I know it’s been 3 years since this was asked, but I just remembered reading this book a while ago and was looking for it too. I think the book is Smiles to GoSmiles to Go by Jerry Spinelli.
The novel begins with a short prologue in which Will, on a visit to an eccentric neighbor, learns about protons, a microscopic particle of matter which, the neighbor tells him, is the indestructible building block of all matter. The impressionable Will wonders whether, because he is matter, he too is indestructible, narration soon revealing that, over the years since, he has come to believe exactly that. One day, however, Will learns that protons are, in fact, finite and do come to an end. His belief that part of him is indestructible is profoundly shaken, and he goes to school wondering what this news now means for his life, which up to that moment had been safe, secure, and predictable.
