In 440 BCE Democritus theorized that everything was made up of tiny particles surrounded by empty space. He also theorized that atoms were all different shapes and sizes. He called his discovery atomos, Greek for inadvisable. Aristotle however disagreed, he said matter made of 4 elements. Earth, wind, water, and fire.
In 1808 John Dalton took substances and discovered that they broke into the same elements in same proportions and that they must be made up of combination of atoms.
In 1897 JJ Thomson discovered the electron. He theorized that atoms have electrons which were thinly spread out that made up an object.
1911 Ernest Rutherford shot alpha particles at gold foil and discovered that because some ripped through and some deflected that matter had a nucleus where more atoms were more densely packed.
1913 Niels Bohr discovered that electrons orbit the nucleus at a fixed energy and distances and can jump between them.
1925 Werner Heisenberg discovered it is impossible to determine exact place or speed of atoms and electrons.