Democritus (Greek: Δημόκριτος, Dēmókritos, chosen of the people) (c. 460 – c. 370 BCE) was an Ancient Greek philosopher born in Abdera, Thrace, Greece.[1] A pupil ofLeucippus, he was an influential pre-Socratic philosopher who formulated an atomic theoryfor the universe.[2] His exact contributions are difficult to disentangle from those of his mentor Leucippus, as they are often mentioned together in texts. Their speculation on atoms, taken from Leucippus, bears a passing and partial resemblance to the nineteenth-century understanding of atomic structure that has led some to regard Democritus as more of a scientist than other Greek philosophers; however, their ideas rested on very different bases.[3] Largely ignored in ancient Athens, Democritus was nevertheless well known to his fellow northern-born philosopher Aristotle. Plato is said to have disliked him so much that he wished all his books burned.[4] Many consider Democritus to be the father of modern science.[5] Atomic theory In 1800, Dalton became a secretary of the Manchester Literary and Philosophical Society, and in the following year he orally presented an important series of papers, entitled Experimental Essays on the constitution of mixed gases; on the pressure of steam and other vapours at different temperatures, both in a vacuum and in air; on evaporation; and on the thermal expansion of gases. These four essays were published in the Memoirs of the Lit & Phil in 1802 Ernest Rutherford publishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. This experiment involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide (a scintillator). Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of empty space. In 1897 the British physicist Joseph John (J. J.) Thomson (1856–1940) discovered the electronin a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by numerous scientists at the time. Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of bodies much smaller than atoms that he calculated as having a very large value for the charge-to-mass ratio. Later he estimated the value of the charge itself. Niels Bohr was one of the foremost scientists of modern physics, best known for his substantial contributions to quantum theory and his Nobel Prize-winning research on the structure of atoms.Born in Copenhagen in 1885 to well-educated parents, Bohr became interested in physics at a young age. He studied the subject throughout his undergraduate and graduate years and earned a doctorate in physics in 1911 from Copenhagen University.It was while conducting research for his doctoral thesis on the electron theory of metals that Bohr first came across Max Plancks early quantum theory, which described energy as tiny particles, or quanta. A Quantum Theory for Atomic Structure The first model of the atom was developed by J.J. Thomson and Lord Kelvin in 1904. They called it the plum pudding model because the negative electrons (the plums) were embedded in a sphere of uniform positive charge (the pudding). This model was soon replaced. In 1911, Ernest Rutherford suggested that his students Geiger and Marsden fire alpha particles (produced by some naturally radioactive materials) at a thin gold foil. They observed that some of the alpha particles were scattered through large angles, indicating that the positive charge in the atom was concentrated in a small volume – the nucleus. Rutherford subsequently proposed a model of atom based on a central nucleus of positive charge circled by negative electrons – like planets orbiting the sun. Rutherford was awarded the Nobel Prize in Chemistry in 1908 for his work on understanding radioactive decay.
Posted on: Thu, 28 Nov 2013 14:31:45 +0000
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