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Jan 19, 1500
Atom is Discovered - 450 B.C.
Democritus Atomic ModelDemocritus was an Ancient Greek philosopher born in Abdera, Thrace, Greece. He was an influential philosopher who formulated what many consider to be the first atomic theory. His hypothesis on atoms is similar to modern science's understanding of atomic structure, and steered clear of many of the errors of people later on in history. Many people today consider Democritus to be the "Father of the Atomic Theory". -
Conservation of Matter
Antoine Lavoisier was a French Nobleman whose most noteable addition to Chemistry was the theory of "Conservation of Mass." He proved this by measurign the weight of a log, burning it, and then weighing the burnt remains of the log. The weights were equivilant showing that shape or form can change, but the mass will remain constant. -
Law of Constant Composition
Water Molecule ProportionsJoseph Proust developed the Law of Constant Composition. This law states that a compound is composed of exact proportions of elements by mass regardless of how the compound was created. For example, to make water there is a specific ratio of grams of hydrogen to grams of oxygen regardless of where your sample of water was found. All water has this particular proportion. The ratio is two hydrogen atoms for ever oxygen atom. The ratio is definite or fixed. -
First useful Atomic Theory
John Dalton developed the first useful atomic theory of matter. He developed the hypothesis that the sizes of the particles making up different gases must be different. The postulates are as follows:
1.All matter consists of tiny particles.2.Atoms are indestructible and unchangeable.3.Elements are characterized by the mass of there atoms.4.When Elements react, their atoms combine in simple, whole-number ratios. -
J.J. Thomson creates the Plum Pudding Model
Plum Pudding ModelCathode Ray TubeScientists believed atoms were indivisible, the ultimate particles of matter, but Thomson proved them wrong when he discovered that atoms contained particles known as electrons. Thomson discovered this through his explorations on the properties of cathode rays. Thomson found that the rays could be deflected by an electric field. -
Electrons have an electric charge.
Oil Drop ExperimentAs a result of Thomson's experiments, Milikan was able to conduct his own experiment. The measurement of the electron's charge independently, was achieved by his famous experiment from 1909 and with Thomson's results also a value for the electron mass was obtained. This experiment is called the "oil-drop experiment" and it was the first successful scientific attempt to detect and measure the effect of an individual subatomic particle. -
Electrons surround the Nucleus.
Detailed Description of Gold-Foil Experiment with Interactive ModelErnest Rutherford is responsible for discovering that electrons surround something called a Nucleus. He preformed the Gold-Foil experiment in which he sent Hellium through a thin strip of gold foil. When the hellium would go the whole way through it would hit a strip of Zinc-sulfide and emit a flash of light. When it didn't go, it bounced back to the lead screen. This was due to the positive particles in the gold and hellium pushing away from each other, showing that there was more to atoms. -
The Planetary Model
More InformationPanetary ModelNiels Bohr suggested that electrons could only have certain classical motions:1. The electrons can only travel in special orbits: at a certain discrete set of distances from the nucleus with specific energies.2. The electrons of an atom revolve around the nucleus in orbits. These orbits are associated with definite energy levels. They can only gain and lose energy as they travel in a particular orbit. They can only gain and lose energy by jumping orbits by a change in electromagnetic energy.
