Who said most of the mass is in tiny dense charged nucleus?

The concept that most of an atom's mass is concentrated in a tiny, dense, positively charged nucleus was primarily established by Ernest Rutherford through his gold foil experiment in 1909. Rutherford's findings demonstrated that atoms consist of a small nucleus surrounded by electrons, leading to the understanding of the nuclear structure of atoms. This was a pivotal moment in the development of atomic theory, reshaping our understanding of atomic structure.