Before the atom was ever seen, it was developed empirically through the work of many successful scientists. The development of the atom began with the work of John Dalton. He produced the first comprehensive theory of the atom, which included several postulates. Dalton proposed that matter is composed of indivisible particles, as we know them, atoms. He thought they were like little marbles, and was later proved wrong on the appearance side, but he developed the idea nonetheless.
Above: Dalton’s theory of the atom- a small marble. http://2011period5group5.wikispaces.com/John+Dalton
The first scientist to challenge Dalton’s theories was J.J. Thomson. He proposed that there are charges in atoms by experimenting with cathode rays. When evidence showed that these rays were impacted by magnetic and electrical fields, Thomson determined that there must be a negative particle in an atom, what we now know as electrons. Consequently, he decided that these negatively charge particles must be surrounded by an oppositely charged substance. From here, Thomson developed the “plum pudding model”, as seen below.
Above: Thomson’s plum pudding model of the atom. http://www.nobelprize.org/educational/physics/quantised_world/structure-1.html
As a result of his work, Thomson was credited with being the first to the existence of subatomic particles. In addition, he determined the mass to charge ratio of an electron.
The next scientist to assist on the empirical development of the atom was Ernest Rutherford. Rutherford performed an experiment in which he bombarded a sheet of gold foil with alpha particles. He expected the particles to pass through, but discovered that some of the particles went through while some bounced in different directions. This observation resulted in the idea of the nucleus, or a heavy dense mass at the center of the atom.
Above: The result of Rutherford’s experiment. http://tap.iop.org/atoms/rutherford/
The last contributor to the empirical image of the atom was Chadwick. By observing that the mass of an atom did not match what would be expected as a result of the weight protons and electrons, he discovered the existence of neutrons. As a result of having no charge, they were the last subatomic particle to be discovered.
In conclusion, the visual idea of an atom was developed through scientific research and observation without ever actually viewing the smallest building block of matter. While the contributing scientists did not have the microscopic power to visually see an atom, they were able to create an idea of its structure through experimental evidence. Through hard work and thorough research, these admirable scientists developed atom empirically before it was ever seen by the human eye.