A particle is an atom which cant be seen by the naked human eye, everything is made up of: in solids; gasses and liquids. Each atom contains electrons, protons and neutrons, which is what gives each particle its properties. Solids tend to have more compact particles where as gasses have less.
The smallest particle that can be seen with a light microscope is about 0. 2 microns. If an object is smaller than that, the light's wavelength cannot traverse it causing it to fall out of the visible spectrum.