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Basic Structure of the Atom Evidence Supporting the Atomic Theory Democritus Ancient Greek Matter made up of atomos Atoms can not be: Created Destroyed Further Divided Antoine Lavoisier (Late 1700’s) Founder of Modern Chemistry Explained O2 role in combustion Concept of element as basic substance Conservation of matter theory Measured the mass of substances before and after a chemical reaction Performed carefully controlled experiments in closed systems Beheaded during French Revolution William Proust (1799) Devised the law of constant composition A given compound always contains the same elements in the same proportions Dalton Proposed Atomic Theory in 1803 Half a Century until well accepted 100 years until proven Each element : Composed of indivisible and indestructible atoms Atoms of different elements are different; atoms of the same element are the same Atoms of different elements combine to form compound atoms (molecules) Michael Faraday (1839) Developed fundamental theories of electricity, magnetism, and light Theorized the structure of atoms are somehow related to electricity 1832 - Electrochemistry J. J. Thomson Discovered electrons in 1897 Plum Pudding Model – Positive and negative charges scattered randomly throughout atom Instantly accepted Experiment Passed a cathode ray from a cathode to an anode with a hole in it that allows a small amount of the cathode ray to pass through Cathode ray passes through a tube surrounded by both poles of a magnet and electrically charged plates Magnetic field turned on, the ray is deflected upwards Electric field is turned on, the ray is deflected downward Magnetic and electric fields are off, the ray travels straight Becquerl (1896) Discovered spontaneous emission of radiation from an element - radioactivity Accidently placed uranium salts on top of an unexposed photographic plate which was wrapped in paper and in a dark desk drawer Developed plate – saw silhouette of salts Led Pierre and Marie Curie to discover radium and polonium by isolating them from pitchblende Millikan (1909) Measured charge and mass of the electron Experiment measured the effect of an electric field on the rate at which charged oil drops fell under the effect of gravity used x-rays near the droplet to charge them the oil droplets fall could be accelerated, retarded, or even reversed, depending on the charge on the droplet and the polarity voltage of the plate Rutherford Gold foil experiment 1909 – Mass concentrated in very small core at the atom’s center (nucleus) Nucleus positive, negative electrons moving around it Contributions: Field of Nuclear Physics (1898, alpha and beta particles) Radioactive decay Rectify Periodic Table Experiment Fired alpha particles at gold foil Most passed through the foil, a few were deflected When alpha particles (+2) closely approaches the gold nucleus (+79), it undergoes a strongly repulsive interaction Bohr Doctoral Thesis 1911– Theory of Electrons In 1913, discovered electrons revolve around the nucleus in energy levels (Einstein and Planck) Energy levels closest to nucleus have low energy Energy levels increase in energy with distance from the nucleus Electrons gain and lose energy by moving between energy levels (quantum) “This is an enormous achievement” Einstein Moseley (1914) Determined the atomic numbers of each element bombarded different elements with energetic electrons and studied the x-rays they emitted observed that the frequency of the x-rays were different for each element arranged the frequencies in order by assigning each element a unique, integral number, which he called the atomic number Modern Atomic Model The atom consists of three main particles: Protons (positive) Neutrons (neutral) Electrons (negative) Two main parts: Nucleus Electron cloud Atomic Model (cont.) Nucleus contains: Protons (+) Neutrons (0) Nucleus surrounded by: electron cloud Negative charge due to electrons Atomic Structure Atomic Structure Atomic Mass Units Mass measured in atomic mass units For protons and neutrons 1 amu is defined as 1/12 the mass of a carbon atom containing 6 protons and 6 neutrons 1 amu is also the mass of 1 proton or 1 neutron An electron has a mass of 1/2000 amu Key Terms Atomic number: the number of protons in the nucleus of an atom. Mass number: the sum of the number of protons AND the number of neutrons in the nucleus. Mass # = # protons + # neutrons Atomic Symbol and Mass Number Isotopes Isotopes are atoms of the same element with differing numbers of neutrons. Isotopes have different masses Isotopes Isotopes of Carbon How do you record the mass of a group of isotopes? Because most elements have more than one isotope, each element is given an average atomic mass The average atomic mass is the average mass of the mixtures of its isotopes How do you calculate the average atomic mass of an atom? The number of naturally occurring isotopes, their masses, and their percent abundances must be known. Example: Lithium has 2 isotopes: Li-6 (mass 6.015 amu and 7.5% abundance), and Li-7 (mass 7.017 amu and 92.5% abundance). What is its average atomic mass? How do you calculate the average atomic mass of an atom? Calculate the average atomic mass of silicon. The three silicon atoms have masses of 27.98 amu, 28.98 amu, and 29.97 amu with relative abundances of 92.23%, 4.67%, and 3.10%, respectively. What is radioactivity? Emission of high energy radiation or particles from the nucleus of a radioactive atom The atoms of radioactive elements are held together less securely than nonradioactive elements Particles of energy can escape from all nuclei with atomic numbers 84 or higher (radioactive decay) The nuclei of these elements are unstable In elements < 20 Atomic Number, n:p 1:1 In elements > 20 Atomic Number, n:p 1.5:1 The atom’s nucleus is held together by the strong force How do you write the symbol for a nuclide? The symbol gives atomic #, mass #, and chemical symbol mass # 39 atomic # 19 K chemical symbol What is nuclear radiation? Radiation given off by radioactive nuclides There are three types: alpha particles ( particles) beta particles ( particles) gamma rays ( rays) Only gamma rays are a type of electromagnetic radiation!! What are alpha particles? Given off when a nucleus releases 2 neutrons and 2 protons Same thing as a helium nucleus Has a charge of +2 and an atomic mass of 4 Largest and slowest form of radiation Least penetrating – can be stopped by a sheet of paper Used by smoke alarms (americium) What are beta particles? Neutrons can spontaneously decay into a proton and an electron The electron is the beta particle The proton can decay into a neutron and a positron What is a positron? A positron is similar to an electron, only with a positive charge Positrons are considered beta particles too Beta particles are much faster and more penetrating than an particle What are gamma waves? Most penetrating and potentially dangerous form of radiation Not made of particles Are electromagnetic waves with high frequency and energy Have no mass, no charge, and travel at the speed of light Usually released along with and particles thick blocks of lead and concrete are commonly used for barriers What is transmutation? Process of changing one element to another through nuclear decay Atomic mass # of the decayed nuclide equals the sum of the mass # of the newly formed nuclide and the emitted particle How do you determine the mass of the new nuclide? If the particle is an alpha particle, subtract the mass of the ejected particle from the mass of the old nuclide. Alpha particle emission: 218 84 Po 214 82 4 2 Pb + He How do I calculate the mass of the nuclide when it loses a beta particle? Because a beta particle is the product of the decay of a neutron, a proton will be left behind when the –e is ejected. 214 82 Pb 214 83 Bi + 0 -1 e Charged Atoms In a neutral atom, the number of protons equals the number of electrons. The positive and negative charges balance out, leaving the atom with 0 net charge In a charged atom or ion, there is an uneven number of protons and electrons, so the atom will have either a positive or negative net charge Ions What is half life? Some nuclides of radioactive isotopes may require a long time to decay Half life is the amount of time it takes for half the nuclides in a sample of a given radioactive isotope to decay It can vary widely among the radioactive isotopes Can determine amount of a radioactive sample that will remain after a given amount of time with the half life Example Carbon 14: At the beginning, there is 100%. It’s half life is 5730 years. So, after 5730 years, there will be only half, or 50%, left. After another 5730 years (11,460 total), there will be half of 50% left, or 25%. After another 5730 years (17,190 total), there will be half of 25% left, or only 12.5% of the original amount remaining. Is there a formula for halflife calculations? Amount remaining = (initial amount)(1/2)n n = number of half-lives that have passed n also can equal t/T, where t = the elapsed time, T = length of half-life Both t and T have to be in the same units What is carbon 14 dating? Radioactive materials - In your body Carbon 14 emits beta particles and decays into nitrogen Measuring % carbon 14 to carbon 12 allows determination of approximate age of material How can you measure radioactivity? Cloud Chamber – contains a gas cooled to a temperature below its condensation point; droplets of the gas condense around the radioactive particles, which leave a trail that shows up along the chamber lining. Geiger counter – produces an electric current in the presence of a radioactive substance. What is fission? Nuclear fission is the splitting of an atomic nucleus into two smaller nuclei Word “fission” means to divide Large nuclei with atomic numbers above 90 can undergo nuclear fission U 235, when bombarded by a neutron, splits to produce Ba 141, Kr 92, three neutrons and ENERGY! What is a nuclear chain reaction? Neutrons released from one fission reaction collide with another atom to cause another fission reaction. A continuous series of fission reactions is called a chain reaction. Huge quantities of energy are released with many simultaneous nuclear reactions. An uncontrolled chain reaction causes a nuclear explosion. Nuclear Chain Reaction Nuclear Reactor Nuclear reaction controlled with cadmium and boron control rods that absorb neutrons Generate heat (energy) from U-235 and heats the coolant water The “hot” coolant water then heats water that is used to drive steam-driven turbines, which produce electricity Problem – Spent fuel rods are VERY hazardous waste and buried underground ½ life of U-235 is 713,000,000 years Schematic of a Nuclear Power Plant What happened at Chernobyl? What is Fusion? Fusion – Bind together Joining of 2 less stable nuclei (<60) into one stable nuclei Example: Sun 4 1H + 2 e --> 4He + 2 neutrinos + 6 photons A temperature of 5,000,000 K required to overcome electrostatic repulsions between the nuclei Fusion Reaction In the picture to the right, two types of hydrogen atoms, deuterium and tritium, combine to make a helium atom and an extra particle called a neutron This process releases four times as much energy as the fission of a uranium nucleus Medical Uses for Radiation Treating Cancer – Kills cancer and healthy cells as well Radiotracers – emits non-ionizing radiation and is used to signal the presence of an element Used in studying blood flow patterns, uptake of thyroid gland, emptying rate of gallbladder Used in research experiments to trace amounts of chemicals in the system ( tertiary oil recovery)