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He decided that, fundamentally, everything must be made of the same thing – much as today we believe that all matter is made of atoms. His idea was that in its most fundamental form, all matter is water. It took about 200 years for Thales' idea to be transformed by his compatriot Democritus into “all matter is atoms.”
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Greek philosopher Democritus first suggested that all substances consisted of tiny indestructible particles called atoms
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In Aristotle's time, atomists held that matter was fundamentally constructed out of atoms. These atoms were indivisible and uniform, of various sizes and shapes, and capable only of change in respect of position and motion, but not intrinsic qualities.
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Isaac Newton contributed to atomic theory through his systematic development of calculus-based object modeling concepts e.g. displacement, velocity, acceleration and energy, as well as gravity, uniform circular motion and angular momentum
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Dalton's atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass.
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Michael Faraday, a British physicist, made one of the most significant discoveries that led to the idea that atoms had an electrical component. Faraday placed two opposite electrodes in a solution of water containing a dissolved compound.
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Dmitri Mendeleev devised the periodic classification of the chemical elements, in which the elements were arranged in order of increasing atomic weight.
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It was the Irishman George Johnstone Stoney who in 1874 proposed that there were elemental units of electricity in the atom, for which he invented the word “electron” in 1891.
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Eugene Goldstein discovered positive particles by using a tube filled with hydrogen gas (this tube was similar to Thomson's tube). This resulted in The positive particle had a charge equal and opposite to the electron. The positive particle was named the proton.
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J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."
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Curie conducted her own experiments on uranium rays and discovered that they remained constant, no matter the condition or form of the uranium. The rays, she theorised, came from the element's atomic structure. This revolutionary idea created the field of atomic physics
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Planck made many contributions to theoretical physics, but his fame rests primarily on his role as originator of the quantum theory. This theory revolutionized our understanding of atomic and subatomic processes, just as Albert Einstein's theory of relativity revolutionized our understanding of space and time.
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Abegg is known best for his research recognizing the role that valence had with respect to chemical interactions. He found that some elements were less likely to combine into molecules, and from this concluded that the more stable elements had what are now called full electron shells.
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Ernest Rutherford is known for his pioneering studies of radioactivity and the atom. He discovered that there are two types of radiation, alpha and beta particles, coming from uranium. He found that the atom consists mostly of empty space, with its mass concentrated in a central positively charged nucleus.
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Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values. ... Bohr's model explained why atoms only emit light of fixed wavelengths, and later incorporated the theories on light quanta.