We are All Made of Molecules by Susin Nielsen

Meet Stewart. He’s geeky, gifted and sees things a bit differently to most people. His mum has died and he misses her all the more now he and Dad have moved in with Ashley and her mum.
Meet Ashley. She’s popular, cool and sees things very differently to her new family. Her dad has come out and moved out – but not far enough. And now she has to live with a freakazoid step-brother.
Stewart can’t quite fit in at his new school, and Ashley can’t quite get used to her totally awkward home, which is now filled with some rather questionable decor. And things are about to get a whole lot more mixed up when these two very different people attract the attention of school hunk Jared. . .