I have no idea how to rate this one. 
 I really enjoyed Simone Elkeles other books, which I read one week when I was home sick earlier this year, but Chain Reaction bothered me. My issue was with the premise. In this novel, the youngest of the Fuentes boys, Luis, returns with his mother to Chicago--not only Chicago, but his old neighborhood and old house--and gets wrapped up with the same violent guys that almost killed his brother Alex. And this is where I got tripped up. My internal monologue when something like this, 
 "What, what?! Mama Fuentes is taking her kid back to the gang-infested neighborhood?! But he only has his senior year left! Why is she doing this?" 
 Now, her reason is that she doesn't want to be far away from her grandchild (Alex & Brittany have had to return to Illinois because Brittany's sister is now in state care--but at least they have the sense to live in Evanston, where Alex is going to grad school). However, the notion that she couldn't wait a year until Luis, straight-A student and good boy, went to college is absurd to me. I mean, she could Luis and Carlos back to Mexico to escape the very environment she returned to in this book! Either their mother is an idiot or completely selfish. (Her actions and general cluelessness regarding Luis and the neighborhood throughout the book lead me to think it's probably a combination of the two.) 
 This stuck with me while I read the book and it really kept me from enjoying it like I'd enjoyed the first two. However, I will say that I thought Nikki was a very compelling character, with far more depth than either Brittany or Kiara, who were narrators in the first two novels in the series, respectively. 
 I guess my hesitation with the third install of this series is that the premise is hard to swallow, that Luis' story could have been told from Colorado, where he was thriving. If you can get beyond that premise, and enjoyed the other books in this series, you'll likely enjoy this one as well. I believe the Perfect Chemistry novels are very well-researched, and that really does come through in the scenes dealing with gang culture--it's heart-breakingly realistic. Like all of Elkeles novels, the pace is fast, there's the perfect amount of heightened teen emotions and drama that will appeal to high school age readers tremendously. (I would have loved these books so much when I was 15 or so!) Adults who enjoy YA may have a harder time overlooking some of the issues, however. 
 Also, the epilogues in this series drive me crazy. I know most folks love them, but I don't.