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300
Democritus
Democritus, an acient Greek philosopher, was the first person to concieve of the idea of an atom. He made his contribution to the theory of the atom sometime around the turn of the 3rd century BCE, although it is difficult to know for sure.
An atom, acording to Democritus, would be a very small spherical particle. -
350
Aristotle
Aristotle was influential during the 3rd century BCE.
Aristotle said all matter was one of 4 four elements: earth,fire,air,and water. He did not believe in atoms at all, but did contribute to the foundations of sciencs and empiricism. -
Antione Lavioser
Antione Lavioser,considered the father of mdern chemistry, was a french scientist who discovered the law of the conservation of matter, among numerous other discoveries, such as that hydrogen and oxygen combine to form water. However, he did not contribute significantly to the atomic theory of matter. -
John Dalton
John Dalton was an English schoolteacher who can be regarded as the founder of the scientific discipline of chemistry.In 1805, Dalton published papers envisioning an atom to be a small, spherical marble-like object. Each element would have its own unique atom, and molocules would be composed of atoms bounded together. Previous models of the atom had not accorded each chemical element an atom, and did not envision atoms being bound together. -
Henri Becquerel
Henri Bequerel found in 1896 that uranium emitted powerful rays, thus discovering the principle of radioactivity. When Bequerel placed uranium slats near a photographic plate, the plates became fogged, indicating electromagnetic energy of some sort. -
J.J. Thomson
J. J. Thomson conducted an experiments that resulted in the discovery of the eletron. Thomson constructed a device called a "cathode ray tube", which fires electrons through a vacuum tube, and used this device to prove that the rays inside of it were in fact particles. Thomson theorized that electrons were uniformily distributed throughout the atom, and were the only particles in the atom. The postive charge would be uniformly distributed throughout the atom. This was the plum-pudding model -
Marie & Pierre Curie
Marie Curie and her husband, Pierre were distinguished physicists who made great advances researching radioactivity.
In 1898, Marie Curie was expirementing with the ore, pitchblend, which contanined uranium but was more radioactive than what the quantities of uranium present warranted. The Curies managed to isolate a new element, polonium, from the ore. Later, radium was also isolated, but the Curies famously did not patent the radium-isolating process. -
Max Planck
Max Planck created the field of quantum physics with his discovery that energy is, at the atomic level, quantized. Energy,Plank found, can exist only in multiples of the fundumentall unit E = hν, where h is Planck's constant, 6.6262x10^-34 J-s, and v is the frequency of the raditaion in question. -
Robert Milikan
Robert Milikan discovered tthe charge of an electron using an oil-drop expirement,applying a charge to a single drop of oil and measuring how strong an applied electric field had to be in order to keep the drop from falling.Milikan calculated the mass of the oil drop, and from this deduced its charge. Milikan was also able to claculate teh mass of the electron. -
Ernest Rutherford
Ernest Rutherford, along with his assistants Hans Geiger and Ernest Marsden, conducted an expirement in which a thin gold plate was bombarded with alpha particles.The result indicatede thatthe positive charge of the atom was concentrated in the nucleus of the atom. Rutherford's revised model incorpated a positively charged nucleus,orbited by electrons. This was an improvemnet on the plum pudding model, which did not include the nucleus. Rutherford had studied under J.J. Thomson. -
Neils Bohr
Neils Bohr, a student of J.J tompson, and a colleage of Ernest Rutherford, through work in theoretical physics, developed a model of the atom (called the Bohr model) which gave each electron in the atom an energy level. This improved upon the previous Rutherford model by solving the problem of electrons losing energy in orbit, by allowing the electron to reduce its energy level. -
John Mosely
A British chemist, Mosely studied under Rutherford. Mosely discovered an empirical law, dubbed Mosely's law, that cretain x-rays emitted by atoms have a frquency proportional the atomic number squared. This justified other physicists' theories involving the atomic number of elements, since the atomic number was now a measurable quantity, as opposed to a theoretical position on the periodic table -
Werner Heisenberg
In 1925, Heisenberg developed sophisticated mathematics to discover the uncertainity principle, that one cannot measure both the position and momentum of a particle to an arbitrarily accurate degree. Heisenberg's work marked the beggining of the development of the quantum model of the atom, which says that electrons are found somwhere inside an "electron cloud". This improves upon the Bohr model by incorparting the uncertainty principle. -
Erwin Schrodinger
Schrodinger's namesake equation descirbes how the quantum state of a physical system changes over time. This equation allowed him to calculate the probability distribution of the electron's distance from the nucleus of the atom, and refined and improved the quantum theory of the atom. -
James Chadwick
Chadwick discovered the neutron by examining the radiation resulting form bombarding berrylium with alpha-particles. This resolved the problem of the discrepancy bewteen atomic mass and atomic number in earlier models. Now tthe atom consisted of a nucleus of protons and neutrons surronded by electrons in an electron cloud. Chadwick had studied under Rutherford.
