subatom

from The Century Dictionary.

• noun A definite part of an atom, if this assumed unit, for the most part looked upon as indivisible, is supposed capable under special conditions of undergoing division; a corpuscle. Also called atomicule.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chem.) A hypothetical component of a chemical atom, on the theory that the elements themselves are complex substances; -- called also atomicule.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any particle that is smaller than an atom.