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History of the Atom Democritus of Abdera Democritus: 460-370 BCE Greek mathematician and philosopher Co-author (with Leucippus) of the idea atoma Proposed that the behavior of substances was due to the type of atom Aristotle Greek Philosopher: 384-322 BCE Not an ‘atomist’ Proposed all matter made up of 5 elements: Fire Air Water Earth Aether Robert Boyle Robert Boyle: 1627-1691 Performed experiments with controls and published detailed results 1661: Published claiming that Aristotle's ideas were incorrect Some 'elements' (like mercury) did not contain the 4 Greek elements (earth, air, fire, water) Antoine Lavoisier Lavoisier: 1743-1794 Experimented and established the Law of Conservation of Mass Investigated combustion and cellular respiration in terms of energy and mass John Dalton Dalton: 1766-1844 Law of Multiple Proportions (1803): elements combine in mass ratios of small whole numbers. Small whole numbers reflect the atoms involved Elements may combine with the same element in different proportions to make a different compound. Atomic theory contains 4 hypotheses Dalton’s Atomic Theory 1. Elements are made of small particles called atoms. (Dalton viewed atoms as small spheres of differing characteristics.) 2.All atoms of an element are identical. All atoms of one element are different from atoms of other elements. 3.Compounds are made of atoms of more than one element. The ratio of the elements is a simple fraction. 4.A chemical reaction involves separation, combination or rearrangement of atoms. Joseph John Thomson 1856-1940 Used a cathode ray tube to determine the presence of electrons 1897 Suggested the plum pudding model of the atom and the existence of isotopes Won the Nobel Prize in Physics in 1906 Robert A. Millikan 1858-1953 Used the 'falling drop method' to determine the charge of the electron (-1.6022 x 10-19 C) and mass of electron as 9.10 x 10-28 g Investigated photoelectric effect and spectroscopy of elements Won the Nobel Prize in Physics in 1923 Wilhelm Röntgen 1825-1923 Rays produced by cathode ray tube interaction with other matter could pass through objects, affect photographic plates and cause some materials to fluoresce (X-rays) Nobel Prize in Physics in 1901 Antoine Henri Becquerel 1852-1908 Investigated possible link between X-rays and natural phosphorescence Discovered that the rays emitted by uranium were different from X-rays Nobel Prize in Physics 1903 (shared with Pierre and Marie Curie) Marie Curie 1867-1934 Worked with husband Pierre to study radioactivity Isolated elements polonium and radium Shared Nobel Prize in Physics 1903, won Nobel Prize in Chemistry 1911 for work with radioactive elements. Ernest Rutherford 1871-1937 Investigated release and measurement of different forms of radiation from elements (α, β, γ) Gold foil experiment with Geiger and Marsden led to new model of an atom Model:ruther14.swf Nobel Prize in Chemistry 1908 Model of the atom in the early 1900s Nucleus: has positive charge, very small, but very dense Proton: same charge as electron, but opposite. Mass=1.67262 x 10-24 g Vast empty space around nucleus contains electrons Where are the electrons? Niels Bohr: 1885-1962 Collaborated with Rutherford Used emission spectra to propose Bohr model of atom 1913 Electrons found in specific ‘orbits’ around nucleus Most of atom is empty space Nobel Prize Physics 1922 James Chadwick 1891-1974 Discovered the neutron and worked on fission of atoms Discovery of neutron (mass = 1.67493 x 10-24 g) explained the mass problem of many atoms Nobel Prize in Physics 1935 Current Model Based on the previous work, as well as Einstein, Heisenberg, Born, Schrödinger and others Nucleus contains p+ and no Made of smaller particles called quarks uud, ddu (baryons) Electron cloud e- are in areas of probability, predicted by wave equations leptons Mass and Atomic Number Each element's atoms have a specific number of protons in the nucleus (Z=atomic number) For a neutral atom, the number of protons is equal to the number of electrons The mass number (A) is the number of protons plus the number of neutrons (the number of particles in the nucleus). Some atoms of the same element have different A values. This means they have different numbers of neutrons. They are called isotopes. Isotope notation Isotope notation is used to represent an isotope. Z, A and symbol of element are represented. Isotope notation can be used to determine the number of protons, neutrons and electrons in an isotope.