Something that has to do with atoms is atomic. Atomic structure, for example, means the way an atom is organized and what it's made of.
You'll often find the word atomic specifically describing power or weapons — in this case, atomic means the process of either splitting an atom, known as fission, or joining two atoms, fusion, to create energy. Also, every chemical element has its own atomic number, which is the number of protons in the nucleus of one of the element's atoms. Atomic comes from the Latin atomus, "indivisible particle," from the Greek atomos, "uncut" or "indivisible."