The current theory on atomic structure came about following many earlier ideas.
In 1803 JOHN DALTON came up with the idea that all matter was made up of tiny particles called ATOMS. He imagined them as tiny spheres that could not be divided.
In the early 1900’s J.J. THOMPSON came up with the PLUM PUDDING model of the atom. He thought that the atom was a ball of positive charge with negative electrons embedded in it.
In 1909 ERNEST RUTHERFORD designed an experiment to test the plum pudding model. He fired positively charged alpha particles at thin gold foil. Most alpha particles went straight through. A few were scattered in different directions.
This evidence led to the nuclear model of the atom, identifying that the mass of an atom is concentrated in the nucleus and that the nucleus is positively charged.
NIELS BOHR adapted Rutherford’s nuclear model and suggested that electrons orbit the nucleus in shells. The shells are at certain distances from the nucleus.
In 1932 JAMES CHADWICK suggested the existence of particles in the nucleus with mass but no charge. These he called neutrons.
All of the above reserach and discovery led to the development of the modern atomic model that is so familar to us today.