[n] (nontechnical usage) a tiny piece of anything [n] (physics and chemistry) the simplest structural unit of an element or compound
atom, mote, particle, speck
ace, acid, acidity, agent, air, alkali, alkalinity, alloisomer, anion, antacid, atom, atomic cluster, atomic particles, base, benzene ring, biochemical, bit, branched chain, brute matter, building block, cation, chain, chemical, chemical element, chromois