Benchmark 10G
Historical Perspectives: Splitting the Atom
Grades 9-12, page 253

The Curies made radium available to researchers all over the world, increasing the study of radioactivity and leading to the realization that one kind of atom may change into another kind, and so must be made up of smaller parts. These parts were demonstrated by other scientists to be a small, dense nucleus that contains protons and neutrons and is surrounded by a cloud of electrons.

See Content Standard G History and Nature of Science (grades 9-12): Historical Perspectives