-
440
Democritus
Democritus was a Greek philosopher and speculative scientist. He believed that all matter is composed of single, indivisable atoms exactly alike qualitatively. He was also the first one to use the term atom. He had the earliest atomic model of which there is written evidence. (He discovered this in 440 B.C) -
Antoine Lavoisier
Lavoisier created the Law of Conservtion of Mass which sid that mass is neither created nor destroyed in chemical reactions. The mass of any one element at the beginning of a reaction will equal the mass of that reactant at the end of that reaction. -
Law of Conservation of Mass
The Law of Conservation of Mass was discovered by Antoine Lavoisier. It states that mass is neither created nor destroyed in chemical reaction. -
John Dalton
Dalton believed that atoms of one element differed from those of other elements by their relative weights. He also believed that atoms of two or more different elements combine together to form chemical compounds. -
Dalton's Atomic Theory
Dalton performed a series of experiments on mixtures of gases to determine what effect properties of the individual gases had on the properties of the mixture as a whole. While trying to explain the results of those experiments, Dalton developed the hypothesis that the sizes of the particles making up different gases must be different. -
Dmitri Mendeleev
By arranging all 63 elements then known by their atomic weights, he managed to organize them into groups possessing similar properties. -
J.J Thomson
Thomson discovered the electron. His work put forward a new theory, the atom was made up of small particles, which led him to discover electrons. HE proved his theory by using the cathode ray tube. -
Cathode Ray Tube
J.J Thomson did three experiments with the cathode ray tube, and from the experiments he concluded three things: cathode rays are charged particles (which he called corpuscles), these corpuscles are constiuents of the atom, and these corpsucles are the only constituents of the atom. -
Plum Pudding Atomic Model
The Plum Pudding Model was proposed before the discovery of the atomic nucleus in order to add the electron to the atomic model. In this model the atom is composed of electrons surrounded by a cloud of positive charge to balance the electrons' negative charges. The electrons were thought to be positioned throughout the atom. -
Gold Foil Experiment
Rutherford noticed that alpha particles would not always behave in accordance to the plum pudding model of an atom when fired at a piece of gold foil. These observations stimulated further research that was eventually published in 1911 and has been known ever since as Rutherfords's Gold Foil Experiment. -
Robert Millikan
Millikan did an oil-drop experiment to accurately determine both the charge and, by virtue of the charge-to-mass ratio, the mass of the electron. Both numbers allowed Neils Bohr to finally calculate Rydberg's constant and provided the first and most important proof of the new atomic theory. -
Rutherford Model
Rutehrford's model suggested taht most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space.Ruther came to this conclusion following his gold foil experiment. -
Ernest Rutherford
Ernest Rutherford's atomic theory describes the atom as having a central positive nucleus surrounded by negative orbiting electrons. His model suggests that most of the mass of the atom was contained in the small nucleus and the rest of the atom was mostly empty space. -
Neils Bohr
Bohr published a theory about the structure of the atom based on the earlier model of Rutherford's. Bohr expanded his theory by proposing that electrons travel in only certain succecively larger orbits. He suggested that outer orbits could hold more electrons than the inner ones, and that these outer orbits determine the atom's chemical properties. -
Henry Moseley
Henry Moseley published the results of his measurements of the wavelengths of the X-ray spectral lines of a number of elements which showed that the ordering of the wavelengths of the X-ray emissions of the elements coincided with the ordering of the elements by atomic number. -
Bohr Planetary Model
The Bohr Model has an atom consisting of a small, positively-charged nucleus orbited by negatively-charged electrons. The modern model of the atom is based on quantum mechanics. The Bohr Model contains some errors, but it is important because it describes most of the accepted features of the atomic theory without all of the high-level math of the modern version. -
Quantum Mechanical Model
The Quantum Mechanical Model was proposed by Erwin Schrodinger. This model predicts the odds of the location of the electron. This introduced the concept of sub-energy levels. -
Electron Cloud Model
The electron cloud model is an atom model where electrons are no longer depicted as particles moving around the nucleus in a fixed orbit. -
Erwin Schrodinger
Schrodinger used mathematical equations to describe the likelihood of finding an electron in a certain position. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of teh location of the electron. This model can be portrayed as a nucleus surrounded by an electron cloud. Where the cloud is more dense, the probability of finding the electron is greatest. This model introduced the concept of sub-energy levels. -
James Chadwick
James Chadwick proved that the atomic nucleus contained a neutral particle which had been proposed more than a decade ago by Ernest Rutherford.
