Democritus believed that if you continuously break matter in half how many breaks can it make. Aristotle believed in the 5 elements. water, fire, air, earth, and aether.
For more than 2,000 years nobody even thought of it until an English Chemist named John Dalton performed experiments with various chemicals to show that matter did consist of particles (atoms).
In 1897 the English Physicists, J.J. Thomson discovered the electron and proposed a model for the structure of the atom. His model was later called the Plum Pudding Model.
in 1900 Max Planck, A professor of theoretical physics, proposed that when you vibrate atoms strong enough you can measure the energy only in discrete units.
Ernest Rutherford thought it would fascinating to blast atoms with these alpha rays. He thought this experiment could examine the inside of the atom sort of like a Probe. He used radium as the source of the alpha particles and shinned them onto the atoms of gold foil, behind the gold foil was a florescent screen so he could observe. the results were unexpected.
The oil drop expieriment entail balancing the Gravitational force with the upward drag and electrics forces on tiny charged droplets of oil suspended between two metal elctrodes.
In 1912 A Danish Physicist, Neils Bohr, came up with a Theory that electrons will not spiral into the nucleus and also came up with two rules: Rule 1, Electrons can orbit only at a certain distance from the nucleus. Rule 2, Atoms radiate energy when an electrons jump from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high energy orbit.
Finally he identifies the particles of the nucleus as discrete positive charges of matter. Using alpha particles like bullets he knocked hydrogen nuclei out of atoms of six elements : boron, fluorine, sodium, aluminum, phosphorous, and nitrogen. He named them protons.
He thought that if light can exist as both particles and waves, why couldn't atom particles behave the same way?
Before Schrodinger, a German Physicists named Werner Heisenberg had a theory called Matrix Mechanics. Which also explained the behaviors of atoms. The two ideas were based on different things. Heisenberg based on mathematical quantities and Schrodinger based his on waves. but results proved mathematically the same.
His Idea was "why not go all the way with particle waves and form a model of the atom?" His theory worked.
He found it to measure slightly heavier then the proton with a mass of 1840 electrons and no charge (neutral).