Democritus was a street philosopher who was the first to suggest the existence of atoms. He believed that matter consisted of tiny,indivisible, unchangeable atoms.
Aristotle, a well known philosopher, dismissed Democritus' theory of atoms. He believed matter consisted of only four elements: earth, fire, air, and water. Most believed him to be right.
French chemist, Antoine Lavoisier, found that mass is conserved in a chemical reaction. His findings led to one of the fundamental laws of chemical behavior: the law of conservation of matter.
Joseph Proust first published his Law of Definite Proportions, which states that a compound is composed of exact proportions of elements by mass regardless of how the compound was created.
In 1803, Dalton created the first chart of atomic weights. In 1808, he introduced his belief that atoms of different elements could be universally distinguished based on their varying atomic weights. In 1810, he discovered that the atoms within a given element are all exactly the same size and weight, even while the atoms of different elements look, and are, different from one other. Dalton eventually made a table listing the atomic weights of all known elements.
Michael Faraday, a British physicist, made one of the most significant discoveries that led to the idea that atoms had an electrical component.
Becquerel, a French scientist, discovered radioactivity. He discovered this phenomenon while experimenting with uranium and a photographic plate.
J.J. Thomson discovered the electron. He also created the "Plum Pudding" atomic model.
Marie & Pierre Curie discovered the elements radium and polonium. They also greatly contributed to the discovery of radiation and radioactivity.
Max Planck, a German physicist, is the originator of the quantum theory of energy. Planck's work in thermodynamics led to the formulation of his quantum theory. To explain the colors of hot glowing matter, he proposed that energy is radiated in very minute and discrete quantized amounts, rather than in a continuous unbroken wave.
Albert Einstein is well known for the theory of relativity, which laid the basis for the release of atomic energy. He established law of mass- energy equivalence through his famous formula E=mc².
Robert Milikan's oil drop experiment helped to quantify the charge of an electron. In these experiments, the atomizer from a perfume bottle was used to spray water or oil droplets into a sample chamber. Some of these droplets fell through a pinhole between two plates of an electric field, where they could be observed through a microscope.
Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes. This experiment demonstrated that atoms have tiny, heavy nucleuses.
Neils Bohr developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons.
James Chadwick discovered neutrons. He discovered them when he bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton.