Aristotle (350 B.C.) disagreed with Democritus's model of the atom in Aristotle was a Greek philosopher. Many of his ideas were more thought based than scientifcially based. For this reason, Aristotle strongly disagreed with Democritus. He felt that there was no smallest part of matter and that different substances were made of earth, fire, air, and water. Aristotle did not have an atomic model due to the fact that he thought atoms did not exist.
Democritus was the first scientist to create a model of the atom. He was the first one to discover that all matter is made up of invisible particles called atoms. He created the name "atom" from the Greek word "atomos", which means uncuttable. He also discovered that atoms are solid, insdestructable, and unique. HIs model was just a round solid ball. Democritus didn't know about a nucleus or electrons, all he knew was that everything is made of atoms.
Lavoisier was a French nobleman that founded several elements and put the first table of elements together. He used Aristotle's ideas of fire, earth, air, and water to create experiments invesigating combustion and oxidation. By using previous knowledge of atomic bonding, he discovered important elements like oxygen, hydrogen, and sulfur. He discovered that water was made of oxygen and hydrogen, and air included nitrogen. Lavoisier also created the first chemistry textbooks and tables.
John Dalton was an English chemist that created the Atomic Theory of Matter, a composition of previous findings by Democritus and his own findings. He included in this theory that all matter is made of atoms, that atoms cannot be created nor destroyed and also, atoms of different elements combine in whole ratios to form chemical compunds. His theory would later contribute to an advance in the atomic model.
Henri Becquerel was born into a family of scientists. With influences from his father and grandfather, Bequerel worked with properties of the atom, such as magnetism and radioactivity. His biggest achievements were in the field of radioactivity. In his earlier works, Bequerel worked with light and the absorption of light by crystals. He also looked at the mechanics of X-rays. His discovery of radioactivity allowed later scientists to perfect the atomic model.
J.J. Thomson was a very important scientist when it came to the atomic model. Up until his time, all models of the atom looked like a big solild ball. J.J. Thomson discovered the electron, which led him to create the "plum pudding" atomic model. In this model, he thought that the atom was mostly positive, and negative electrons wandered around the atom. The "plum pudding" model influenced other scientists to make better atomic models.
Marie and Pierre Curie
Marie and Pierre Curie were a European couple that contributed to atomic chemistry by exploring the mysteries of radioactivity. After radiation was discovered by Henri Baquerel, Marie decided to look further into this discovery. Through this she and her husband discovered the elements radium and polonium and won the Nobel Peace Prize for their works in radioactivity. Her discovery later added to the atomic model.
Max Planck was a German scientist that created the Quantum Theory. In this theory, Planck stated that energy was given off in little packets of energy. These were called photons when talking about light. He discovered that the energy in wave form is restricted to specific quantaties. This discovery led to the understanding of energy levels in atoms, since quantums are leaps in the atom. This discovery later added to the advance in the atomic model.
Robert Millikan was an American scientist that was very interested in J.J. Thomson's finding of the electron. J.J. Thomson predicted that the electron was 1000 time smaller than the atom. Millikan wanted to prove this hypothesis. He preformed an "oil-drop experiment" in which he found that J.J. Thomson was correct. Millikan was also involved in the Quantum Theory after he was inspired by Max Planck. Millikan inspired other scientists to explore parts of the atom.
Ernest Rutherford was another scientist that changed the atomic model. He felt that J.J. Thomson's model was incorrect, so he created a new one. He created the nucleus, and said that instead of the positive matter being the whole atom, it was just in the middle. He said the atom was mostly empty space and that the electrons surrounded the positive nucleus. This model influenced one of his own students to perfect the atomic model later on.
Niels Bohr was a Danish scientist that was a student of Rutherford. He decided to make a new model based off of Rutherford's model, but changed the orbit of the electron. Also, he created energy levels in the atom, where only a certain amount of electrons could fit on one energy level of the atom. Bohr also used Planck's ideas in order to create quantum mechanics, his new concept regarding energy. This model is still used to this day.
Henry Mosely was an English scientist who worked with Niels Bohr in order to create the real atomic number. Mosely used X-rays to find the frquencies of elements on the periodic table. Before his discovery, the atomic number was just an assigned number to a random element. Mosely used these frequencies to find that the number of protons in the nucleus correlated with the atomic number. This created Mosley's Law. The image to the left is one of Mosely's periodic tables.
Erwin Scrhodinger was an Austrian scientist that worked with the Quantum model of the atom. He disagreed with Bohr's theory, so he created his own. He thought that the only way to find the location and energy of an electron in an atom was to calculate its probability of being a certain distance from the nucleus. This equation influenced the Quantum mechanical model of the atom.
Werner Heisenberg was a German scientist that proposed the uncertainties of the Quantum model. He said that you can't know the exact velocity and momentum of the electron at the same time, which means you can't know the exact location of the electron. This principle proves error in Bohr's model because of the uncertainty of the location of an electron.
Jame Chadwick was an English scientist that discovered the neutron. Before this discovery, Rutherford had concluded that the nucleus was made of positive matter. It made sense that the atom was neutral because the negative electrons and the positive protons cancelled out. But, Chadwick started to question why there was a difference between the atomic mass and the number of protons. Chadwick then found that the missing component was a neutral part: the neutron.