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Democritus - (5th century BC.) Greek Philosopher that believed changes in matter were a result of dissociations or combinations of the atoms as they moved throughout the void.
J.J. Thomson, in 1897, showed that electrons have negative electric charge and come from ordinary matter by using a cathode rays
John Dalton, in 1803, was the English chemist who put the pieces of the puzzle together and developed an atomic theory
In 1908-1913 Rutherford's Gold Foil Experiment proved the existence of a small massive center to atoms, which would later known as the nucleus of an atom.
Cathode Ray Tube Experiment was performed in 1838, Faraday noted that when passing a current through such a tube, an arc of electricity was observed. The arc started at the negative plate (known as the cathode) and traveled through the tube to the oppositely charged anode.
Antoine Lavoisier recognized and named oxygen (1778)
Antoine Lavoisier recognized hydrogen (1783)
James Chadwick In 1932, James Chadwick, a British physicist who had studied with, and was working for, Ernest Rutherford at the time, set out to solve the problem. They found that by using polonium as a source of alpha particles, they could cause beryllium to emit radiation that, in turn, could be used to knock protons out of a piece of paraffin wax.
Erwin Schrodinger His great discovery, Schrödinger’s wave equation, was made at the end of this epoch-during the first half of 1926. It came as a result of his dissatisfaction with the quantum condition in Bohr’s orbit theory and his belief that atomic spectra should really be determined by some kind of eigenvalue problem.
Niels Bohr This stability problem was solved by Niels Bohr in 1913 with a new model in which there are particular orbits in which the electrons do not lose energy and therefore do not spiral into the nucleus. This model was the beginning of quantum mechanics, which successfully explains many properties of atoms.
Oil Drop Experiment An experiment performed by Robert Millikan in 1909 determined the size of the charge on an electron. What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have.
Laws of Conservation of Mass The law of conservation of mass was crucial to the progression of chemistry, as it helped scientists understand that substances did not disappear as result of a reaction (as they may appear to do); rather, they transform into another substance of equal mass.
Ernest Rutherford A monumental breakthrough came in 1911 when Ernest Rutherford and his coworkers conducted an experiment intended to determine the angles through which a beam of alpha particles (helium nuclei) would scatter after passing through a thin foil of gold. ( Used the gold foil experiment to discover the proton)
The Millikan Oil Drop Experiment. An experiment performed by Robert Millikan in 1909 determined the size of the charge on an electron
Plum Pudding Atomic Model - 1904 (the Thomson, or "plum-pudding," atom) proposed that the negatively charged electrons (the plums) were mixed with smeared-out positive charges (the pudding). This model explained the neutrality of bulk material, yet still allowed the description of the flow of electric charges.
Bohr Planetary Introduced by Niels Bohr and Ernest Rutherford in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus
Dalton Atomic Theory By the late 1800’s, John Dalton’s view of atoms as the smallest particles that made up all matter had held sway for about 100 years, but that idea was about to be challenged. Several scientists working on atomic models found that atoms were not the smallest possible particles that made up matter, and that different parts of the atom had very distinct characteristics.
Rutherford Model In Rutherford’s model, the electrons orbit the nucleus similar to the orbit of planets about the sun.
Electron Cloud Model Chemists use the electron cloud model to map out the atomic orbitals for electrons. These probability maps are not all spherical. Their shapes help predict the trends seen in the periodic table.
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