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327
327 BC-Alexander the Great's empire reaches India
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460
Democritus is born in 460 BC (exact date not known)
Within his lifetime, no one knows exactly when, Democritus suggested the existence of atoms. Even though he did not have experimental support, Democritus was the first to sugest their existence. He believed atoms were indivisible and indestructible. -
Jan 1, 730
Printing Invented in China 730 BC
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Oct 11, 1492
Christopher Columbus reaches Caribbean Islands
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Declaration of Independence adopted by Second Continental Congress
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Treaty of Paris ends Revolutionary War
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Joseph Proust gives evidence for law of constant composition
The law of constant composition said that: there are fixed and constant proportions (by weirght) of elements in every chemical compound. It was a radical idea at the time, but it defined the fact that elements can chemically combine to make compounds, but there are rules that apply. -
John Dalton turns Democritus's ideas into a scientific theory
By putting together "Dalton's Atomic Theory", John Dalton opened the doors to a new perception of atomic science even though he wasn't completely correct when his hypothesis stated that atoms have no internal structure. -
Dalton's Atomic Theory
1.) All elements are composed of tiny, indivisible particles called atoms.
2.) Elements' atoms are identical but each elements' atoms are unique to other elements' atoms.
3.) Atoms of different elements can mix to make mixtures or chemicall combine in whole-number ratios to make compounds.
4.) Atoms can separate, join, or be rearranged. When this occurs, it is called a chemical reaction. -
Henri Becquerel
From his experiments with uranium, Becquerel discovered radioactivity because of uranium's radiation without an outside source (the sun). This paved the way for a new understading of the atomic model because he discovered that there was energy that comes out of an element, meaning something had to give off that energy. -
JJ Thomson announces discovery of electron at Royal Institution
JJ Thomson's discovery of electrons was an important event since it proved that atoms could be divided into smaller means--something not known to the public before. He made this discovery though his study of cathode rays: a process of passing electric currents through gasses at low pressure.His atomic model (plum pudding model) where electrons are stuck in a lump of positive charge like raisins stuck in dough, was later replaced by his student's, Ernest Rutherford's, model. -
Max Planck devises quantum theory
Planck came up with the quantum theory that said: energy does not flow in a steady continuum, but comes out in quanta. His interest in radiation from hot materials and why these hot materials could glow in more than one color changed physics and allowed future scientists to build onto his knowledge. -
Ernest Rutherford conducts Gold-Foil Experiment
From the gold-foil experiment, Rutherford was able to conclude that an atom is composed mostly of empty space. Most of the mass and all of the positive charge of an atom are confined to a small region of the atom--the nucleus--and the elctrons move around the nucleus. This suggested a new angle of looking at the atomic theory since it was previously believed that the positive charge was spread out in atoms. -
Niels Bohr changes Rutherford's atomic model
Rutherford's model, though revolutional, only explained a few simple properties of atoms and could not explain the chemical properties of elements. His student, Bohr, changed Rutherford's model by proposing that an electron is found only in specific circular orbits at certain distances from the nucleus. He went further by saying that electrons can gain or lose energy and their quantum of energy brings them to a higher or lower energy level. Quantum energy levels are whole numbers (ie. n=1) -
Robert A. Millikan calculates mass and charge of an electron
Millikan carried out experiments to find out more about the sublevels of an atom--something that was not quite an area of expertise in the science field. He discovered that the charge of an electron is -1 units and its mass is 1/1840 of a hydrogen atom. -
Period: to
World War I
Germany signs armistice agreement with the Allies outside Complegne, France. -
Louis de Broglie proposes electrons move like waves
While studying the new quantum mechanical model, Broglie considers the new fact that people were talking about--ligth waves could behave like particles. He proposed that the opposite could be true too; moving particles, like electrons, could have properties like waves. This theory was to be later incorporated into mathematical equations by Schrodinger. -
Werner Heisenberg publishes theory of quantum mechanics
Werner Heisenberg used math in attempt to explain electron orbits. He said electrons have quantum behaviors like discontinuities and quantum jumps (matrix mechanics). He lacked a physical model which didn't bring him much popularity but his model states the same thing as Erwin Schrödinger's physical wave model does. -
Erwin Schrodinger describes motion of electrons with mathematical equations
Since Bohr made his model based on hydrogen, it failed to explain the movement of energy in atoms that contain more than one electron. Schrodinger's mathematical (wave) equation formed a basis for the quantum mechanical model which describes the behavior of electrons in atoms. He said that there is a cloud of electrons around the nucleus and wherever the cloud is more dense the probability of finding an electron is high; this leads to the formation of energy levels, each with a different shape. -
Werner Heisenberg publishes Uncertainty Principle
Heisenberg states that by nature, the more precisely you determine the position of a particle, the less precisely you will know its magnitude (uncertainty principle). -
James Chadwick discovers neutron
English Physicist James Chadwick confirmed the existence of neutrons, establishing that they are subatomic particles with no charge but has a mass that is 0.1 percent larger than a proton's. This was revolutionary because people were starting to think that protons weren't the only thing inside of an atom's nucleus and Rutherford came so far as making a hypothesis that there were such particles called neutrons; however, Chadwick was the one who confirmed the hypothesis. -
Period: to
World War II
Note that the exact dates of start and finish of the war may vary from historian to historian. -
Birth Control Pill Made Available for Women
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Period: to
Collapse of Communist Regime marks end of communist era in Europe
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First use of modern paper: 105 AD
Modern paper replaced papyrus, slate, stone, and vellum which made writing more convenient and cheaper. -
Aristotle Lived (384-322BC)
Within his lifetime, Aristotle contributed to the Atomic Theory, laying a base along with Democritus that later scientists would use to work off of around 2000 years later. Aristotle proposed that no matter how many times someone cut a form of matter in half, they could always make it smaller, which would later lead to discoveries of atoms and subatomic particles.
