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Chapter 8: Composition of the Atom • The Discovery of Subatomic Particles • Rutherford’s Model of the Atom • Atomic Number and Isotopes Chapter 8: Composition of the Atom Th Discovery The Di off Subatomic S b t i Particles Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- Early Models of the Atom • Democritus – Suggested the existence of atoms – Thought that atoms were indivisible and indestructible – Problems • Didn’t explain chemical behavior • No N experimental i t l supportt • Dalton’s Model – Studied ratios in which elements combine in chemical reactions – Dalton’s Atomic Theory • All elements are made of tiny indivisible particles called atoms. • Atoms of the same element are the same but are different from those of other elements. • Atoms of different elements can physically mix or can chemically combine with each other in simple, wholenumber ratios to form compounds. Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- • Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element never change into other kinds of atoms in chemical reactions. – Inaccuracies in Dalton’s Theory • Atoms are divisible. • Atoms of the same element aren’t entirely the same always. y • Current theory of the atom – Smallest particle of an element that still has all its properties – Extremely small and typically measured in angstroms (1 x 10-10 m) • Electrons – Negatively charged subatomic particle – Extremely light compared to other subatomic particles – Experiments • J.J. Thomson – Use of the cathode ray tube – Attraction Att ti and d repulsion l i – Determined the mass of an electron is about 1/2000 the mass of a hydrogen atom • Robert A. Millikan Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- – Calculated an accurate value for the mass of an electron – Discovered the electron carries exactly one unit of negative charge – Mass of an electron is 1/1840 of a hydrogen atom • Protons and Neutrons – Properties of atoms and charge • Atoms have no net electric charge; they are neutral • Electric charges are carried by matter • Electric El t i charges h always l exist i t in i whole-number h l b multiples of a single basic unit • Charges have to cancel each other out to be neutral – Protons • Positively charged subatomic particles • Discovered by E. E Goldstein in cathode ray tube • 1840 times more massive than an electron • Proton carries exactly one unit of positive charge – Neutrons N t • Neutrally charged subatomic particles • Discovered by James Chadwick • As massive as a proton Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- • The Atomic Nucleus – Original theory of uniform distribution – Ernest Rutherford • Atoms have a lot of empty space • Core of atom is the nucleus composed of protons and neutrons Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- • Discovery of the electron – William Crookes • Use of a partially evacuated tube containing low pressure gas called a cathode ray g y tube • Two electrodes (conductors of electricity) in the tube – Anode (positive electrode) – Cathode (negative electrode) • Applied a voltage and noticed a green beam • Applied a magnetic field and noticed a deflection of the beam • Conclusion: charge of the particle must be negative C k ’ C th d R b Crookes’s Cathode Ray T Tube Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- C k ’ C th d R b Crookes’s Cathode Ray T Tube – J.J. Thomson • Use of a partially evacuated tube containing low pressure gas called a cathode ray tube • Coated the end with a fluorescent material that glowed g where particles struck • Observations – No magnetic field: straight-line straight line path of beam – Magnetic field: deflection of the beam – Magnetic field and charged plates: D fl ti Deflection off beam b and d movementt toward t d positive iti charge • Considerations – Mass of the particle » Heavier particles deflect less than lighter. Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- – Velocity of the particle » Faster particles deflect less than slower particles. – Electric charge of the particle » More highly charged particles have more of a bent path. – Strength of the magnet » Stronger magnets caused more bending. bending – Amount of charge on the plates » Stronger charges on plates cause more bend, and t type off charge h on plates l t determines d t i effects ff t off bend. J J Th ’ C th d R b J.J. Thomson’s Cathode Ray T Tube Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- • Conclusions – Negatively charged subatomic particle common to all atoms » Based on charge g to mass ratio from experiments, p , regardless of gas in tube » Deflection pattern was always the same. – Robert Millikan • Used an apparatus to measure the mass and specific charge of an electron • Conclusions – Electron charge: 1.602 x 10–19 coulombs – Electron mass: 9.10953 x 10–28 grams • Discovery of the proton – J.J. Thomson • Use of a partially evacuated tube containing low pressure hydrogen gas in a cathode ray tube (as he did to discover electrons) Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -- -- The Discovery of Subatomic Particles -- • Observations – Second beam (aside from electrons) moving toward negatively charged cathode – Deflection was greater g • Conclusions – Hydrogen atoms ionized into electrons and positively charged hydrogen ions – Since hydrogen contains only one electron, there had to be a positively-charged particle with equal and opposite charge to the electron. electron – Hydrogen ions are protons, common to all atoms. – Development of the “plum-pudding” model of the atom. atom » Electrons present in a positively-charged “pudding” of protons. » Revision to Dalton’s model of the atom First atom (left): ( f ) boron atom (5 ( protons balancing 5 electrons)) Second atom (upper right): B+ (neutral boron atom losing 1 electron) Chapter 8: Composition of the Atom -- The Discovery of Subatomic Particles -– E. Goldstein • Similar equipment as that used by Thomson • Observations – Second beam (aside from electrons) moving toward negatively g y charged g cathode – Deflection was greater • Conclusions – Referred to these “rays” rays as “canal rays” and later dubbed them as protons – 1840 times more massive than an electron (based on deflection) Chapter 8: Composition of the Atom R th f d’ Model Rutherford’s M d l off the th Atom At Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- Rutherford’s Rutherford s Model of the Atom -- -- Rutherford Rutherford’s s Model of the Atom -- • Radiation – Energy given off from a source and traveling through space – Originally discovered by Henri Becquerel (1896) • Use of photographic plates and uranium (a radioactive element) • Strong images of uranium material remaining on plates – Led to the concept of radioactivity • Radioactivity – Spontaneous emission of radiation from the nucleus of an atom – Processes by which unstable atomic nuclei achieve stability – Later discovered by y Marie and Pierre Curie in radium and polonium • Types of radiation – Alpha radiation • Alpha particles (nucleus of a helium atom – 2 protons and 2 neutrons) emitted from a radioactive source • Also identified as helium ions • One-tenth the speed of light (c = 3.0 x 108 m/s) • Cannot penetrate paper or clothes – Beta radiation • Fast moving beta particles (electrons formed when a neutron decomposes into a proton and electron) Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- Rutherford Rutherford’s s Model of the Atom -- -- Rutherford Rutherford’s s Model of the Atom -- • Approaches the speed of light • Greater energy and penetration because of speed • Cannot penetrate a few millimeters of aluminum – Gamma radiation • High energy electromagnetic radiation emitted by certain radioactive nuclei • More energy than X-rays • Travels at the speed of light because of no mass and no electric charge • Cannot penetrate several centimeters of lead or even thicker amounts of concrete • Discovery of the nucleus – Ernest Rutherford • Bombarded gold foil with alpha particles and masked the source with a lead block to allow particles only to emit out through the one opening • Fluorescent screen placed around target with sensors around • Observations – Most particles went through the foil. – A handful of particles were deflected back. p Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- Rutherford’s Rutherford s Model of the Atom -- -- Rutherford’s Rutherford s Model of the Atom -- • Conclusions – Deflections caused by a small, dense center of positive charge – More than 99.9% of the mass of the atom located in what he called the nucleus – Electrons moving around the nucleus – Slight deflections caused by charges repelling – “A nucleus is to an atom what a pea is to a football field.” Rutherford’s Model of the Atom Chapter 8: Composition of the Atom -- Atomic Number and Isotopes -- Chapter 8: Composition of the Atom At i N Atomic Number b and d Nucleus N l • Discovery of the neutron – Irene and Frederic JoliotCurie • Bombarded beryllium with alpha particles • Radiation struck material containing hydrogen and gave off protons but emitted high, neutral energy (assumed to be gamma rays) – James Chadwick • Repeated the experiment i t off the th Joliot-Curies but also used a cloud chamber Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- Atomic Number and Isotopes -- -- Atomic Number and Isotopes -- • Observations – Mass present to what was colliding with paraffin – Electrically neutral particle that was not a gamma ray • Conclusions – A neutron has approximately the same mass as a proton. – A neutron has no charge. • Atomic Number – Number of protons in an atom of an element – Same as the number of electrons in a neutral atom (since protons cancel out electron charges in a neutral atom) • Mass Number – Total number of protons and neutrons in an atom – Mass of an atom does not come from electrons because of how light g they y are – # of neutrons = Mass number - atomic number Summary of Subatomic Particles Relative charge Relative mass (amu) Actual mass (g) Electron (e–) Proton (p+) Neutron (n0) 1– 1+ 0 0.000549 1.007 1.009 9.11 x 10–28 1.673 x 10–24 1.675 x 10–24 Also known as oxygen-16 (“element” - mass number) Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- Atomic Number and Isotopes -- -- Atomic Number and Isotopes -- • Isotopes – Atoms with the same number of protons but different numbers of neutrons – Have different mass numbers because of the same number of protons and electrons but differing neutrons – Chemically Ch i ll alike lik because b off protons and d electrons l – Some very stable while other extremely unstable because of proton-to-neutron ratio – Isotopes of hydrogen • Atomic Mass – Possible to predict using a mass spectrometer – Atomic mass units • Abbreviated “amu” • 1/12 the mass of a carbon-12 atom – Relative abundance • Each isotope of an element has a fixed mass and occurs in a certain percentage in nature, like how nitrogen makes up a certain percentage of air, oxygen another percentage, etc. • Of approximately 1500 known isotopes, only 264 are stable and therefore have larger relative abundances. – Calculating (average) atomic mass • Weighted average of masses • Average atomic mass = (% abundance1)(mass1) + (% abundance2)(mass2) + … Chapter 8: Composition of the Atom Chapter 8: Composition of the Atom -- Atomic Number and Isotopes -- -- Atomic Number and Isotopes -- Natural Percent Abundance of Oxygen Average atomic mass ((“weighted weighted average ): average”): = 99.759% 15.995 + 0.037 16.995 + 0.204 17.999 = 0.99759 15.995 + 0.00037 16.995 + 0.00204 17.999 = 15.956 + 0.0063 + 0.0367 = 15.999 amu Chapter 8: Composition of the Atom -- Atomic Number and Isotopes -• Atomic mass inferences – Average atomic masses are never whole numbers – Example: average atomic mass of copper • Average atomic mass of copper is 63.546 among isotopes p of copper-63 pp and copper-65 pp • Since 63 is closer to 63.546 than 65, copper-63 must be more abundant than copper-65 Chapter 9: Nuclear Chemistry • Exploring Radioactivity • Using Nuclear Reactions for Research • Nuclear Reactions for Energy Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -- Chapter 9: Nuclear Chemistry E l i Radioactivity Exploring R di ti it • Nuclide – General name for nucleus of atom – Neutrons are the only things that change in nuclides of atoms of the same element • Types of radiation – Alpha radiation () • Alpha particles emitted by a radioactive source – Positively charged particle emitted from certain radioactive nuclei – Consists of two protons and two neutrons and is identical to the nucleus of a helium atom 4 2 Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -204 82 Al h particle ti l He Alpha Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -• Examples 4 Pb 200 80 Hg 2 He • Atomic numbers must add up, and mass numbers must add up. up The new atomic number identifies the element as mercury (Hg). • Properties of alpha radiation – Involves a helium nucleus – Charge: 2+ (no electrons to balance protons) – Mass: 4 amu – Penetrating power: 0.05 mm body tissue – Shielding: paper, clothing – Can be dangerous when ingested Parent nuclide Daughter nuclide • Parent and daughter nuclides – Parent nuclide: initial nucleus – Daugher nuclide: resulting nucleus (aside from emitted particle) » Less energetic than parent nuclide » Formed for stability Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -– Beta radiation () • Beta particles emitted by a radioactive source – Fast-moving electron emitted from certain radioactive nuclei – Formed when a neutron decomposes into a proton and an electron 0 1 14 6 C C 147 N 0 1 e Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -- Positron 15 8 -- Exploring Radioactivity -– Penetrating power: 4 mm body tissue – Shielding: metal foil or thin pieces of wood – Process used in carbon-14 dating • Examples e Beta particle • Properties of beta radiation – Involves an electron – Charge: 1- (charge of an electron) – Mass: 1/1837 amu 8 5 Chapter 9: Nuclear Chemistry B 84 Be O 157 N 0 1 e 0 1 e – Done when too few of neutrons are present in the atom – Charge: 1+ (charge of a proton) – Mass: 1/1837 amu • Positron emission – Some nuclei have too few neutrons for a ratio of stability and need to be balanced by converting protons to neutrons by emitting positrons (particle that has the same mass as an electron but that has a positive charge). Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -– Gamma radiation () • Gamma rays emitted by a radioactive source – High energy rays – Have no mass and no electrical charge • Often emitted along with alpha or beta radiation by the nuclei of decaying radioactive atoms Gamma ray 4 Th 226 88 Ra 2 He + 230 90 • Properties of gamma radiation – Involves rays having no charge or mass – Charge: 0 (no charge in and of itself) – Mass: 0 amu Chapter 9: Nuclear Chemistry Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -– Penetrating power: through entire body – Shielding: lead, lead concrete (incomplete shielding) – Most dangerous of all radiation – Usually accompanied by alpha or beta particles – Similar in properties to X-rays • Ionizing vs. nonionizing radiation – Ionizing radiation • Has enough energy to change atoms and molecules into ions • Examples – Alpha radiation – Beta radiation – Gamma radiation Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -– Nonionizing radiation • Does not have enough g energy gy to change g atoms and molecules into ions • Examples – Microwaves – Light waves – Radio waves • Sources S off radiation di ti – The sun (cosmic radiation) – Food – Building materials – Background radiation (natural radiation) • Carbon-14 • Hydrogen-3 (tritium) Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -• Effects of ionizing radiation – Genetic mutations/annihilations • Sensitivity of DNA • Carcinogenic consequences – Threshold for radiation • Below: minimal damage • Above: radiation sickness – Hair loss – Deformities – Cellular degradation/mutation -- Exploring Radioactivity -• Examples Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -• Examples Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -• Mass conversion – Antimatter • Antiparticles of matter than can annihilate each other – Every particle has an antiparticle. – When a particle meets an antiparticle antiparticle, they disappear in a process known as pair annihilation. – Their mass-energy is used to create photons or other particles. ti l – “Mass-energy equivalence” – Extremely y short existence because it is annihilated when it comes into contact with ordinary matter – Four basic forces of the universe • Gravitational force • Electromagnetic force Chapter 9: Nuclear Chemistry Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -- -- Exploring Radioactivity -- • Weak nuclear force • Strong nuclear force – Short range force keeping protons and neutrons together – Gets extremely y weak as p protons and neutrons g get farther apart – When strong force does not exist, radioactivity occurs. – Elementary El t particles ti l • Leptons (particles involved in the weak and electromagnetic forces) – Electron – Muon – Tau – Neutrino • Hadrons (particles that take part in the nuclear forces, including weak and electromagnetic forces) – Mesons M ((particles ti l that th t decay d into i t photons h t and d leptons) l t ) » Pion » Kaon » Eta – Baryons (particles that decay into protons and/or other thi things) ) » Proton » Neutron » Lambda » Sigma » Xi » Omega Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -Table of Elementary Particles Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -– Albert Einstein • Mass is lost in nuclear reactions. • Nuclear reactions cause mass to be converted into energy. • Equation: q E = mc2 – E: energy (J) – m: mass (kg) – c: speed of light (3.0 108 m/s) – Mass is derived by how much mass is lost in the nuclear reaction. • Half-life – The time required for one-half of the atoms of a radioisotope to emit radiation and decay to products Chapter 9: Nuclear Chemistry Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -- -- Exploring Radioactivity -- – Symbol: T – A=A0(0.5)t/T where A is the final amount, A0 is the initial amount, t is the time, and T is the half-life in the same units – Example • A sample initially contains 50.0 g of cobalt-60. After 2.00 years, the sample contains 38.4 g of cobalt-60. Calculate th half-life the h lf lif or cobalt-60. b lt 60 – A0 = 50.0 g – A = 38.4 g – t = 2 years Chapter 9: Nuclear Chemistry -- Exploring Radioactivity -• Decay chain – Chain of decay in which unstable isotopes decay into stable iisotopes t