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200
400 B.C.
Democritus, a Greek philosopher, suggested that matter was really minute particles and could not be broken down further. He entitled these particles atoms, derived from the Greek word "atomos", meaning "indivisible". This discovery proved that elements were composed of different types of atoms, an idea unheard of at the time. -
300
About 350 B.C.
Yet another Greek philosopher, Aristotle, favored Empedocles "element" model over Democritus' "atom" model. Due to the fact that Aristotle was so influential, the "element" model was accepted for almost 2000 years. -
Period: 500 to
A.D. 500-1600
Alchemists, a quadruple threat consisting of philosopher, magician, mystic and chemist, believed that metals behaved exactly like plants. Centuries past, and during that timespan they performed numerous experiments attempting to create gold from other metals. They came up with chemical symbols for substances now recognized as elements and compounds. They also invented labratory tools still used to this day. -
1650
Robert Boyle, an English scientist, denied the "four-element" model and devised a new definition for the word "element". His definition later became the modern definiton still used today. He also believed that air, formerly recognized as an element, was actually a mixture. -
Late 1700's
Joseph Priestley became the first person to isolate oxygen scientifically, without knowing that oxygen was an element. Antoine Lavoisier, the father of modern chemistry, discovered that fact while experimenting with Joseph's oxygen. He concluded that air must be a mixture containing at least two gases, one being oxygen, just like Robert Boyle. Until this time, scholars had still believed that water was an element. -
Period: to
!800's
Dalton forgot to mention that matter is able to develop positive and negative charges. A new model was then devoloped, introducing tiny negatively charged partickes that could be sperated from thier atoms and moved to other ones. -
1808
By this time the fact that matter was indeed composed of elements was generally accepted, meaning that both the "four-element" model and the "atomic" model were correct. An English chemist by the name of John Dalton published a theory explaing why elements differ from themselves and non-elements. His atomic model stated four main points: All matter is made up of atoms, each element has its own kind of atom and mass, compounds are created when atoms from seperate elements link to form molecules.. -
1904
J.J. Thomson managed to revise the atomic model even further, explaining his discovery of very light negative particles, called electrons. This new model was nicknamed the "raisin-bun model". Some main concepts he stated are: atoms contain particles called electrons, electrons have a small mass and a negative charge, the rest of the atom is a sphere of positive charge and the electrons are embedded in this sphere, so that the resulting atoms are neutral and uncharged. -
H. Nagaoka
H. Nagoaka, a Japanese scientist, modelled the atom as a large sphere with a positive charge envelloped by a ring of negatively-charged electrons. -
1911
Ernest Rutherford created an experiment to test both Thomson's and Nagoaka's models. He aimed a kind of radiation named alpha particles at a thin sheet of gold foil. He hypothesized, based on Thomson's model, that the particles would travel through the foil. Most of them did, however, a small amount of the alpha particles bounced off of the foil. Rutherford then came up with another model entitled the "nuclear model". -
About 450 B.C.
Empedocles, a Greek scholar, came up with an idea stating that matter was made up of four different elements. He called these elements earth, air, water and fire and when mixed together, depending on the porportions, could make a multitude of substances. What made Empedocles stand out among his colleagues was that he actually tested his theories experimentally.
