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Notes ch 3.2 Discovery of subatomic particles Modern atomic theory Discovery of electrons JJ Thomson Nucleus Ideas • JJ Thompson :( (Plum Pudding Model, 1904) =ball of positive charge with electrons throughout • Ernest Rutherford (Nuclear Model, 1909) =dense, positive charged mass, concentrated in a small nuclear area Discovery of the nucleus Today • Daltons basic ideas survive. • We now know there are versions of elements (isotopes) • We now know that atoms can be subdivided. (They have parts p, n, e) • Atoms became visible to researchers in the 1980’s with the use of a scanning tunneling microscope. What did each of the following contribute to development of atomic theory? JJ Thompson: Ernest Rutherford: Democritus: Nuclear atom or plum pudding? • • JJ Thompson’s • model Ernest Rutherford’s “Plum Pudding” Nuclear Atom Nuclear particles • Proton • Neutron – Positive charge • Mass 1.67 x10-24g • • Is the defining particle of • an atom. • Gives the atom its identity. – No electric charge Mass = to a proton Adds to the mass of the atom Gives nucleus stability (separation of + charges) Unit of mass used is the AMU Atomic Mass Unit= ~ the mass of a proton. Really, 1/12 of a carbon-12 atom Proton = element defining particle • # of protons = atomic # Ex: Oxygen = 8 protons, has atomic number of 8 Neon = 10 protons, has atomic number of 10 • How many protons does carbon have? • Calcium? Sub atomic particles Particle Charge Symbol Electron -1 e- mass (amu) 1/2000 Proton +1 p+ 1 0 n 1 Neutron Essentially all of the mass of an atom comes from the nuclear particles (protons and neutrons) Summary • Mass number (A) = nucleus number – Protons + Neutrons • Atomic number (Z) = # of protons • Number of Neutrons = mass number – atomic number (A-Z) • An element is neutral (no net charge) if the # of electrons= # of protons