Was Democritus theory wrong?
It all depends how Democritus assumptions were interpreted in modern theories of physics. Even though atomic mass structure has been basically determined as being divisible and the periodic table of elements describes the architecture of mass structure; The basic structure of matter is not fully quite understrood as which particle is indivisible. The Greek word atom signifies somthing that is universal that cannot be cut any further. Basically the smallest Atom which is Hydrogen consits of a bulk volume which is called space that is structured as part of the Atom.
