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Oct 1, 1400
Democritus-442 BC
Democritus was an Ancient Greek philosopher who began the search for a description of matter. After much questioning about if matter could continue to be divided into smaller pieces, he finally came up with a theory. His theory stated that matter could not keep being divided (which proved Aristotle wrong). At some point, matter would reach a point where it would be indivisible. He named it "atomos", meaning "not to be cut." -
Oct 1, 1500
Aristotle- 384 BC
Like Democritus, Aristotle was a greek philsopher as well. He was a student of Plato and taught Alexander the Great. Aristotle provided the method of gathering scientific facts. This became and still holds the position of being the basis for all scientific work. He believed that all matter was made up of one of the four elements of water, fire, air, or earth. Many believe this did nothing but delay the development of the Atomic Theory. However, Aristotle did discover there are elements. -
Antoine Lavoisier
Lavoisier was a french chemist who was credited with being the founder of modern chemistry. He discovered that the concept of a an element was so simple that no method of chemical analysis could break it down further. He also provided the formula for the conservation of matter in chemical reactions. -
John Dalton
With deductive reasonsing and experimentation, John Dalton came up with the Atomic Theory of Matter. It consisted of 5 theorems that said, all matter is composed of atoms, atoms of same elements are identical, elements can be identified by their atomic weight, atoms of elements unite to form chemical compounds, and atoms can not be created or destroyed in chemical reactions. He was the first give a true model of the atom which he believed was like a solid wooden bead. -
Marie & Pierre Curie
Pierre and his wife Marie, known and the twins. They are known for having found radioactive materials cause atoms to "break down" and release raidiation in the form of subatomic particles. -
Max Planck
Planck discovered and stated that energy is radiated in small units. He called these units "quanta". He used this discrete unit of energy to explain hot glowing matter (Quantum Theory). Einsein used his theories and found particle properites of light. In 1913, Niels Bohr used Planck's theories as well, to develop a new and enhanced model of the atom. -
Henri Becquerel
Henri is famous for his discovery of radioactivity. He recieved the Nobel Prize in physics, hand-in-hand with Marie and Pierre Curie. -
J.J. Thomson
Joseph John Thomson was a British physicist who was credited with the discovery of electrons in 1897. Thus, enhancing John Dalton's first true model of the atom in which he believed the atom was solid (no such thing as electrons/protons). Thomson proposed a new model of the atom in 1904 and called it the plum pudding model. In this model, the atom is composed of electrons, surrounded by positive charge to balance (negative charged plums surrounded by positive charged pudding). -
Ernest Rutherford
Rutherford conducted the Gold Foil Experiment and proved the structure of the atom. While Thomson believed there was a "pudding" of positive charge that surrounded the electrons, Rutherford, one of his students, found this was incorrect. The fact that some of the alpha particles were deflected/reflected through the gold foil experiment of directing a narrow beam of light through the foil meant the atom had a concentrated center (nucleus) of positive charge. -
Niels Bohr
Bohr had planned to spend a year at the Labratory at Cambridge for research on electromagnetism and atomic theory. But, Thomson (the director at this time) was not interested in Bohrs ideas. However, Rutherford did express interest in his ideas so Bohr traveled to Manchester to participate in his nuclear model of the atom.There, he developed a new theory of atom where electrons only travel in certain orbit. -
Henry Mosely
Mosely is known for writing that the atomic number of an element is equal to the number of protons in the nucleus. This truly established the scientific basis of the Periodic table, which then sorted teh elements in order of their atomic numbers. -
Robert Millikan
Millikan is known for his experiments on the charge carried by an electron.He conducted the oil-drop experiment and gave credit to Bohrs quantum theory of the atom. As many other physicists tried to use this experiment to find the charge of an electron, they werenot successful like Millikan. He observed single drops of a liquid (water/oil). Each drop showed a different evaporation than water. So, he found (after repeating) an electrons charge is constant. -
Erwin Schrodinger
Schrodinger made advances by devloping the groundwork for wave mechanics and took Bohrs model one step further.Schrödinger used mathematical equations to show the chance of finding an electron in a certain position. This is known as the quantum mechanical model of the atom. Bohrs model predicted more of the path an electron moves, whereas Schrodingers predicts the location of the electron. -
Werner Heisenberg
Heisenberg was best known for his Principle of Indeterminancy, which stated that both the exact energy and location of an electron can not be determined (at the same time). He said this meant that electrons did not travel in neat orbits, which enhanced Bohr's model which only stated they travel in certain orbits. His fcontributions to the atomic theory included using quantum mechanics to find and interpret the behavior of electrons that make up an atom. -
James Chadwick
Chadwick used alpha particles to measure the energy coming out of protons in hydrogen atoms. This led to the discovery of a neutral atomic particle that had a mass very close to that of a proton. Thus, the neutron was discovered.
