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300
about 450 B.C.
A Greek scholar named Empedocles proposed that matter was composed of four "elements": earth, fire, water and air. Thes elements mixed together in different proportions to yeild different substances. Rust might be two part fire one part air. Unlikemost philosophers of his time, Empedocles checked some of his theories experimentally. He demonstrated that, it is not just "nothing." Because it takes up space, it must be a form of matter. -
400
about 400 B.C.
A second Greek, Democritus, Suggested that matter is made of small particles that can not be broken down further. He called the particles atoms, after the greek word atomos, wich means "invisible." thus different elements were composed of different kinds of atoms, a revolutionary concept at the time. However, Democritus' ideas were never widely accepted because Socrates, a very influential figure at the time did not accept them. -
450
about 350 B.C.
Ther philosospher Aristotle delieved in Empedocles "four-element" model despite the more recent "atomic" model. Aristotle's influence was so great, and his writings, that the "four-element" model was accepted for almost 2000 years. -
500
A.D. 500-1600
Do metals grow like plants, ripening into gold? Many alchemists (combonation of philosopher, mystic, magician and chemist) belived that thay did. For centuries that preformed numerous experiments attempting to make gold from cheap metals such as elements and compounds. They also invented many labroatory tolls that we still use today: beakers, filters, stirring rods, and distillation apparatus. However, despite finding many new substances, they still accepted the four-element model. -
1650
An english scientist, Robert Boyle, did not belive in the four-element model. he devised a new definition for the word element: "I mean by element simple unmitigated bodies." this became thr modern definition of an element: a pure substance that cannot be chemically broken down into simpler substances. Boyle also belived that air was not an element, but rather a mixture. -
1700s
Joseph Proestly was the first person to isolate oxygen scientifically, but he did not know that oxygen is an element This fact was soon reconized by Antoine Lavoisier. Experimenting with Prieestly's oxygen, Lavoisier found that air must be a mixture of tow gases. one of wich was oxygen. Henery Cavendish experimented by mixing metal and acid, wich resulted in a flammable gas that was lighter than air. He did not know that he had created hydrogen gas, but found that his gas would burn in oxygen -
1800s
However Dalton's atomic model cannot explain why, on a dry winter day you get a spark when you toch a metal doorknob, matter is able to devlop negative and posative charges-quantities of electricity that can build up on an object. in 1831, Micheal Faraday found that a electric current could cause chemic changes in some soloutions and compounds. -
1808
By this time it was generally accepted that matter was made from elements: the town models had come together. English chemist John Dalton published a theory of why elements are different from each other and from non-elements -
1904
J.J. Thomson revised the atomic model further, to explain his discovery of very light negative particles, called electrons. He also did experiments with beams of much heavier -
1911
Ernest Ruthfor, working at McGill University in Montreal, designed an experiment to test Thomson's and Nagaoka's models. He aimed a type of radation called alpha partices smaller than most atoms at a thin sheet of gold foil. He perdicted, based on Thomson's raisin-bun model, that the particles would pass straight through the gold foil, as indeed most of the alpha particles bounced almost straight back from the gold foil.
