Atom atomos (Greek) Indivisible, individual, a unit; among the Greek Atomists what in theosophy is called a monad. Atomic theories of the constitution of the universe or of matter are many and ancient. In modern physics the atom is a small particle once thought indivisible, but now resolved into component units. In some philosophies, as that of Leibniz, the atoms (which he calls monads) are psychological rather than physical units -- unitary beings of diverse kinds and grades, composing the universe.
In theosophy, atoms have to be considered in relation to monads; in The Secret Doctrine gods, monads, and atoms are a triad like spirit, soul, and body. A monad is a divine-spiritual life-atom, a living being, evolving on its own plane, and a life-atom is the vehicle of the monad which ensouls it, and in turn ensouls a physical atom. The ultimates of nature are atoms on the material side, monads on the energic side; monads are indivisible, atoms divisible (a departure from the etymological meaning). Thus there is a quaternary of gods, monads, life-atoms, and physical atoms. "An atom may be compared to (and is for the Occultist) the seventh principle of a body or rather of a molecule. The physical or chemical molecule is composed of an infinity of finer molecules and these in their turn of innumerable and still finer molecules. Take for instance a molecule of iron and so resolve it that it becomes non-molecular; it is then, at once transformed into one of its seven principles, viz., its astral body; the seventh of these is the atom. The analogy between a molecule of iron, before it is broken up, and this same molecule after resolution, is the same as that between a physical body before and after death. The principle remains minus the body. Of course this is occult alchemy, not modern chemistry" (TBL 84).