Atoms are composed of a central nucleus, containing positively-charged protons and neutrally-charged neutrons. Smaller, negatively-charged electrons travel around the nucleus in layered orbits called shells.

Light a match and chemical change happens in a one-way process: Reactants are transformed into products. But there are many chemical reactions called "equilibrium reactions" that operate in both directions: with reactants and products always present. The Unit 9 video will show how chemical equilibrium works, the essential role it plays in the function of the human body, and how it is exploited in chemical processes such as ammonia synthesis, a process that provides food for up to half the world's population.