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Quiz 1) 2) 3) 4) 5) 6) Compare Aristotle’s and Democritus’s ideas about Matter Why was Aristotle held to be correct? List 3 ideas from Dalton’s Atomic Theory Who discovered electrons? Draw a model of Thomson’s atom. What allowed John Dalton to verify his atomic theory? 7) 8) 9) 10) 11) 12) Why did the existence of electrons account for the existence of protons? Describe Rutherford’s experiment thoroughly Explain Rutherford’s model of the atom What did Millikan discover? What do opposite charges do to each other? What do like charges do to each other? Chemistry February 26, 2007 1) 2) 3) 4) 5) Review for Quiz on Factor Label Quiz on Factor Label Notes on History of Atomic Theory HW: WS 4.1 Turn in Vocabulary in box now, please Chemistry Unit 4: Atomic Structure Spring 2007 1st, 2nd, & 4th Periods Answer the following questions in your notes 1) Write your definition of an atom 2) If you could see atoms, sketch (or describe) a model of the atom Chapter Objectives 1. Discuss early theories about matter 2. Discuss the discovery of subatomic particles 3. Use terms such as mass number, atomic number, atomic mass to describe elements Early Theories About Matter Democritus (460-370 BC) Greek Philosopher Around 420 BC, he proposed that matter was discontinuous There was some point at which matter could NOT be divided further Early Theories About Matter Aristotle (384-322 BC) Around 360 BC, he proposed matter was continuous Matter could be subdivided indefinitely without ever reaching a limit He believed there was no ultimate underlying structure to matter Early Theories About Matter Aristotle’s hypothesis was held to be correct In ancient Greece, hypotheses were not tested Hypotheses were accepted or rejected based on the reputation of the philosopher Early Theories About Matter In the 1500s and 1600s, a basic change in the way science was done occurred. Hypotheses were now TESTED! Development of the Modern Atomic Theory • In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. • He observed that the mass of reactants in the container before a chemical reaction was equal to the mass of the products after the reaction. Development of the Modern Atomic Theory • Lavoisier concluded that when a chemical reaction occurs, mass is neither created nor destroyed but only changed. • Lavoisier’s conclusion became known as the law of conservation of mass. Development of the Modern Atomic Theory Click box to view movie clip. Development of the Modern Atomic Theory • In 1799, another French chemist, Joseph Proust, observed that the composition of water is always 11 percent hydrogen and 89 percent oxygen by mass. • Regardless of the source of the water, it always contains these same percentages of hydrogen and oxygen. H H O Development of the Modern Atomic Theory • Proust studied many other compounds and observed that the elements that composed the compounds were always in a certain proportion by mass. This principle is now referred to as the law of definite proportions. H H O Dalton’s Atomic Theory • John Dalton (17661844), an English schoolteacher and chemist, studied the results of experiments by Lavoisier, Proust, and many other scientists. Dalton’s Atomic Theory John Dalton conducted various experiments and formulated his atomic theory. He found that: 1) All matter is composed of extremely small particles called atoms Dalton’s Atomic Theory 2) Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties 3) Atoms cannot be subdivided, created, or destroyed Dalton’s Atomic Theory 4) Atoms of different elements can combine in simple, whole number ratios to form chemical compounds 5) And finally, Dalton found that in chemical reactions, atoms are combined, separated, or rearranged Dalton’s Atomic Theory Dalton was able to develop his theory based on the invention of the chemical balance Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that it has remained essentially intact up to the present time. Copy & Answer in Notes Answer the following questions on a separate sheet of paper 1) Compare and contrast Democritus’s and Aristotle’s early theories about the atom 2) Which philosopher (Democritus or Aristotle) was correct {Hint: Look at Dalton’s Atomic Theory} Questions Continued 3) On what basis did Aristotle and Democritus propose hypotheses about the nature of matter? 4) How did Dalton’s Atomic Theory differ from Democritus’s or Aristotle’s Theories? Homework Construct & complete a chart with the following 1) Individuals Who Proposed Various Atomic Theories 2) Approximate Date Theory Proposed 3) Theory 4) Description of Experiment Used 5) Any discoveries made 6) Other important information Discovery of the Electron Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. • In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate. JJ Thomson’s Experiment JJ Thomson (1856-1940)-British Physicist Credited with discovery of electrons in 1897 Reasoned that negative particles resided in the atom He called these particles electrons The Electron • Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. • In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate. • Thomson’s experiments used a vacuum tube. The Electron • A vacuum tube has had all gases pumped out of it. • At each end of the tube is a metal piece called an electrode, which is connected through the glass to a metal terminal outside the tube. • These electrodes become electrically charged when they are connected to a high-voltage electrical source. Cathode-Ray Tube • When the electrodes are charged, rays travel in the tube from the negative electrode, which is • Because these rays originate at the the cathode, to the cathode, they are positive electrode, called cathode rays. the anode. Video of Thomson’s Experiment Click box to view movie clip. Cathode-Ray Tube • Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate. • He knew that objects with like charges repel each other, and objects with unlike charges attract each other. Cathode-Ray Tube • Thomson concluded that cathode rays are made up of invisible, negatively charged particles referred to as electrons. • These electrons had to come from the matter (atoms) of the negative electrode. Animation of Thomson’s Experiment http://www.dlt.ncssm.edu/core/Chapter3Atomic_Str_%20Part1/Chapter3Animations/Canal_Ray-CRT.html He reasoned that since atoms are NEUTRAL, there must be some positive charge to balance out this negative charge He proposed a dense cloud of positive charge with the negative charge randomly embedded within the cloud His model is called the “plum pudding model” Similar to a pepperoni pizza model Thomson’s Plum Pudding Model The dense cloud of positive charge Electrons Thomson’s Experiment 1) What did he discover? 2) In what year did he make his discovery? 3) Adequately describe the experiment that he used. 4) How did his discovery support or disprove Dalton’s atomic theory? Thomson’s Experiment 5) Why were the electrons in the cathode tube attracted to the positive plates? 6) Draw a model of what Thomson believed the atom looked like. Millikan’s Oil Drop Experiment Robert Millikan (1868-1953) American scientist, University of Chicago Discovered the charge on an electron using the oil drop experiment Animation of Millikan’s Experiment http://www.dlt.ncssm.edu/core/Chapter3Atomic_Str_%20Part1/Chapter3Animations/OilDrop.html Millikan’s Experiment Liquid goes through an atomizer to produce droplets These droplets enter a chamber The bottom of the chamber has a hole in the center The droplets fall through the hole due to gravity An electrical field is produced using electricity This field offsets the effect of gravity causing the droplets to be suspended in mid air Through a series of known relationships and mathematical equations, Millikan calculates the charge of the electron in Coulombs. Thomson’s and Millikan’s work allowed for the mass of the electron to be determined using known relationships and equations Discovery of Neutron James Chadwick (1891-1974) is credited with the discovery of the neutron in 1932 Rutherford’s Model of the Atom Ernest Rutherford (1871-1937) from New Zealand Thomson’s atomic model had not yet been tested due to the tiny size of the atom Rutherford’s Model of the Atom Rutherford had been studying radioactivity In 1911, he bombarded a thin sheet of metal with alpha particles Alpha particles are positively charged The Gold Foil Experiment http://micro.magnet.fsu.edu/electromag/java/r utherford/ Video of Rutherford’s Experiment Click box to view movie clip. If Thomson’s “plum pudding model” was correct, he expected the alpha particles to crash through the gold foil (with only minor deflections) He placed a screen around the foil. After the alpha particles passed through the gold foil, they would light up the screen. Most of the particles should pass through unaffected Rutherford’s Results Some of the alpha particles passed through and hit the screen as predicted BUT a high number of alpha particles were deflected. This indicated that there must be something in the center of the atom that caused deflection Rutherford’s Conclusion He reasoned that a positive charge center rested in the atom He called this center the nucleus Thomson’s Movie Video of Rutherford’s Experiment Click box to view movie clip. Rutherford’s Experiment Click box to view movie clip. Neils Bohr’s Model His model is called the “solar system”model The center of the atom contains protons and neutrons This area is collectively called the nucleus The electrons orbit around the nucleus like the planets do the sun Question (copy and answer in your notes) The planets orbit around the sun due to gravity. Why do electrons orbit the nucleus? The Three Subatomic Particles After various molecular models of the atoms had been tested, it was determined that three subatomic particles made up the atom Protons Neutrons Electrons Protons Protons are found in the nucleus Protons have an actual charge of +1.6 x 10-19 C and a relative charge of +1 The actual mass of a proton is 1.67 x 10-24 g The relative mass of a proton is 1 atomic mass unit (amu) The symbol is p+ Neutrons Neutrons are found in the nucleus Neutrons have an actual charge of 0 C and a relative charge of 0 The actual mass of a neutron is 1.67 x 10-24 g The relative mass of a neutron is 1 atomic mass unit (amu) The symbol is n0 Electrons Electrons are found outside the nucleus Electrons have an actual charge of -1.6 x 10-19 C and a relative charge of -1 The actual mass of an electron is 9.11 x 10-28 g Electrons The relative mass of a electron is 1/1840 atomic mass unit (amu) The electron has a very small mass The symbol is e Nuclear Forces Neutrons exert an attraction toward protons and other neutrons Overall the forces produce enough attraction to keep protons relative close together in the nucleus Construct and complete a chart with the following information List the particles of the atoms List the particles’ location in the atom List the relative and actual charge of the particle List the relative and actual mass of the particles List the symbol of the particle Atomic Number Each element has a certain number of protons in its nucleus The number of protons in the nucleus is called the atomic number Each element has its own atomic number because each element has its own, unique number of protons An element has certain chemical properties based on the number of protons in its nucleus Elements are numbered in the periodic table based on the atomic number Example: If an element has two protons, what is its atomic number? What is the identity of the element? On Your Own Which element has a) 12 protons b) 35 protons c) 50 protons d) 92 protons Since atoms are neutral, the number of protons must equal the number of electrons Example: If an element (atom) has 7 electrons, how many protons does it have? What is the elements identity? On Your Own: Determine the number of protons in the following atoms as well as each atom’s identity a) 6 electrons b) 14 electrons c) 72 electrons d) 55 electrons Mass Number Mass Number: the number of protons and neutrons in an atom added together Mathematically Mass Number = Protons + Neutrons Example What is the mass number of an atom with 16 protons and 16 neutrons? On Your Own Determine the Mass Number for the following atoms a) 17 protons and 18 neutrons b) 11 protons and 12 neutrons c) 1 proton and NO neutrons d) 3 protons and 4 neutrons Nuclide Symbols Mass Numbers are written in the upper left preceding the chemical symbol Atomic Number is written directly under the mass number Example: Write the correct nuclide symbol for an element with 79 p+ and 118 n0 Mass 197 Element Number Symbol 79 Au Atomic Number Copy and Answer in Notes 1. List the three subatomic particles, their relative charge, relative mass, and location 2. What does atomic number mean? 3. What atom has an atomic number of 32? 4. Determine the number of protons and electrons in a. Ne b. Poc. Pt Review-Complete the chart Element Atomic Mass p+ # # O n0 e- 17 53 74 235 92 Isotopes Atoms of the same elements have the same number of protons HOWEVER there may be different numbers of neutrons Isotopes When an element’s atom has different numbers of neutrons, it is said to have isotopes Isotopes are common in nature Hydrogen’s Three Isotopes Hydrogen has the following isotopes: Protium-a hydrogen atom with one proton and NO neutrons Hydrogen’s Three Isotopes Deuterium-a hydrogen atom with one proton and only one neutron Tritium-a hydrogen atom with one proton and two neutrons On Your Own Determine the mass and atomic numbers for each of hydrogen’s isotopes. Put this information into nuclide symbol form Average Atomic Mass Atomic mass is the mass of an atom expressed in atomic mass units or amu The atomic mass unit is based in relation to the standard of carbon-12 Average Atomic Mass Continued Carbon-12 has a mass of 12.000 00 amu If an atoms weighs half as much as carbon-12, its atomic mass will be 6.000 00 amu If an atom weighs four times as much as carbon-12, it will have a mass of 48.000 00 amu The atomic mass that is reported in the periodic table is a weighted average based on the relative abundance of each element Review - Answer in Notes 1. What mass would an atom have that has a mass equal to 1/12 that of carbon-12? What element is this? 2. Determine the mass each atom would have if: a. It weighed 12 times as much as C-12 b. It weighed 3/17 as much as C-12 3. How do all isotopes differ from each other? Relative abundance refers to how commonly the isotope occurs in nature Certain isotopes will occur more commonly than other isotopes To Determine Avg. Atomic Mass 1) First convert relative abundance (%) to decimal equivalent 2) Multiply mass (in amu) by decimal equivalent 3) Add the numbers together 4) The sum (in amu) is the average atomic mass Copy & Answer in Notes 1. What is an isotope? 2. How is mass number different from atomic number? 3. How is relative (%) abundance determined? For example, an element has two naturally occurring isotopes. One isotope has a relative abundance of 19.91% and a mass of 10.012 amu. A second isotope has a relative abundance of 80.08% and a mass of 11.009 amu. Calculate the atomic mass Additional Example Calculate the average atomic masses for the following: Isotope: Rel. Abund. Rel. Mass hydrogen-1 99.985% 1.007 825 hydrogen-2 0.015% 2.0140 Ans) 1.008 amu Copy & Answer in Notes 3. Determine Avg. Atomic Mass for oxygen: Isotope Rel. Abund. Actual Mass O-16 99.762 15.994 915 O-17 0.038 16.999 13 O-18 0.200 17.999 160 Isotope rel. abund. (%) C-12 98.90 1.10 U-235 0.720 U-238 99.280 O-16 99.762 O-17 0.038 O-18 0.200 b. 13 c. d. rel. mass (in amu) 12.0 C13.003 355 235.043 238.050 78 15.994 915 16.999 131 17.999 160