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465 BCE
Democritus
Democritus developed the first idea of atoms. His theory stated all matter consists of invisible particles called atoms, which are indestructible, solid but invisible, homogenous, and they differ in size, shape, mass, position, and arrangement. His model was a simple ball. -
335 BCE
Aristotle
Aristotle didn't believe in atomic theory. He believed everything was made of earth, wind, fire, and water. His ideas were generally accepted, causing Democritus's ideas to be overlooked for 2,000 years. -
Antoine Lavoisier
Lavoisier theorized that mass is conserved in chemical reactions. His work resulted in the law of conservation of matter, which states that matter is neither created nor destroyed in chemical reactions. Following up on Lavoisiers's work, we now know that matter is conserved becuase atoms are neither created nor destroyed. -
John Dalton
Although Democritus first suggested atoms existed, it was John Dalton that created the first atommic theory. Over 200 years old, his theory is still valid today. He stated that all matter is made of atoms, atoms are indivisible and indestructible, all atoms of a given element are identical in mass and properties, compounds are formed by a combination of two or more different kinds of atoms, and a chemical reaction is a rearrangement of atoms. -
Henri Becquerel
Becquerel is credited with discovering radioactivity. Inspired by the discovery of X rays, Becquerel thought that phosphorescent elements like uranium might emit radiation when exposed to light. He tested this theory byt placing uranium salts on a photographic plate wrapped in black paper. One day, he put the experiment in a drawer becasue it was overcast and left it there for multipe days. When he developed the plate, he saw a very strong image. -
J.J. Thomson
J.J. Thomson helped form the modern atom by discovering the electron. He conducted an experiment in a cathode ray tube to show cathode rays were negatively charged. The experiment semonstrated that the rays could be deflected by a magnetic field, and that this was not a seperate phenomenon. He stated the atom to be like "plum pudding," with electrons scattered throughout a ball of positive charge. -
Marie & Pierre Curie
Marie and Pierre Curie are well known for their study of radioactivity and their discovery of elemnts polonium and radium.After doing multiple experiments involving uranium and thorium, she drew the conclusion that pitchblende contains a small amount of a radiating element. She, her husband, and Henri Becquerel won a nobel prize for the joint discovery of radiation. She ultimately ended up dying from cancer, caused by prolonged exposure to radiation. -
Max Planck
Planck, who is besk known for the original quantum theory or energy, significantly contributed to atomic and subatomic processes. Albert Einstein demonstrated that electromagnetic radiation has the characteristics of both a wave and, consistent with Planck's theory, a particle. These particles were later named photons. Niehls Bohr used this knowledge to develop a new model of the atom -
Robert Millikan
Millikan made many discoveries as a scientist, but his most important to atomic structure was the finding that electrons are negatively charged. He determined this with the Oil Drop experiment. To do this, he put a charge on a small drop of oil and measured how strong an applied electric field had to be in order to stop the oil drop from falling. He could then determine the electric charge that the drop must have. By varying the charge on different drops, he found the charge of an electron. -
Ernest Rutherford
Rutherford, a student of J.J. Thomson's, decided to challege the structure of the atom. He devised an experiment to shoot high velocity alpha particles at a sheet of god foil. If the "plum pudding" model was correct, all the particles would pass through. However, a few of the particles bounced stright back. For this to happen, he determined there had to be a nuclues that contains most of the mass. He thus developed the planetary model, where electrons orbit protons in the nucleus. -
Henry Moseley
Mosely, who worked with Ernest Rutherford, found a flaw in Mendeleev's periodic table. The position predicted by an element’s atomic weight did not always match the position predicted by its chemical properties. To try and solve this dilemma, he trie to use X-rays to learn what goes on inside of atoms. His results supported a previous hypothesis that atomic number was equal to the charge in the nucleus. This led to the discovery that an element is defined by its number of protons. -
Niels Bohr
Bohr combined Ernest Rutherford's description about the nucleus and Max Planck's quantum theory to explain what happened inside an atom and develop a model of the atomic structure. His model shows a small, positively chraged nucleus orbited by electrons. He was also the first to discover that electrons travel in seperate orbits. His work won him a Nobel Prize in 1922. -
Werner Heisenberg
Heisenberg is well known for his discovery of the uncertainty principle. This means that electrons do not travel in neat orbits, and that all electron that contain photons will then change momentum. This helped clarify the modern view of the atom because scientists can compare atoms by their electron movements. -
Erwin Schrodinger
Schrodinger took the atomic model proposed by Neils Bohr one step further. He described the likelyhood of finding an electron in a certain postion using mathematical equations. This model is known as the quantum mechanical model. It does not define the path of an electron, but rather predicts its location.Where the cloud is most dense, the probability of finding the electron is greatest, and vice versa. -
James Chadwick
Until 1932, atoms were thought to be composed of just protons and electrons. Chadwick discovered the neutron by shooting alpha particle at beryllium atoms, which produced an unknown radiation. Chadwick interpreted this radiation as being from neutral particles with an approximate mass of a proton. This discovery finally made an adequate model of the atom available to scientists.
