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The First Thery Of an Atom!
Democritus of Adbera (northern Greece) asserted that all material things are composed of extremely small irreducible particles called atoms. “Nothing exists except atoms and empty space. Everything else is opinion”. The atomic theory was roundly rejected by Aristotle, and, thus, by almost everybody else for the next two millennia. -
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Boye puts Math to Chemestry!
Robert Boyle (England) extended mathematics to chemistry and revived atomic theory. -
Conservation of Matter!
Antoine Lavoisier (France) demonstrated the conservation of matter (matter can be neither created nor destroyed) in a chemical reaction and defined the difference between an element and a compound. -
The Coulomb Formulia!
Charles Coulomb (France) described the force between two electric charges with a mathematical formula which looked very much like Newton’s law of gravity:
where F is the force q1 and q2 are two charges and r is the distance between them. The electrical force is the chief force involved in atomic reactions. This force is attractive when charges q1 and q2 have opposite signs and repulsive when the charges have the same sign. -
The modern version of the atomic theory
John Dalton (England) formulated the modern version of the atomic theory. In his model all atoms in a given chemical element are exactly alike, while the atoms of different elements differ by atomic weight -
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James Clark Maxwell Predicts...
James Clark Maxwell (England) showed that electricity and magnetism are two aspects of the same phenomena, and predicted that accelerating charges radiate waves traveling at the speed of light. These waves are known generically as electromagnetic waves of which visible light is one example. -
electron Discovery!
J.J. Thompson (England) discovered the electron, the component of the atom with negative charge. His model of the atom had the negatively charged electron evenly distributed throughout a sphere of positively charged material. This is known as the “plum pudding” model of the atom. -
The Quantum Theory...
Max Planck (Germany) introduced the quantum theory to explain the shape of the temperature versus color curve of a glowing solid. Briefly, he found that light cannot be converted into heat (energy) by any arbitrary amount, but only as discrete packets which he called quanta (known as photons today). For light of wavelength λ, the energy per quanta is given by: where h is a constant which we now call planck’s constant. -
E=Mc^2 !
Albert Einstein (Germany, USA) published papers on special relativity which included the famous equation relating energy E to mass m:
Here, c is the speed of light. Thus, the mass of any particle has an equivalent energy and a photon, viewed by Planck as a packet of pure energy, has an equivalent mass. -
Name Them Protons!
Ernest Rutherford (England) demonstrated that the atom is mostly empty space with a small positively charged nucleus containing most of the mass and low mass negatively charged particles (Thompson’s electrons) orbiting this nucleus. Rutherford could experimentally identify nuclear particles with positive charge that he called protons. Although he could explain the charge of atomic nuclei with the right number of protons, the mass of the nucleus for large atoms was always larger than the sum o -
This is such a Bohr!
Neils Bohr (Denmark) developed the first successful model of the atom. Since we still use Bohr’s model to explain many aspects of physical phenomena such as the appearance of spectra, it is worthwhile to spend some time describing it. -
Principle of Complementarity
Louis de Broglie (France) hypothesized that the electrons in Bohr’s model were confined to discrete orbits because they had the properties of standing waves. He proposed that any particle with a momentum p (p = mv)
. Here m is the mass of the particle and v is its velocity. Calculations based on the assumption that matter at the atomic level can be viewed as waves agreed so well with experiments that it became a cornerstone of quantum mechanics.Principle of Complementarity -
A Girl and Her Atom!
Cecilia Payne (England, USA), using the new model of the atom, showed that the sun and stars are composed almost entirely of hydrogen and helium, with only trace amounts of more familiar, heavier, elements. She came to this conclusion by studying the spectral data which had been accumulated at Harvard Observatory over the past quarter century. As was pointed out in the discussion of the Bohr atom, both hot stars (O and B) and cool stars (K and M) do not exhibit strong hydrogen lines for opposi -
Nuclear Phycics!
Sir Arthur Eddington (England) produced the first model of stellar structure based on nuclear physics. The energy source of the sun and stars had been a mystery for centuries. It is easy to determine the total energy output of the sun. Basically, you measure how much heat is transferred to a square meter of water (or other calibrated material) in a given amount of time, then multiply this number times the surface area of a sphere centered on the sun with a radius of 1 A.U.
