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460
Democritus
In 460 B.C., Democritus, develop the idea of atoms. He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms. -
Antoine Lavoisier
Lavoisier showed the phlogiston theory to be inconsistent. In Methods of Chemical Nomenclature, he invented the system of chemical nomenclature still largely in use today, including names such as sulfuric acid, sulfates, and sulfites. His Elementary Treatise of Chemistry, was the first modern chemical textbook, and presented a unified view of new theories of chemistry, contained a clear statement of the Law of Conservation of Mass, and denied the existence of phlogiston. -
Law of Conservation of Mass
Established in 1789 by French Chemist Antoine Lavoisier. -
John Dalton
John Dalton was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory. -
Dalton's Atomic Theory
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. A chemical reaction is a rearrangement of atoms. -
Dmitri Mendeleev
Mendeleev is best known for his work on the periodic table; arranging the 63 known elements into a Periodic Table based on atomic mass, which he published in Principles of Chemistry in 1869. His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties. -
J.J. Thomson
Thomson, in 1897, was the first to suggest that the fundamental unit was over 1000 times smaller than an atom, suggesting the subatomic particles now known as electrons. -
Cathode Ray Tube
The cathode ray tube is a vacuum tube containing an electron gun (a source of electrons or electron emitter) and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. -
Ernest Rutherford
In 1898 Rutherford was appointed to succeed Hugh Longbourne Callendar in the chair of Macdonald Professor of physics at McGill University in Montreal, Canada, where he did the work that gained him the Nobel Prize in Chemistry in 1908. -
Plum Pudding Atomic Model
The plum pudding model of the atom by J. J. Thomson, who discovered the electron in 1897, was proposed in 1904 before the discovery of the atomic nucleus in order to add the electron to the atomic model. -
Robert Millikan
Starting in 1908, while a professor at the University of Chicago, Millikan worked on an oil-drop experiment in which he measured the charge on a single electron. J.J. Thomson had already discovered the charge-to-mass ratio of the electron. -
Gold Foil Experiments
It was an experiment to probe the structure of the atom performed by Hans Geiger and Ernest Marsden in 1909. -
Rutherford Model
Rutherford's model for the atom, based on the experimental results, contained the new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and with this central volume also containing the bulk of the atomic mass of the atom. This region would be named the "nucleus" of the atom in later years. -
Bohr Planetary Model
The most important properties of atomic and molecular structure may be exemplified using a simplified picture of an atom that is called the Bohr Model. This model was proposed by Niels Bohr in 1915. -
James Chadwich
In Cambridge, Chadwick joined Rutherford in accomplishing the transmutation of other light elements by bombardment with alpha particles, and in making studies of the properties and structure of atomic nuclei. -
Niels Bohr
In 1922, Bohr was awarded the Nobel Prize in physics for his services in the investigation of the structure of atoms and of the radiation emanating from them. Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a photon (light quantum) of discrete energy. This became a basis for quantum theory. -
Erwin Schrodinger
For his 1926 introduction of Schrödinger's wave, the mathematical equation of wave mechanics that is still the most widely used piece of mathematics in modern quantum theory. It posits a non-relativistic wave equation that governs how electrons behave within the hydrogen atom -
Electron Cloud Model
The electron cloud model is an atom model wherein electrons are no longer depicted as particles moving around the nucleus in a fixed orbit. Instead, as a quantum mechanically-influenced model, we shouldn’t know exactly where they are. -
Quantum Mechanical Model
The quantum mechanical model is based on quantum theory, which says matter also has properties associated with waves. According to quantum theory, it’s impossible to know the exact position and momentum of an electron at the same time. -
Henry Moseley
Moseley's outstanding contribution to the science of physics was the justification from physical laws of the previous empirical and chemical concept of the atomic number.
