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340
Aristotle's Concept of Matter
Aristotle was born 384 BC in Stagira, Chalcidice, Greece. He died in 322 BC. Exact years of his work are not known. Around 340 BC, Aristotle proposed his concept of matter. He claimed that all matter is made up of one or a combination of elements. He claimed that there were only 5 elements: air, water, fire, earth, and aether (which only made up matter in space, such as stars). This idea was originally accepted, delaying scientists' understanding of atomic structure. -
460
Concept of the Atom
Democritus was born 460 BC in Abdera, Thrace, Greece. He died 370 BC, and worked with atomic structure throughout his life (exact years are unknown). Democritus was the first scientist to consider the existence of atoms; he said that matter is made up of atoms (in Greek, "atomon" or "atomos") moving through space. Democritus claimed that these atoms are small, solid, and indivisible. He also believed that all matter is made of the same kind of atoms. -
Law of Conservation of Mass
Antoine-Laurent Lavoisier was born in 1743 in Paris, France. He died in 1794, and worked with atomic structure from about 1775-1789. Lavoisier worked on determining the masses of reagents and products of chemical reactions. In his Law of Conservation of Mass (discovered 1789), he stated that chemical reactions cause neither the gain nor the loss of mass, in matter. This law is true, because naturally occurring elements on Earth are stable (so their atoms do not transfer to other elements). -
Law of Definite Proportions
Joseph-Louis Proust was born September 26, 1754 in Angers, France. He died July 5, 1826 after working with his Law of Definite Proportions from about 1793-1797. Proust proposed the law in 1793. It stated that every chemical compound has a specific (fixed) proportion of its components, in mass. Although this concept was criticized by some scientists (such as Claude-Louis Berthollet), it was eventually accepted. Dalton's atomic theory later helped to support the law. -
Spherical Model
In 1803, Dalton worked on a table of elements by atomic weight, and needed a way to convey his ideas. After various readings, (including one on the fact that Democritus called the balls he believed form water "atoms"), Dalton adopted the term "atom". He claimed that all matter consisted of hard, round "atoms". Dalton soon developed symbols to represent atoms and their chemical reactions. -
Atomic Theory
John Dalton was born September 6, 1766 in Eaglesfield, Cumberland, England. He died July 27, 1844. Dalton worked with atomic structure from 1802-1816. In 1808, Dalton proposed his Atomic Theory. He claimed that each element has a different type of atom (rather than the idea, beginning with Democritus, that all matter is alike). Dalton also worked on finding the masses of each type of atom, using the number of atoms in various chemical compounds. -
Cathode Ray Tube Experiment
In 1897, Thomson studied, electricity in high-vacuum cathode ray tubes (important equipment for his experiments). While experimenting, he observed the bend of cathode rays, off of plates and magnets with charge. To Thomson, this suggested that cathode rays contain particles smaller than atoms, "corpuscules", which he claimed could form atoms. -
Discovery of Electrons
Joseph John Thomson was born December 18, 1856 in Manchester England. He died August 30, 1940. Thomson studied atomic structure from 1884-1913. When observing electricity in his Cathode ray tube experiment, Thomson proposed that cathode rays contain particles smaller than atoms. These particles were negatively charged electrons. Thomson was one of the first scientists to recognize the existence of subatomic particles. -
Quantum Theory
Max Planck was born April 23, 1858 in Kiel, Germany. He died October 4, 1947 and studied energy and the atom from 1897-1906. Planck proposed his Quantum Theory in 1900, after studying thermodynamics. He claimed that energy is emitted in small amounts (quanta), and that the energy of each quantum is equivalent to the frequency of emission multiplied by a constant (Planck's constant). This theory helped future developments, such as Einstein's studies of light. -
Plum Pudding Model
In 1904, Thomson proposed his Plum Pudding model of the atom. He believed that the atom is made of positive matter, containing electrons (whose positions are affected by electrostatic force). Thomson's idea contradicted popular belief at the time; most people thought that the atom was indivisible. Thomson, however, claimed that it contained smaller particles (electrons). -
Period: to
Gold Foil Experiment
In Rutherford's gold foil experiment, he focused radiation emitting positive alpha particles towards gold foil. Most particles passed straight through, but some turned, and others bounced back. He concluded that some particles were repelled by the positive charge of the nucleus, while most passed through due to empty space in the atom (and a small nucleus). The experiment required gold foil, a protective lead shield, a screen with a zinc sulfide coating, a lead screen, and a source of radiation. -
Nuclear Model/ Planetary Model
Ernest Rutherford was born August 30, 1871 in Nelson New Zealand. He died October 19, 1937 and worked from 1896-1919. In 1911, Rutherford developed his Nuclear Model of the atom (also called the Planetary Model). He believed that the nucleus was small, dense (containing most of the atom's mass), and positive. Rutherford claimed that negative electrons move around the nucleus (like planets revolving around the Sun). -
Discovery of the Nucleus
The results of Rutherford's Gold Foil Experiment allowed him to discover the nucleus of the atom. In the experiment, some alpha particles sent towards gold foil bounced back, rather than passing through. Since the alpha particles were positive, Rutherford reasoned that, since like charges repel, these particles must change direction due to another, small, positively charged object. As a result, Rutherford concluded that the atom has a positive center: the nucleus. -
Bohr Model
Niels Bohr was born October 7, 1885 in Copenhagen, Denmark. He died on November 18, 1962. Bohr worked from 1911-1962. In 1913, Bohr developed the Bohr Model, which is similar to Rutherford's Nuclear Model. In both models, electrons move, or orbit, around the nucleus. In the Bohr Model, these electrons orbit in set layers; the number of electrons in the outer orbit affects the properties of the element. -
Modified Nuclear Model
The Modified Nuclear Model is also known as the Bohr Model. In 1913, Bohr altered Rutherford's Nuclear Model to consider Quantum Theory, and to account for the emission of light by hydrogen atoms. In this model, there are several layers of orbits for electrons. Bohr claimed that light is only emitted from the atom when an electron moves to an orbit layer closer to the nucleus (due to a loss in that electron's energy). -
Discovery of the Proton
During Rutherford's Gold Foil experiment (1910-1911), some alpha particles bounced off of gold foil, rather than passing through. Since these particles were positive, and like charges repel, Rutherford claimed that the nucleus was positively charged. Later, in 1920, Rutherford named the positively charged particles of the nucleus "protons". -
Theory of Quantum Mechanics
Werner Heisenberg was born December 5th, 1901 in Würzburg, Germany; he died February 1, 1976. Heisenberg worked with the atomic model from 1924-1976. Werner Heisenberg developed his Theory of Quantum Mechanics in 1925. He stated that scientists cannot know where an electron is, within an atom, at any point in time. Therefore, scientists cannot know that electrons follow an orbit (as Niels Bohr believed). -
Quantum Mechanical Model/ Electron Cloud Model
Erwin Schrödinger was born in 1887, in Austria. He died in 1926, after studying atomic structure from 1925-1956. In 1926, Schrödinger explained his Quantum Mechanical Model ( the Electron Cloud Model). It was similar to the Bohr Model, but instead of showing the exact orbit of an electron, it determined the probability of that electron being in a certain space. The model shows a nucleus surrounded by an electron cloud, which is denser where there is a greater probability of finding an electron. -
Uncertainty Principle
In 1927, Heisenberg developed the Uncertainty Principle. This stated that there are always errors in measuring the position and momentum of a moving subatomic particle, because the method used (bouncing photons off of the particle) affects its behavior. The theory stated that when the accuracy of measured position increases, the measured momentum is less accurate (and vice versa). -
Discovery of Neutrons
James Chadwick was born October 20, 1891 in Manchester, England. He died July 24, 1974, and worked with atomic structure from 1922-1946. In 1932, Chadwick discovered the neutron. At the time, scientists realized that the number of protons in an atom is different than its mass; since electrons have very little mass, Chadwick inferred that atoms have a neutral particle with mass: the neutron. He shot these neutrons at various elements to study their collisions and, later, to determine their mass.
