9th edition. I saw some reviews that were bewildered to see the low ratings. Here is why I give this book a low rating:
I have gone through this whole book over the last year. As I read it, and especially towards chapters 17,18,19,20,21, things feel really jumbled around. I will go through a passage, feel like I understand it, and then I see an example using definitions that it had not previously introduced. It often skips many steps. I was doing Ch 15 homework, and the equation I was supposed to use for one of them was from Ch 16. I'm not there yet! The homework used equations or forms of the primary equation that were not introduced and not listed in the end-chapter formulas list. If it did, then it would just be printed with nothing really being said about it, but I'm expected to apply it. These were common problems for me. I am predisposed towards biochemistry, and physical chemistry just doesn't work for me. This book doesn't work for me.
At the very end of all my physical chemistry courses, I have been fortunate enough to find the classic big red book: McQuarrie & Simon. That book has been a lifesaver. I have been learning physical chemistry for the first time in my life, albeit poorly. It uses words to keep you on track, followed by the equations. I feel like McQuarrie puts things in to English much better than Atkins. I also feel like McQuarrie is better at putting things in to the overall context of what we are doing and what our end goal is. The flow of the chapters is very tight... they roll into each other nicely. When McQuarrie talks about derivative or integrated forms of these equations, there is context and it explains more deeply what is happening when you apply the derivative forms.
I have gone through the entire Atkins book during my courses, and I have hardly learned anything from it. McQuarrie & Simon lifts the veil. If Atkins works for you, then that's great. If it isn't working for you, try McQuarrie. It's a little more reading than Atkins but I get through it way faster because I spend less time trying to decode everything.