elementary particle

n. subatomic particle, individual elemental particle that compose the whole atom

In particle physics, an elementary particle or fundamental particle is a not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic particles of the universe from which all larger particles are made. In the Standard Model, the quarks, leptons, and gauge bosons are elementary particles.
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The particles which form the building blocks of atoms and those which carry energy.

Noun
1. a particle that is less complex than an atom; regarded as constituents of all matter
(synonym) fundamental particle, subatomic particle
(hypernym) particle
(hyponym) antilepton
(part-holonym) atom