My daughter is a 3rd grader, but she reads at 8th grade level, and she is 2 grade levels ahead in math.
She reads everything she gets her hands on.
She took some chemistry activities, and she loved them. ("Mad Science", "Ooo, goo and stinky too",...)
She knows that matter is made of atoms, and that atoms are made of protons and electrons.
I'd like to find some chemistry books that would be appropriate for her. A good book would be one that starts from beginning, help her connect together the stuff she already knows, and then take her further.
My reason is that I don't want school to be too easy for her. I want to nurture her curiosity, love of challenge, and motivation to study.
On the other hand, I don't want to push her even further ahead in the subjects that she has to take in emementary or middle school anyway -- because that would make our dealing with the school system even more complicated... So, I think Chemistry and Physics are good subjects.