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Matter and substance Breaking up a substance / an element S3 “Atomic Structure” Defining the Atom Democritus’s ideas about atoms. Defining the Atom The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”) He believed that atoms were indivisible and indestructible Defining the Atom Dalton’s atomic theory. (Dalton 1766 – 1844) Dalton’s Atomic Theory John Dalton (1766 – 1844) Sizing up the Atom Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element. If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long. Structure of the Nuclear Atom OBJECTIVES: Identify three types of subatomic particles. Structure of the Nuclear Atom One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles: E.g. in Nuclear fission Electrons, protons, and neutrons are examples of these fundamental particles Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron Modern Cathode Ray Tubes Television Computer Monitor Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. Cathode ray are negatively charged – can be deflected by magnets or charged plates Cathode ray are negatively charged – can be deflected by magnets or charged plates Mass of the Electron Mass of the electron is 9.11 x 10-28 g The oil drop apparatus 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge Conclusions from the Study of the Electron: a) Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. b) Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons c) Electrons have so little mass that atoms must contain other particles that account for most of the mass Conclusions from the Study of the Electron: Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron) 1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton Subatomic Particles Particle Charge Electron (e-) Proton (p+) Neutron (n) -1 Mass (g) / Relative mass 9.11 x 10-28 1/1840 (negligible) Location Electron cloud 1.67 x 10-24 +1 1 Nucleus 1.67 x 10-24 0 1 Nucleus Proposed models of atomic structure….???? Thomson’s Atomic Model J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. the “plum pudding” model of JJ Thomson Ernest Rutherford’s Gold Foil Experiment - 1911 Rutherford was a student of Thomson and decided to see if there was more evidence for the 'plum pudding /blueberry muffin' theory. His idea: to bombard the atom with dense alpha particles alpha particle = He atom with no electrons - this was like shooting bullets at a blueberry muffin Ernest Rutherford’s Gold Foil Experiment - 1911 this was like shooting bullets at a blueberry muffin…. So what did Rutherford expect? Surprising result of Rutherford’s Gold Foil Experiment - 1911 http://www.youtube.com/watch ?v=Q8RuO2ekNGw Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like gun shells bouncing off of tissue paper!” Rutherford’s Findings and deductions Findings Deductions Most of the particles passed right through an atom was much more than just empty space and scattered electrons A few particles were deflected, Since alpha particles are relatively heavy, positively charged particles, the fact that the occasional particle would be deflected by either a small or large extent, an atom must have a positively charged center that contains most of its mass (which Rutherford termed as the nucleus). very few were greatly deflected Rutherford’s Findings and deductions An illustration that compares the plum pudding model of an atom to what Rutherford observed in his experiment. The top structure shows how the alpha particles would have passed through the gold foil atoms if the plum pudding model was correct in its assumptions. The bottom structure shows what Rutherford and his collogues observed and is the true depiction of an atom's structure. The nucleus as described by Rutherford: a) The nucleus is small b) The nucleus is dense c) The nucleus is positively charged Modern understanding of an atom a) The nucleus is small b) The nucleus is dense – contain most of the mass of an atom c) The nucleus is positively charged d) The majority of the volume an atom is just empty space Evolution of atomic model…… Changing definition of an atom Dalton’s Atomic Theory The atom is the smallest unit of an element that cannot be divide any more (indivisible). Changing definition of an atom Dalton’s Atomic Theory The atom is the smallest unit of an element that cannot be divide any more (indivisible). – proven wrong Modern definition The atom is the smallest unit of an element and that still retains the properties of an element.