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John Dalton was an early English chemist who first developed an atomic theory. He postulated that elements were composed of units called atoms. He thought that atoms could not be subdivided, created, or destroyed and that atoms of the same element were all of the same size and mass. Finally, he felt that chemical reactions were the combining, separating, or rearranging of chemical atoms.
JJ Thomson was a British physicist who came along a few decades later. He first discovered the existence of electrons by using cathode rays. He discovered that there was a mass that was much smaller than a hydrogen atom and was negatively charged. He also discovered the existence of isotopes where different atoms of the same element can have different atomic masses. He developed what is called the "plum pudding" theory of the atom which includes a distribution of electrons (plums) dispersed throughout a general cloud of positive charge (pudding). This clearly disproves the part of Dalton's theory that claims that atoms cannot be subdivided and that all atoms of the same element have the same mass.
Ernest Rutherford was a British chemist and physicist at about this same time. He was the father of nuclear physics and first discovered alpha and beta decay as two types of radioactive decay. He also first discovered the phenomenon of the radioactive half life. After his famous gold foil experiments with the radioactive decay of atoms, he developed his own model of the atom that included a positively charged nucleus which contains most of the atom's mass that is orbited by electrons like the planets around the sun. This progressed the view of the atom a bit forward and also clashed with Dalton's theory that atoms cannot be created or destroyed. Nuclear (not chemical) reactions can turn one element into another element through fission. So while mass cannot be destroyed, an element can be broken down into smaller elements.
Finally, Niels Bohr was a 20th century Danish physicist. His model of the atom incorporated some of the earliest thought of quantum mechanics. He postulated that the orbits of electrons around the nucleus are at discrete energy levels and that when electrons jump from a higher orbit to a lower orbit, the atom emits a discrete photon of energy. His theory differs from Dalton in a new way in that chemical reactivity is determined much more by outer (valence) shell electrons than by atomic nuclei.
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