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Elements, Atoms, and Ions • • • • • • Element names and symbols Atomic structure, history and nucleus Isotopes and average atomic mass Formation of ions Nuclear Radioactivity and nuclear transformation The Atom – A History • Ancient Greece • Democritus • All matter consists of tiny particles or combinations of particles of four types. The Atom- A History • Alchemists • • • • Quest for gold Some sincere scientists Lacked a quantitative approach Resulted in progress none the less • Mercury, sulfur, antimony, acids • Robert Boyle (1627-1691) • Quantitative experimental approach • Gases • Defined element as a substance that could not be broken down into two or more simpler substances The Atom- A History • John Dalton (1766-1844) • What was already clear? • Most natural materials are mixtures of pure substances. • Pure substances are either elements or combinations of elements called compounds. • A given compound always contains the same proportions by mass of the elements which constitute it. • ie, water always contains 8g of oxygen for every 1g of hydrogen The Atom- A History • Dalton’s Atomic Theory 1. 2. 3. 4. Elements are made of tiny particles called atoms All atoms of a given element are identical The atoms of a given element differ from those of other elements Atoms of one element can combine with those of other elements to form compounds, and a given compound always has the same relative numbers (ratios) of types of atoms. • ie, H2O is a 2:1 ratio by mass and that ratio will not change 5. Atoms are indivisible in chemical processes, but the way in which they may be grouped can change • Allowed prediction of formation of multiple compounds between two elements and simple formulas The Atom- A History • Billiard Ball Model • What was still not clear? • What does an atom look like? • Can it be broken into small pieces? • J.J. Thomson • 1897 • Discovery of electron • Cathode ray tube The Atom- A History • J.J. Thomson • Cathode ray was deflected away from a negatively charged plate. • What conclusion could he draw from this? • Plum pudding model The Atom- A History • Ernest Rutherford • Gold Foil Experiment (1911) • Emission of positively charged particles from radioactive elements was somewhat clear already. A radioactive element served as Rutherford’s source of alpha particles. The Atom- A History • Gold Foil Experiment Rutherford Scattering What conclusions could be drawn from this experiment? The Atom- A History • Rutherford’s Nuclear Atom • Other conclusions • Proton – same size charge as electron but positive • Most of the atom actually consists of what? • Neutron • Rutherford later discovered existence of neutral particle with slightly larger mass than the proton. - + Defining the Atom • Atom – smallest particle of an element that retains its identity in a chemical reaction. • Protons p+ (+) • Mass 1840 times larger than electron • Neutrons n0 (no charge) • Mass nearly equal to that of a proton • Electrons e(-) • Can be shared or exchanged between atoms • Mass is 1/1840 the mass of a hydrogen atom • occupy almost all the volume of the atom. Distinguishing Among Atoms • How are atoms of hydrogen different than atoms of oxygen? • Elements are different because they contain different numbers of protons. • Atomic number = number of protons = electrons in a neutral atom • Mass number = the total number of protons and neutrons in an atom • Number of neutrons = mass # - atomic # Isotopes • Isotopes – atoms that have the same number of protons but different numbers of neutrons. C C Atomic # = 6 Atomic # = 6 Mass # = 12 Mass # = 13 Mass # = protons + neutrons Estimation of actual mass or isotopic mass Slightly different than mass number due to a small difference in the masses of protons and neutrons. • In nature, most elements are mixtures of isotopes. • So what about the mass on the periodic table? Average Atomic Mass • Atomic mass – weighted average mass of the atoms in a naturally occurring sample of the element. • Which method(s) would result in the correct average atomic mass? Average Atomic Mass • Avg Atomic Mass= (mass1)(abundance1) + (mass2)(abundance2) … 100 Both valid Periodic Table • Groups and Periods • Metals, Nonmetals, Metalloids • Electrons and Ions The Modern Periodic Table • The Periodic Table is a listing of all the known elements. • The elements are organized by: • Atomic number • Chemical Properties • Group( or Family): a vertical column. Elements in groups have similar chemical properties. • Period: a horizontal row. Metals, Nonmetals and Metalloids • Elements can be classified into: • Metals • • • • • • Good conductors of electricity and heat. Malleable (can be hammered into sheets). Ductile (can be drawn into wires). Lustrous. Easily lose electrons. Form positive (+) ions. (cations) • Nonmetals • They do not have the properties of metals. • Gain electrons. • Form negative ions. (anions) • Metalloids (semimetals) • Display properties of both Nonmetals Metals Metalloids Actinides and Lanthanides Actinides and Lanthanides Electrons and Ions Bohr’s Model – Energy Levels Electrons and Ions • The periods (rows) of the periodic table indicate the highest energy level occupied by one or more electrons. • The “A” groups are numbered 18. • The number of the “A” groups correspond to the number of valence electrons. • Valence electrons – electrons in outermost, occupied energy level Charges- Noble Gas Configuration Nuclear Chemistry • • • • Radioactive Decay - types Nuclear Equations Half-life calculations Nuclear Energy-fission and fusion Radioactive Decay What affects nuclear stability? • Neutron:Proton ratio A/Z • Even or odd number of protons • "magic" numbers 2, 8, 20, 28, 50, 82, and 126 • Unknown factors Radioactive Decay Radioactive Decay • Types Radioactive Decay Positron Decay • Antiparticles • Every particle has a corresponding antiparticle Positron Decay • Antimatter • Hydrogen and anti-hydrogen Positron Decay • “Going beyond the basics, we can say that an antiparticle is related to the particle by charge conjugation. This includes more than just electric charge; it inverts all internal quantum numbers such as baryon number, lepton number, and strangeness.” • http://hyperphysics.phy-astr.gsu.edu/hbase/particles/antimatter.html Positron Decay • Beta Decay = neutron electron + proton • Positron Decay = proton neutron +positron p+ no + What is the effect of positron decay on the mass and atomic numbers of the parent isotope? Electron Capture Electron Capture • A proton absorbs an electron to become a neutron. no p+ What is the effect of electron capture on the mass and atomic numbers of the parent isotope? Predicting Decay Type Radioactive Decay Nuclear Equations Uranium-238 decays by alpha emission into thorium-234 • Carbon-11 produces a betaparticle • Neptunium-237 produces an alpha particle Half-Life • How long does it take for unstable nuclei to become stable? • Rate of Decay • Measured in half-lives • Half-life (t1/2) – amount of time it takes for half of the radioactive nuclei to decay Half-Life