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Unit 2: Atomic Structure and Nuclear Chemistry Chemistry I Goals 1 – 3 • • • Summarize Dalton’s atomic theory. Distinguish among protons, electrons, and neutrons in terms of mass and charge. Describe the structure of the atom. What discoveries lead to the Atomic Theory? • Atom: – Smallest particle of an element that retains the properties of the element • Democritus – Father of the Atom – 400 B.C. – Atomos: Greek for “indivisible” – Philosophy – not Science – No measured or experimental support John Dalton – Father of the Modern Atom • • 1808 Four Statements 1. 2. 3. 4. All elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. Atoms of different elements are different. Atoms combine in simple whole number ratios. Atoms of one element are never changed to atoms of another element during chemical reactions. Sizing up the Atom • What instruments are used to observe individual atoms? – Despite their small size, atoms are observable with scanning electron microscopes – Radii fall between 5 x 10-11 m – 2 x 10-10 m 50 – 200 pm (picometer) Example 1 Subatomic Particles • • • • Electrons Protons Neutrons Nucleus Electrons • J.J. Thomson – 1897 • Discovered the electron using a cathode ray tube (CRT) • Negatively charged particles Protons • E. Goldstein – 1886 • Discovered the proton by careful observations of another ray in the Cathode Ray tube • Positively charged particles Neutrons • James Chadwick – 1932 • Discovers the neutron • Neutrons are the “glue” that holds the nucleus of the atom together. Subatomic Particles • • • • Electrons Protons Neutrons Nucleus Atomic Charges: Neutral atom: Equal # of Protons and Electrons Element dependant on # of Protons Number of Electrons can change – creates charged atom (called IONs) “Plum Pudding Model” • Electrons evenly distributed in an atom filled with uniform positive chare • First Atomic Model • Example The Atomic Nucleus • • Ernest Rutherford – 1911 Using the “Gold Foil” experiment, Rutherford determines: 1. 2. 3. 4. An atom is mostly space. There is a nucleus in the middle of the atom containing the protons The electrons orbit a large distance away from the nucleus. Proposes a “Solar System” or “Orbital” model of the atom Cute Summary of History Atomic Particle Summary Name Electron proton neutron Symbol Charge Mass (amu) e- -1 0.000543 Practically 0 p+ +1 1 n 0 1 Atomic Number • Number of protons in the nucleus of an atom • Each element has a unique atomic number • The number of electrons usually equals the atomic number in a neutral atom, but doesn’t need to Mass Number • The total number of protons and neutrons in an atom • Atoms of the same element can have different mass numbers • Mass number is NOT normally found on the periodic table Mass# • Shorthand notation Atomic# Symbol – Example: Gold 197 79 Au Number of Neutrons Number of Neutrons = Mass # - Atomic # Sample Problem 4.2 How many protons, electrons, and neutrons are in each atom? a. 9 Be 4 b. 20 10 c. 23 11 Ne Na Isotopes • Atoms with the same number of protons, but different numbers of neutrons • Isotopes of an element have the same atomic number, but different mass numbers • Nuclear Symbol or Isotopic symbol: shows number of protons & mass number of an atom Nuclear or Isotopic Symbols • Examples – Carbon -12 – Carbon -14 • Nuclear Symbol: Name - Mass # • Isotopic Symbols – 12C Mass Number 6 – 14 6 C Atomic Number Atomic Mass: A Weighted Average • Atomic Mass Unit: (amu) – Unit of mass for atomic nuclei – 1 amu = 1/12 the mass of Carbon-12 • Atomic Mass: – Weighted average mass of all the naturally occurring isotopes in a sample of an element Sample Problem 4.5 • Element X has two naturally occurring isotopes. The isotope with a mass of 10.012 amu has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu has a relative abundance of 80.09%. Calculate the atomic mass of element X. Chapter 25: Nuclear Chemistry Regular Chemistry • Deals with the arrangement of atoms to form compounds • Never about the changing of the nucleus • Elements NEVER change to other Elements Nuclear Chemistry • Deals with the decay of unstable isotopes to form a stable nucleus • About the changing of the nucleus • Elements CAN change to other Elements 25.1 Nuclear Radiation • Radiation – The penetrating light rays and/or particles emitted by a radioactive source • Radioactive decay: – The spontaneous emission of radiation by an unstable nucleus • Radioisotope: – An isotope that has an unstable nucleus and undergoes radioactive decay Types of Radiation Property Composition Alpha Beta Gamma Alpha Particle (helium nucleus) Beta Particle (electron) High-Energy Radiation Symbol α β γ Charge 2+ 1- 0 Mass (amu) 4 1/1840 0 Low Tissue Paper (0.05 mm) Moderate Skin or Foil (4 mm) Very high Lead/Concrete Entire body Penetrating Power Radiation Examples • Alpha Radiation Uranium-238 to Thorium-234 • Beta Radiation Carbon-14 to Nitrogen-14 Summary: What has really happened? • Gamma Radiation Thorium-230 to Radium-226 Thorium-234 to Protactinium-234 25.2 Nuclear Transformations • Why does the nucleus stay together? • Strong Nuclear Force – Short range, attractive force that acts among nuclear particles – Neutrons and Protons attract one another! – Much stronger than electrical or gravitational force • Electromagnetic Force – Repulsive force between protons. Tries to pull the nucleus apart. • Stable Nucleus – Correct balance of Strong Nuclear & Electromagnetic Forces – Correct p+ to n ratio – MOST ATOMS ARE STABLE!! Band of Stability 1:1 Ratio Line Band of Stability Most common transformation particles Radiation Name Symbol & Iso. Notation Alpha particle α, Beta particle β Gamma radiation γ Positron β+ 0 1 Neutron n 1 0 n Proton p+ 1 1 p 4 2 Charge Mass +2 ~4 amu e -1 1/1840amu (nothing) “none” 0 0 e +1 1/1840amu (nothing) 0 ~1 amu +1 ~1 amu He 0 1 Nuclear Stabilization Equations • Alpha emission: Uranium-238 • Beta emission: Carbon-14 • Positron emission: Oxygen-15 • Neutron emission: Silver-107 U 24He 234 90Th 238 92 C 10 e147N 14 6 O10 e157N 15 8 107 47 Ag n Ag 1 0 106 47 Additional Example • Beta Absorption of Krypton - 81 Transformation Examples Silicon-27 ______ + β 27 14 Si P e 27 15 0 1 Bismuth-214 _____ + α 214 83 Bi Tl He 210 81 4 2 Aluminum-27 + α Silicon-30 + ____ 27 13 Al He Si p 4 2 30 14 1 1 25.3 Fission and Fusion • Fission: – Large nucleus breaking down into smaller nuclei • Fusion: – Smaller nuclei combining to larger nucleus