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J.J. Thomson Webquest By: Ritesh Modi Democritus(460 BC-371 BC ) Co-originator of the belief that all matter is made up of various imperishable indivisible elements which he called “atoms." Atoms were completely solid, homogeneous, indestructible objects and imperceptible to the senses, composed of exactly the same matter but different in size, shape, and weight. Democritus postulated the constant motion of atoms and, on this basis, explained the creation of worlds. The whirling motion caused by the falling of atoms resulted in aggregations–the heavier atoms forming the earth and the lighter ones the heavenly bodies. Antoine Laurent Lavoisier(1783) The father of modern chemistry He stated the first version of the law of conservation of matter, which states that mass is neither created nor destroyed in any ordinary chemical reaction Recognized and named oxygen Helped to reform chemical nomenclature Lavoisier believed that weight was conserved through the course of chemical reactions — even those involving gases. He explained combustion (and respiration) in terms of chemical reactions that involve a component of air which he called oxygen. He explained combustion and thereby discredited the phlogiston theory Joseph Louis Proust(1799) The Law of Constant Composition established by Joseph Louis Proust States that in a pure compound, the elements are always present in the same definite proportion by mass Ratio of H:0 is always 2:16 or 1:8 2Na + Cl2 -----> 2NaCl 2.3 g + 3.55 g -----> 5.85 g 4.6 g + 7.1 g -----> 11.7 g John Dalton(1803) Formalized the discovery of Lavoisier into the "Law of Definite Proportions (when atoms combine to form a particular compound, they always combine in the same ratios by weight) and Proust’s Law of Constant Compostion (States that in a pure compound, the elements are always present in the same definite proportion by mass).” Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. All matter was composed of small indivisible particles termed atoms Atoms of a given element possess unique characteristics and weight Three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules). The work identified chemical elements as a specific type of atom, therefore rejecting Newton's theory of chemical affinities. Dalton inferred proportions of elements in compounds by taking ratios of the weights of reactants, setting the atomic weight of hydrogen to be identically one. He proposed that chemical elements combine in integral ratios. Despite the importance of the work as the first view of atoms as physically real entities and introduction of a system of chemical symbols, New System of Chemical Philosophy devoted almost as much space to the Caloric Theory as to Atomism. The Law of Dalton's Multiple Proportions One of the strongest arguments for Dalton's atomic theory of chemistry was the Law of Multiple Proportions. He found that when carbon combined with oxygen to form a gas, there were two possible outcomes, depending on the conditions - and in one outcome each gram of carbon combined with precisely twice as much oxygen as in the other. He correctly interpreted this as the formation of CO2 and CO respectively. Dmitri Mendeleev(1869) Mendeleev discovered that by sorting the elements ascending of their atomic weight, their chemical properties show characteristic periodicity and this later become known as the Periodic Law. Later this table was named the periodic table of elements. He Arranged elements into 7 groups with similar properties. Mendeleev determined the holes,. the spaces for the missing elements. For example, germanium was discovered upon his guidance. Moreover, he had foretell all the important chemical properties of this element. J.J. Thomson(1897) Subjected cathode rays to magnetic and electric fields and showed that the beam was deflected as would be expected for negatively charged particles. He calculated the ratio of the electron's charge to its mass. Thomson announced that the cathode rays consisted of negatively charged particles, which represented fundamental particles of matter. Discovered the electron. Thomson’s Plum Pudding Model In 1898 Thomson proposed that the atom was a sphere of positive electricity containing most of the mass Small negative electrons were embedded in the surface. He calculated the charge to mass ratio e/m for the electron by careful observations of the curvature of an electron beam in cathode ray tubes in a magnetic field. Ernest Rutherford(1898) Published 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. 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'.