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442 BCE
Democritus
Democritus believed that all matter consisted of invisible particles called atoms, and of empty space. He also theorized that atoms could not be broken down or changed. Democritus thought that atoms had no internal structure and that they were just solid spheres. -
340 BCE
Aristotle
Aristotle’s beliefs were drastically different from current beliefs about elements. Aristotle believed that there were five elements, water, air, light, fire, and earth. He believed that everything was made of one or a mix of these elements. This had a negative contribution on the atomic theory. -
Antoine Lavoisier
Antoine Lavoisier was a French chemist who is well known for discovering The Law of Conservation of Matter. This basically stated that atoms are neither created or destroyed during a chemical reaction. Or in other words, mass is always conserved. -
John Dalton
John Dalton was one of the first people to believe in the existence of the atom. He believed that everything was made of atoms. He also believed that all atoms of the same element had the same mass, and that compounds were a combination of two or more different kinds of atoms. -
Henri Becquerel
Henri Becquerel is credited for discovering radioactivity. This led to the further understanding of protons and neutrons when scientists realized that there were different radioactive waves that consisted of different amounts of protons and neutrons. Based on the experiments of Henri Becquerel and other scientists a new model of the atom was developed. -
J.J. Thomson
J.J. Thomson is most famous for discovering the electron. He discovered this by performing an experiment using a cathode ray. He used this to prove that cathode rays were negatively charged. Thomson also created a model often referred to as the plum pudding model, but it was later disproven by his student Ernest Rutherford. -
Max Planck
Max Planck was a German scientist that is known to be the originator of quantum theory. He came up with this theory to describe the colors of hot glowing matter. He thought that instead of energy being radiated in an unbroken wave, it is radiated in small “pockets” of energy -
Marie & Pierre Curie
Marie and Pierre took Henri Becquerel’s discovery of radioactivity and furthered it. They figured out that other elements also emitted radiation. From their studies, Marie was able to figure out that radioactivity did not depend on how the atoms were arranged in a compound, but rather it comes directly from the atom itself. -
Robert Millikan
Robert Millikan quantified the negative charge of an electron. He did this through his famous oil drop experiment. He would drop oil between two electrodes and measure things like the force of the droplets when they passed through the electric field, and the electric field between the electrodes. With this data Millikan was able to find the charge of a single electron. -
Ernest Rutherford
Ernest Rutherford disproved J.J. Thompson using the “gold foil” experiment. He than created his own model from the findings of this experiment. This consisted most of the mass in the atom being concentrated in the nucleus. The model he created is very similar to today’s model. -
Niels Bohr
Niels Bohr’s greatest contribution to science was his atomic model That he worked on with his teacher, Ernst Rutherford. His model had a nucleus in the center with electrons orbiting it. Niels Bohr was also the first person to discover that electrons travel in separate orbits around the nucleus. -
Henry Mosely
Henry Mosely was an English scientist best known for reorganizing the periodic table by atomic number instead of by atomic weight. However, he also did extensive research on radioactivity specifically on X-Rays. Additionally, he worked a lot on the atomic battery. -
Werner Heisenberg
Werner Heisenberg was a German scientist. He best known to have discovered a method to formulate quantum mechanics in terms of matrices, which he was awarded a Nobel Prize for. In addition, he also developed the uncertainty principle. -
Erwin Schrodinger
Erwin Schrodinger is credited for developing the quantum mechanical atom model. He developed this model to show the likelihood of the location of an electron. He figured out the location by using several mathematical equations. -
James Chadwick
James Chadwick is best known for discovering the neutron. He did this by bombarding a beryllium atom with alpha particles, then an unknown radiation was formed. James Chadwick then discovered the radiation was made of a neutral electrical charge that later became the neutron. His findings were very important to the discovery of nuclear fission which lead to the atomic bomb.
