Neils Bohr and Atomic Theory
In ancient Greek the word atom meant the smallest indivisible particle that could be conceived. The atom was thought of as indestructible; in fact, the Greek word for atom means "not divisible." Knowledge about the size and make up of the atom grew very slowly as scientific theory progressed. What we know/theorize about the atom now began with a core theory devised by Democrotus, a Greek philosopher who proposed that matter consisted of various types of tiny discrete particles and that the properties of matter were determined by the properties of these particles. This core theory was then modified and altered over years by Dalton, Thompson, Rutherford, Bhor, and Chadwick. The atoms original structure was simple, but as more and more research was done the atom became more complex and puzzling The five atomic theories of the past two centuries represent the sudden advancement of science in modern times. Beginning with a basic theory on the behavior of atoms to the current model, some changes have been made, and some ideas are still the same. Ancient Greek philosophers believed that everything was made up of invisible particles called atoms.
These experiments included the emission of radiation from hot, glowing objects (called thermal radiation) and the release of electrons from metal surfaces illuminated with ultraviolet light (the Photoelectric Effect). All atoms of the same element are exactly alike; in particular, they have the same mass. The laws of classical physics work perfectly well on the scale of everyday objects, but on the tiny atomic scale, the laws of quantum mechanics apply. If an atom had four electrons you wouldn't find two in the first, one in the second, and one in the sixth. Some of Dalton's points are still thought to be true, but over time this original theory has been modified. Next, if the center of an atom was composed of protons (+) and the electrons (-) orbited this positive core why didn't the electrons crash into the protons, causing an ultra violet catastrophe. At the same time that Bohr and Rutherford were developing the nuclear model of the atom, other experiments indicated similar failures of classical physics. They called this new approach quantum mechanics (Quantum Theory), and they developed a mathematical basis for it in the 1920s. Atoms of different elements are different; in particular, they have different masses. Also, if the core was composed of just positive protons and opposite charges repel then how did the protons stay together. Using Rutherford's model of the atom as a miniature solar system, Bohr developed a theory by which he could predict the same wavelengths scientists had measured radiating from atoms with a single electron. Compounds are formed by the joining of atoms of two or more elements.
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