As we know it, that is.
A hundred years ago today, Ernest Rutherford published a paper, “The Scattering of α and β Particles by Matter and the Structure of the Atom,” in the Philosophical Magazine. He described atoms as we visualize them today, with most of the mass concentrated in a tiny nucleus. The full story is here.
That's one of those facts that give great power to science. From that discovery came our ability to understand the structure of DNA, the development of most of today's modern medical tools, and, yes, nuclear energy in its explosive and electrical forms.
A hundred years ago, the way scientists thought about atoms was not much different from how the ancient Greeks thought about them. What a change.