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Atomic theory.notebook September 11, 2012 Early Theories of Matter Atomic Structure Chapter 4 • • • • • Greek philosophers (Empedocles 450 B.C.) • air, water, earth, fire • Democritos (420 B.C.) • atoms • John Dalton (17761884) • All matter made of atoms. • Atoms of same element are identical & vice versa. • Atoms cannot be created, divided, or destroyed. • Atoms combine in whole # ratios to make compounds. • Atoms are separated, combined, or rearranged in chemical reactions. Aug 303:53 PM Aug 303:53 PM Defining the Atom Subatomic Particles What is an atom? Size of atom 1 cm ≈ 200 000 000 atoms Scanning tunneling microscope • Discovering the electron • William Crookes: cathode ray tubes • Stream of charged particles • Carries a negative charge Si atoms on face of a crystal Aug 303:53 PM electron cont… • J.J. Thomson – discovered electron (e) • mass of e = 9.11 × 1028 g • Plum pudding model (1897) Aug 303:53 PM The Nuclear Atom • Rutherford – discovered nucleus (1911) • gold foil experiment • small positive core • core contains most of atom’s mass • mostly empty space • Robert Millikan – determined charge of electron • 1.60 × 1019 coulombs Aug 303:53 PM Aug 303:53 PM 1 Atomic theory.notebook September 11, 2012 Bohr Model of the Atom Discovering protons & neutrons • • • • • • Proton (1919) Discovered by Rutherford Positively charged; in nucleus Neutron (1932) discovered by Chadwick Neutral; in nucleus • Neils Bohr – proposed quantum model of the atom • Electrons move about the atom in circular paths of specific energies • Termed these paths energy levels • Later determined to be fundamentally incorrect Nuclear Model of the Atom Aug 303:53 PM Aug 303:53 PM Quantum Mechanical Model • • • • • Summarizing the Atom Louis de Broglie Waveparticle duality Erwin Schrödinger Electrons treated as waves Electrons exist in “clouds” around the nucleus 1 0 1.67x1024 Aug 303:53 PM Aug 319:59 AM 1/1840 9.11x1028 no p+ 1 1+ 1 Aug 303:53 PM Atomic Number (Z) • Number of protons in the atom • Since atoms are neutral, number of protons and number of electrons are the same (if they are not equal, then have ion, cation, anion) • Therefore, Atomic number = # protons = # electrons 1.67x1024 e Mass Number • Mass number is the sum of the protons and neutrons in an atom • Mass number = p+ + n0 p+ n0 e Mass # 11 12 11 23 Potassium 19 20 19 39 Bromine 45 35 80 Sodium 35 Aug 319:59 AM 2 Atomic theory.notebook September 11, 2012 THE STRUCTURE OF THE ATOM To calculate the composition of an atom 23 Proton Neucleons Neutron A mass number Na SYMBOL X 11 Zatomic mass Electron <http://education.jlab.org/qa/atom_model_03.gif> Number of protons + neutrons = A The mass of the atom is concentrated in the nucleus. The volume of the atom is determined by the space that the electrons move in, around the nucleus Number of protons = Z Number of electrons in neutral atom = Z Number of neutrons = A Z Nov 2811:16 AM Nov 2812:50 PM Isotopes • Isotopes are atoms of the same element that have different mass numbers due to a different number of neutrons • Same number of protons, but different number of neutrons • # neutrons = mass number – atomic number Nov 2802:06 PM Aug 319:59 AM Atomic Mass • Atomic mass is the average of all the isotopes of that element Carbon – 12.011 Isotope Protons Electrons Neutrons Hydrogen – 1 (protium) 1 1 0 Hydrogen – 2 (deuterium) 1 1 1 Hydrogen – 3 (tritium) 1 1 2 Aug 319:59 AM Composition % in nature of nucleus Isotope Symbol Carbon – 12 12C 6 protons 6 neutrons 98.89% Carbon – 13 13C 6 protons 7 neutrons 1.11% Carbon - 14 14C 6 protons 8 neutrons <0.01% Aug 319:59 AM 3 Atomic theory.notebook Nuclear Symbol 235 92 September 11, 2012 Calculating Atomic Mass Example on page 103 • Chlorine – 37 • Chlorine – 35 • Atomic mass: 34.969 amu • Atomic mass: 36.966 amu • % abundance: 24.230% • % abundance: 75.770% • Mass contribution: • Mass contribution: (36.966 amu)(24.230%) = 8.957 (34.969 amu)(75.770%) = amu 26.496 amu • Atomic mass of Chlorine = 26.496 amu + 8.957 amu = 35.453 amu Aug 319:59 AM Aug 319:59 AM Summarizing the Atom • Atoms are electrically neutral (p+ = e) • e held in orbit by attraction to nucleus • n0 present to break up p+ repulsion Aug 303:53 PM 4