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460
Atom
Democritus (460 BC-370 BC) formulated the atomic theory or developed the idea of the atom. Using logical reasoning, he concluded that you can not continue dividing something in half forever and will eventually reach a smallest indivisible part. He called this aotmos, the smallest part of matter. -
Law of Conservation of Mass
Antoine Lavoisier stated that the total mass of reactants (starting materials) and the total mass of products (materials produced by the reaction) are equal. In other words, mass is not created or destroyed during a chemical reaction so the mass at the beginning equals the mass at the end. -
Spherical Model
John Dalton (1766-1844) created the spherical model which shows that the atom is shaped like a sphere. Becasue of it's resemblance to a billard ball, he was often referred to as the "Billard Ball Model". Dalto stated along with this model that atoms were tiy invisibly, indestructable particles that each had their own mass, size, and chemical behavior that determined which element it is. Also, this atom did not have any subatomic particles or smaller parts to it. -
Atomic Theory
Joh Dalton 9 1766-1844) created an atomic theory which is still valid today but is more involved. He stated that all matter is made of atoms which are indivisble and indestructable. All atoms of a given element are identical in mass and properties. Compiunds are formed by a combination of two or more different kinds of atoms. Also, a chemical reaction is a rearrangement of atoms. -
Law of Definite Proportions
Joseph Proust (1754-18326) propsed that in any sample of a compound, the masses of the elements are in the same ratio. People argued that the elements could combine in any proportion, but they were wrong. -
Cathode Ray Tube Experiment
When J. J. Thomson conducted the cathode ray tube expperiment, it showed that the chathode ryas were deflected toward a positively charged plate. This meant that the rays must be negative and that they were deflected by magnetic fields. -
Discovery of the Electron
Thomson discovered and identified the electron in 1897 and showed that the the cathode rays were composed of a previously uknown negative charged particle. He suggested that the unit was 1000x smaller than an atom and this particle was one of the building blocks of atoms. Also, the electron was originally called the "corpuscles". -
Planck's equation - Quantum Theory
Max Planck (1858-1947) discovered an equation that explained the results from heating black bodies, E=Nhf. Where E=energy, N=integer, h=constant, and f=frequency. The constant (h) became known as Planck's constant and all contributed to quantum theory. -
Plum Pudding Model
J. J. Thomson 91856-1940) suggested that the atom had an equal number of positive and negative charge, making it neutral. Unlike the nucleus, Thomson claimed that the postitive charge was spread out throughout the atom and the the electrons were scatted in between. -
Gold Foil Experiment
Ernest Rutherford (1871-1937) conducted the gold foil experiment by shooting alpha particles through thin gold foil. He expected most of the alpha particles to be deflected except the majority of them went through except for a few. These results led to the discovery of the nucleus. -
Discovery of the Nucleaus
After conducting the Gold Foil Experiment, Rutherford concluded that the mass of the atom is concentrated in small, dense, positively charged nucleus. This nucleus is positive, is 1/10,000 the size of an atom, and had a significant enough mass to deflect because it was traveling at such a high velocity. -
Nuclear Model
Ernest Rutherford (1871-1937) created a model of the atom which is very simlar to the one used today. It suggested that there is a high central charge in a small volume located in the center of the atom and that that central volume contains the majority of the atomic mass. Although it is very similar to the modern model, the electrons here orbit the nucleaus in a specific path. -
Modified Nuclear Model
The modified nuclear model is based off of Rutherford's nuclear model of the atom except for a few modifications. These include an electron cloud, which is a region surrounding the nucleus where the electrons are found. -
Bohr Model
Niels Bohr (1885-1962) created the Bohr model or "planetary model" in 1913 which is commonly see as the atomic energy symbol. It shows that the protons and neutrons are in the ucleus while the elctrons orbit them. It is commonly referred to as the "planetary model" because the nucleus and orbiting electrons look like the solar system. -
Discovery of Proton
After coducting the gold foil experiment, Rutherford proposed the name "proton" for the positively charged particle in the nucleus. -
Uncertainty Principle
Werner Heisenburg (1901-1976) stated that it was impossible to simultaneously determine the position and momentum of a particle and get accurate results. -
Quantum Theory
Niels Bohr largely contributed to Quantum Theory which describes the universe on very small scales, such as the nulceaus. It states that a patricle of matter such as an elctron is "smeared out" into a wave over a region of space and they're forced into a pattern that minimizes their energy. Each unique "place" in the pattern is called a quantum state and can hold one single electron, which are always found to be in one quantum state or another and no 2 electrons can be in the same one at once. -
Quantum Mechanical Model
Erwin Schrodinger (1887-1961) in 1926 created the Quantum Mecahnical Model which is also know as the electron cloud model. It showed the probability that an electron can be found in a give region of space at a given time. This shows a relative location of an electron, not an exact. This model allowed an electron to occupy three-dimesional space, which required three coordinates or three quantum numbers. Also it describedthe isze, shape, and orientation in space of the orbitals of an atom. -
Dirac Equation
Paul Dirac (1902-1984) created the relative equation for the electron or a relavistic wave equation. It implied the existence of antimatter and reinterpreted to describe quantum fields corresponding to spin 1/2 particles. This led to the discovery of the positron. -
Discovery of the Neutron
Rutherford proposed that the nucleus contained a neutral particle because the atomic number was less than the atomic mass. He had no evidence to prove this. James Chadwick repeated experiments of Frederic and Irene Joliot-Curie, except looked for a neutral particle. He proved that the neutron did exist and that it's mass was about 0.1% more than the proton's.