regardless of the difficulty of this class, Professor Gelbart is the reason to take this class. He is extremely responsive to questions, flexible about his office hour times, and truly cares about student learning. During the pandemic learning format, he was willing to drop quizzes entirely from the grading scale and give more points to the homework sections of the grades, and allow us to choose between 1 and 2 midterms as a class. You will not have a professor who is more willing to work with you for the normal CHEM 20A route.
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CHEM 20AH as a class itself is quite difficult and definitely brings more to the table than other classes. If you are not well versed in math (up to at least multivariable calculus) and physics (you need to understand vectors and energy), do NOT take this class. While certain classes are listed as corequisites, if you have not taken the relevant math/physics classes, you will have a very hard time. The material is extremely calculation focused, and much time is spent proving concepts numerically that would otherwise just be expected to be memorized in CHEM 20A.
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The textbook is quite helpful in helping to understand certain parts of the lecture that weren't as clear, but is certainly not mandatory. Homework takes about 3-6 hours per week, depending on how you feel about the material and math. This does not include studying time and supplemental practice, which you should do a lot of. Discussions are optional but highly recommended; Derek was an excellent TA who covered everything Gelbart didn't get to in lectures. Lectures themselves are also posted on CCLE, including notes, which are very helpful as well.
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The midterm and final were both available for 24 hours, though once it started you only had 3 hours to do it. Both time limits felt quite generous, though a tip is to be VERY careful on steps requiring lots of simplification and algebra, as if you have to redo those it will eat up your time. Quizzes were completely dropped and optional due to the pandemic, which was very accomodating. The homework and practice problems provided were extremely similar to the tests, i.e. tests were very fair and of a reasonable difficulty.
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Overall, there is practically no reason not to take this class if you need it for a major requirement, unless you are not well-versed in the mentioned math/physics concepts. Even if you're not in the honors college, I would recommend taking CHEM 20AH with Gelbart. If you are in the honors college, well, you found the class to take for your credits! If you look at the grade distribution, despite the difficulty of the class, Gelbart is extremely generous in grading/curving. You'll be much less stressed in this class than others.

*Reviewed by someone who didn't do so hot in the class*

In my opinion, I'm a fairly good student (got a 4.0 this quarter bc I took this class p/np) but this class absolutely demolished me. Unless you're a super academic person and you're willing to put in a TON of extra effort, AVOID CHEM 20AH AT ALL COSTS. also, you do need strong PHYSICS AND CALC backgrounds or else it'll feel like you're learning chem, physics, and math at the same time. While it's true that some people do end up doing well in the class, most of the students struggle. A LOT. I think a girl in my class said her sister (who's a 4th year biochem major) said that the material we learn in this class is the same stuff as the hardest class at her school.

However, Prof Gelbart is the kindest soul you will ever meet in your life. He offers a TON of help besides his lectures (this quarter he had 6 discussion/office hour periods every week. that's 9 hours of "lecture/discussion"). We were basically allowed to choose our assessment type and we chose HOMEWORK (50%), ONE MIDTERM (22%), ONE FINAL (28%), and UNGRADED QUIZZESx. He also curves the class in the end, but like we don't really know how much (it's kinda dependant on your effort level) so that's kinda a problem.

One gripe I have about this class is that there's very little structure. He goes at the pace of the students, which is a double-edged sword. While it allows us to not be completely lost, I just found it hard to follow along with the schedule and whatnot. If he taught two sections of this class, I'm almost positive each class would learn a different set of material.

This class was definitely not easy, but in my opinion the workload wasn't as much compared to Chem20 this quarter (might have been the prof, but I had a friend in that class who was constantly stressed and had a lot more assignments than me).

Gelbart is a great professor, really engaging and super helpful, but his lectures can sometimes go off-track and he sometimes assumes everyone has a certain level of prior knowledge before launching into his explanation. In those cases, it's important to ask him to clarify (and he'll go over it in a lot of detail!) or ask the TA in office hours. My TA was fantastic; he knew what topics Gelbart had sort of glossed over in lecture and went over those in office hours, was a really nice grader and generally really approachable.
As a bunch of other people have said, this class is a lot of math and a lot of physics, so be prepared for that.
The problem sets varied from "fine" to "really difficult." Basically, find study partners and meet up a couple times a week (I did 3x 1 hour) to do them and they're not terrible. The problem set questions are very similar to what you'll get in an exam. The exams are fine mainly because they're designed so that you have a lot of extra time and don't feel rushed. Also, Gelbart would tell us in advance what types of questions to expect.
This class is much smaller than 20A (<50 students, approx 35-40 I think) and so you get to know each other and the instructors better than in a bigger class.

Even though the concepts can be confusing sometimes, this class is great because Gelbart is such a passionate professor. He always encourages asking questions, visiting office hours or just stopping by his office to talk. He organizes outside-of-class meetings where you drink coffee and talk about whatever you want, and he's really accommodating if you can't make it to class. He's so insanely excited about science that it's contagious and he encourages all the students to use the material they're learning and apply it to their everyday life and the way they view the world.

Gelbart is amazing. As a life science major, I expected to feel alienated or left behind, as this class is largely physics based, and as such, many of the students are physical science majors. Moreover, I never really had a passion for physics, and I was thus not the best at it either. But Gelbart completely turned this around. I now have a passion for physics. It is sometimes good to take a break from the chaotic and unpredictable nature of humans, and instead observe systems in which we can make incredibly accurate predictions. But perhaps more importantly, he taught me how to question the world around me, and how to ask the right questions.
One more thing I should add: Gelbart makes a point that, in the real world, it is difficult to distinguish between biology, chemistry, physics, and the like. And this is how it should be. For example, he is a professor teaching a chemistry class, which is actually largely physics based, while his research in his lab deals with virology (he even worked on COVID vaccines!!). Our grade in this class was secondary to him wanting us to learn how to question our surroundings, regardless of what label it is given (biology, chemistry, etc.). Make estimates! Don't be afraid to be wrong! Stay curious!
Thanks a million Dr. Gelbart.

Chem 20AH, Fall 2021:
The way Professor Gelbart teaches/lectures makes you think about how it applies to your life, and more. You would show up to class intimated by his receding hairline showing his five head and genius brain, but then realize he shows up with an almost dried out expo marker every time and cracks a joke at the beginning of class making us feel comfortable and welcome. Learning content was definitely physics and calculus focused but nothing you can't learn, especially because his homework (weekly problem sets) has repetitive questions that help you understand the math better. Basically, a class learning about quantum mechanics: dimensional analysis, the hydrogen atom, and helium atom. . .etc. Don't be intimidated I never took a physics class, ever, only some knowledge from AP Chem in high school, but now I think I have just as big a fivehead as him, so I guess it's contagious after taking this class. Also, if you want to go the Chem 20A/honors route don't be surprised if you aren't learning stoichiometry or periodic table trends, that's for 14A. In 20A/H, we learn the fun mad scientist stuff. Grade distribution is heavenly so don't worry about your grade, just go to his office hours, ask questions, and study groups. The workload is manageable. The weekly, about 6 problems, homework is interesting because it's a mix of math/physics problems and then some theoretical questions you would never think of thinking about and realize how much it actually is cool? I went to an office hour one time and he's actually a mad genius so definitely utilize his knowledge and research mastery to your advantage, a very approachable man too. I never showed up to lecture because I had practice and still ended up with a satisfying grade, but even better, I actually learned something more than just staring at the professor and listening to gibberish or trying to read some hieroglyphic stuff, Gelbart made it actually make sense. You can definitely pass this class by just showing up to lectures and reading the notes he provides because I crammed it all the day before each quiz/exam and felt like a mad scientist during the exam.

Maybe it's his aura of loving his job or his joyful personality, but if you want a memorable experience as an undergrad the environment, grade distribution, and content learning is 1000% worth. Just be careful to not get a fivehead like him. ;)

I have a lot of respect for Professor Gelbart and TA Derek Urwin.

A note on what you need to know: I hadn't taken chemistry since my sophomore year of high school's Chemistry Honors class (2017-2018). What became more important was the ability to think abstractly and quickly, and a very strong handle on math. You only need to know differential calculus/calc 1; it'd be very hard to take this class concurrently with 31a. He teaches us spherical polar coordinates and partial diff eq's, which is nice; a bit more advanced than 31a. But, this extra information is not assumed, it's taught to us first.

I'm only adding to the pool of good reviews here but I really did appreciate this class. It was awesome, I learned an incredible amount, and I left class with more questions (in the good way) than answers, which is a nod to how incredibly vast the field of chemistry can be. I felt that I was challenged and treated as more than just a freshman with no experience; any high level questions asked by our class was always treated with respect and an honest answer.

If you have the option to take Chem 20A, 20AH, or any class whatsoever with Professor Gelbart: TAKE IT!!! He is an amazing professor. He doesn't care about finishing the syllabus or keeping strict deadlines as long as his class is understanding his material. He postponed multiple problem sets for us, and even let us have input into the midterm(s), quizzes, and final. All that on top of a generous overall class curve. That being said, he doesn't actually care too much about grades, and is dedicated towards our mastery of the concepts. While the chem involved was difficult, his lecture notes were easy to read and he gave very long, thorough explanations of any questions people had. One weird thing about the class's structure during online learning was that Prof. Gelbart made all of his lectures into discussion sections, and added 6 more, giving us 9 opportunities per week to talk to him. The "lectures" we did have were just reading his notes and the relevant material on our own time. Participation is encouraged, but not mandatory. One piece of advice: since the deadlines are loose and the class is so discussion based, it gets really easy to fall behind if you're not on top of the lectures. Make sure to read them ahead of time, and optionally take notes, or else a lot of what he says will go right over your head and you'll tune out. But if you're on top of everything, and asking questions, Prof. Gelbart is a kind, knowledgeable, and exciting Professor!

Professor Gelbart is easily the kindest professor I've ever had. He is extremely accommodating, qualified, and truly does everything possible to help his students succeed in the course. He was present for almost every discussion, and we had around 6 discussion sections/office hours in addition to 2 hour-long lectures every week. He was incredibly generous with letting us vote for how many exams we wanted and letting us do ungraded quizzes, so the grading in the end was weighted 50% for homework, 22% for one midterm, and 28% for the final.
That said, this course is not easy at all. For context, I consider myself to have a pretty strong understanding in chemistry and multivariable calculus, but not too strong of a background in physics. I was definitely confused (at least in the beginning) by the advanced concepts and derivations we were covering in class. This class is heavily based on quantum mechanics, and I found that my chemistry knowledge did not help me at all in this course. Do not take this course if you aren't well versed in multivariable calculus or physics, because it will feel like you are trying to learn multiple subjects at once. The problem sets are very challenging, but basically the most useful study tool for exams. I spent at least 5 hours on problem sets every week. (They are graded on accuracy.) I didn't really use the textbook, but mostly just relied on his lecture notes because they were the most relevant for exams. Overall, I highly recommend this professor and this course if you are up for a challenge.

Professor Gelbart is a fantastic lecturer; he conveys information clearly and really emphasizes core concepts that are central to your understanding. He also gives you plenty of opportunities to ask questions and get help both during class and in the discussion sections/office hours(they were combined this quarter). Make sure you spend a lot of time on the problem sets, as although lecture notes are clear and relatively easy to understand, the application of concepts is really understood through practice problems. It definitely helps to double-check your understanding with classmates, if not in discussion sections/office hours. The textbook is also helpful when you need to understand a specific concept in greater detail. All in all, I highly recommend this class to anyone who has the option of taking it, the material is fascinating, your peers will be inspiring, and your professor will be amazing!

This is an honors class, so it’s a lot harder, but in my opinion it is well worth it. Gelbart is a very good teacher, and he makes a point to learn everyone’s names and answer all their questions. If you put in the work, you will get a lot out of this class.