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3100 BCE
The Alchemists (3100-2686 B.C.)
They had eventually succeeded in demonstrating that the atoms in radioactive materials disintegrate, so that the atoms of a radioactive element are transformed into another element. -
Period: 3100 BCE to
Atomic Theory
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440 BCE
Plato (440 B.C.)
Plato introduced the atomic theory in which ideal geometric forms serve as atoms, according to which atoms broke down mathematically into triangles. -
400 BCE
Democritus (400 B.C.)
Democritus, theorized that atoms were specific to the material which they composed. In addition, Democritus believed that the atoms differed in size and shape, were in constant motion in a void, collided with each other; and during these collisions, could rebound or stick together. -
Robert Boyle (1661)
He discovered the inverse relationship between pressure and volume in a gas (known as Boyle's law today) contributed to the development of atomic theory by demonstrating that matter is made of tiny particles that he called corpuscles, but that are known as atoms today. -
Antoine Lavoisier (1789)
Antoine Lavoisier's work in defining the law of conservation of mass would help to shape atomic theory. This discovery was influential in atomic theory because it defined that matter was composed of atoms that were not created or destroyed during chemical reactions. -
Billiard Ball Model (1803)
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John Dalton (1808)
Dalton's theory was based on the concept that each element consists of its own unique brand of indivisible atom; atoms of one element are all alike but they differ from atoms of other elements. Importantly, Dalton assigned atomic weights to the atoms of the 20 elements he knew of at the time. -
Dmtri Mendeleev (1869)
Dmitri Mendeleev devised the periodic classification of the chemical elements, in which the elements were arranged in order of increasing atomic weight. -
J.J. Thompson (1897)
J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup." -
The Curies (1898)
Curie discovered an even more radioactive element, radium, and, through observation of radium, made a fundamental discovery: Radiation wasn't dependent on the organisation of atoms at the molecular level; something was happening inside the atom itself. -
Plum Pudding Model (1904)
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Albert Einstein (1905)
Einstein also in 1905 mathematically proved the existence of atoms, and thus helped revolutionize all the sciences through the use of statistics and probability. Atomic theory says that any liquid is made up of molecules (invisible in 1905). Furthermore, these molecules are always in random, ceaseless motion. -
Robert Millikan (1910)
His earliest major success was the accurate determination of the charge carried by an electron, using the elegant “falling-drop method”; he also proved that this quantity was a constant for all electrons (1910), thus demonstrating the atomic structure of electricity. -
Ernest Rutherford (1911)
Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus. Based on these results, Rutherford proposed the nuclear model of the atom. -
Niels Bohr (1913)
In 1913, Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values. Electrons move around a nucleus, but only in prescribed orbits, and If electrons jump to a lower-energy orbit, the difference is sent out as radiation. -
Solar System Model (1913)
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Henry Moseley (1914)
He reported that the frequencies are proportional to the squares of whole numbers that are equal to the atomic number plus a constant. Known as Moseley's law, this fundamental discovery concerning atomic numbers was a milestone in advancing the knowledge of the atom. -
Electron Cloud Model (1926)
