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Chapter 4 The Structure of the Atom Early Theories of Matter Ideas of the Philosophers ë Democritus (460 - 370 B.C.) - matter is composed of tiny particles called atomos ë Aristotle (384 - 322 B.C.) - disagreed with the idea of atoms John Dalton (1766 - 1844) ü marks the beginning of modern atomic theory ü based statements on scientific research Dalton’s Atomic Theory 1. All matter is composed of tiny, indivisible particles called atoms. 2. Atoms of the same element are identical in size, mass and chemical properties. 3. Atoms of different elements are different. 4. Atoms of different elements can physically mix or chemically combine. 5. Chemical reactions occur when atoms are separated, joined or rearranged. Neutrons - neutral (uncharged) subatomic particles • Discovered by Chadwick in 1932 • Found in the nucleus • Relative electrical charge ZERO • Relative mass 1 • Symbol n0 Subatomic Particles • the building blocks of atoms Protons - positively charged subatomic particles • Discovered by Goldstein in 1886 • Found in the nucleus • Relative electrical charge 1+ • Relative mass 1 • Symbol p+ Electrons - negatively charged subatomic particles • Discovered by Thomson in 1897 • He used a cathode ray tube • Mass 1/1840th of a proton • Relative electrical charge 1• Relative mass…ZERO à it is negligible compared to the proton • Symbol e- rutherford-scattering.jar Rutherford’s Gold Foil Experiment ü an experiment designed to verify Thomson's model ü the results led to the definition of the nucleus rutherford animation Nucleus - central core of the atom (1/100,000th diameter) • composed of p+ and n0 • contains all of the atom’s positive charge • contains 99.97 % of the atom’s mass Atomic Number • the number of protons in the nucleus • H. Moseley discovered that each element contains a unique positive charge • determines the place for each element on the periodic table Mass Number • total number of protons and neutrons found in the nucleus • Remember: All of the mass of the atom is found in the nucleus. # n0 = mass # - atomic # Isotopes M atoms with the same number of protons and different number of neutrons M each isotope has essentially the same chemical properties because the protons and electrons are the same Example: Fill in the table Isotope atomic # mass # #p+ #n0 47 60 C-14 #e- Nuclear Chemistry Radioactivity Nuclear reactions ë changes in the nucleus ë identity of the atom changes radiation ë rays or particles emitted by a radioactive substance ë emitted because nucleus was unstable ë radioactive decay - process that the unstable nucleus undergoes as it emits radiation ë decay continues until a stable state is reached radioisotopes ë isotopes of atoms with unstable nuclei Radioactive Decay Nuclear stability ë the protons have electrostatic repulsion pushing them apart ë the neutrons and protons have a strong nuclear force holding them together ë the stability is based on the neutron-to-proton ratio ` Summary of the Band of Stability as it relates to Radioactive Decay Radioactive Decay Series > a series of decay steps > an unstable nucleus decays until a stable nucleus is formed Decay Rates à Half-life ***Time required for ½ of the radioisotope’s nuclei to decay into its products. sample remaining can be calculated: amount remaining = (initial amount)(½)n Radon-222 has a half life of 3.80 days. How much of a 1125 g sample remains after 155 hours? amount remaining = (initial amount) (0.5)n Radiochemical dating > process of determining the age of an object by measuring the amount remaining of a certain isotope Carbon dating ë all living things contain some radioisotope carbon-14 ë when things die, C-14 uptake stops ë C-14 à half-life = 5730 years ë by measuring the remaining amount of C-14, the age can be determined up to 24,000 years Uranium dating > longer half-life: U-238 à 4.5 billion years > useful for dating of rocks Nuclear Reactors ë generate large amounts of energy by controlled nuclear fission ë energy released by 1 kg U-235 = 4,000,000,000 kg coal ë fuel rods contain UO2 (uranium (IV) oxide) ë control rods contain Cd or B to absorb neutrons ë waste must be processed and stored away from nature Nuclear Fission > splitting of a large nucleus into fragments > atoms with a mass number > 60 > releases large amounts of energy nuclear-fission.jar chain reaction M self-sustaining process in which one reaction initiates the next reaction M uncontrolled chain rxn - nuclear bomb explosion M critical mass - the minimum mass of fissionable material that can sustain a chain rxn Nuclear Fusion > combination of small nuclei > fusion of H powers the Sun Fusion for electricity generation? Advantages > more energy released > products are not radioactive > abundant fuelà lightweight elements Disadvantages > extremely high energy to initiate the process > can't contain the temperature: 40,000,000 K Detecting Radiation > film badge > Geiger counter roentgen (R) • measures exposure (ionization) of air by gamma-rays rad (radiation absorbed dose) • different materials with the same exposure may not absorb the same amount of energy • measures energy deposited in any material by all types rem (Roentgen Equivalent to Man) • estimates biological damage or health risk due to absorption of all types of radiation • For gamma rays and beta particles, 1 rad =1 rem of dose. > Scintillation counter Acute Radiation Exposure Effects of Large, Whole-Body Radiation Doses Effect No observable effect Dose (rems) 0-25 Slight blood changes 25-100 Significant reduction in blood platelets and white blood cells (temporary) 100-200 Severe blood damage, nausea, hair loss, hemorrhage, death in many cases 200-500 Death in less than two months for over 80% >600 Biological Effects of Radiation Uses of Radiation > radiotracer - used to track reaction mechanisms > diagnostic medicine - PET & CT scans radiation therapy - destroy cancer cells