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- Dalton presented his atomic theory, proposing: elements are composed of atoms of fixed masses; atoms in different elements are of different size, mass and chemical properties, while atoms in the same element are identical; atoms can't be created, destroyed or changed during a chemical reaction, taking into account the law of conservation of mass; and compounds are formed when more than one element is combined in a specific ratio, taking the law of definite proportions into account
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- Dalton also proposed the 'law of multiple proportion' which states that "whenever two elements form more than one compound, the different masses of one element that combines with a fixed mass of the other element is always in a whole number ratio” While Dalton's atomic theory has shaped our understanding of properties of matter, he was unable to put forward ideas about the structure of atoms.
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Dobereiner proposed the 'law of triads' which implied a relationship between the properties and atomic weights of elements. He also discovered the middle of 3 elements in a triad had a relative atomic mass around the average of the relative atomic masses of the other 2 elements, which was important for the arrangement of elements in the periodic table. However, Dobereiner couldn't prove his hypothesis with enough triads, so his findings were regarded as 'interesting curiosities' in his time.
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Newlands discovered that every eighth element in the grouping by atomic weight shared a resemblance and suggested an analogy with the intervals of the musical scale.
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Mendeleev was awarded the Chair of General Chemistry at the University of Saint Petersburg at age 33
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Mendeleev’s periodic table contained 17 columns, with two nearly complete periods of elements, preceded by two partial periods of seven elements each, and followed by three incomplete periods
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His newly formulated law was announced before the Russian Chemical Society in March 1869 with the statement “elements arranged according to the value of their atomic weights present a clear periodicity of properties.”
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Published paper with obeservation that cathode rays were not a form of radiation but a stream of negatively charged particles with mass, which soon came to be known as electrons
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First quantitative measurements of cathode ray particles (electrons) produced in 1897 by Thomson using modified CRT
Measurements from this experiment enables him to determine the charge to mass ratio for the electrons -
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Rutherford proposed an improved model for the atom, that consisted of empty space occupied by electrons, negatively charged particles
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Mosely discovered that each element emits X-rays at a unique frequency, which confirmed physicist Antonius van den Broek hypothesis he published 2 years earlier; that atomic number might actually be equal to the amount of charge in the atom’s nucleus
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Mosely also discovered x-ray spectroscopy: a new non-destructive method to find out which elements are present in any sample. It worked by bombarding the sample with high-energy electrons and looking at the frequencies of the resulting X-rays, which are like a fingerprint for any elements present in the sample.
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Mosely rearranged the elements on the periodic table by their number of protons rather than their atomic weights. He saw gaps in his new periodic table and predicted the existence of four new elements, with 43, 61, 72, and 75 protons, which were discovered by other scientists years later
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Bohr's model proposed that electrons moved about the central nuclear in circular orbits, first proposed by Rutherford, but that only certain orbit radii are allowed. The electron in each orbit had a specific amount of energy with the lowest orbital radii corresponding to the lowest possible energy. Mosely hypothesised light photon would be emitted from an excited atom when one of its electrons fell from higher energy orbit to lower energy one. However, he gave no reason for atomic stability
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Experimental data by Geiger and Marsden on alpha particle deflection showed that the number of protons in the nucleus equalled half the atom’s relative mass, meaning there must be some other neutral/chargeless particle present in the nucleus
In 1932, Chadwick identified neutrons as the chargeless particle. -
James Chadwick was awarded the Hughes Medal if the Royal Society 1932
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James Chadwick was awarded the Nobel Prize for Physics in1935
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