Section 4 Parts Of The Atom: Electrons And The Nucleus
Fun with the Periodic TableSection 4Parts of the Atom: Electrons and the NucleusWhat Do You See?Do not try thisat home. It mayruin your TV.Learning OutcomesWhat Do You Think?In this section you willEver since Democritus from ancient Greece hypothesized the existenceof atoms, a major question was how atoms of different elements weredifferent. Observe or learn the behaviorof a cathode ray in thepresence of a magnet. Discuss Thomson’s conclusionsfrom 1897 about cathode rays. Simulate an experiment from1911 by Rutherford in whichhe learned more about thestructure of atoms. Organize your understandingof some of the differentparticles that comprise matter. If you could observe a single atom of gold and a single atom of lead,how do you think they would be different? What might they have incommon?Record your ideas about these questions in your Active Chemistrylog. Be prepared to discuss your responses with your small groupand the class.Investigate1. Your teacher will demonstrate the behavior of what were calledcathode rays 100 years ago. They were called cathode rays becausethey were emitted from the negative terminal, or cathode of whatwas known as a cathode-ray tube. This tube was a forerunner ofthe television and the computer monitor. Alternatively, your teachermay show you a video demonstrating the effect of a magnetic fieldon a cathode-ray tube.120
Section 4 Parts of the Atom: Electrons and the Nucleusa) What happens to the path of thecathode rays when a horseshoemagnet is placed near the tube?Record your observation in yourActive Chemistry log.b) Record what happens to thepath of the cathode rays whenthe orientation of the horseshoemagnet is reversed.2. For the particles that make up thecathode rays to change direction,the magnet must be exerting aforce on these electrically chargedparticles. In 1897, Joseph John (J. J.)Thomson showed that the cathoderays were made of electrons.Electrons are negatively charged,tiny particles. He also discoveredthat identical electrons were emittedregardless of the metal of which thecathode was made.Discovery of electrons emergingfrom the atoms of any metal gavescientists new information about theatom. The atom is divisible. It hasinternal parts, one of which is theelectron.a) In a sentence or two, describethe relationship between cathoderays, the electron, and thestructure of atoms.3. To investigate the other componentsof an atom, you will take part in thefollowing simulation. It is similar tothe game Battleship. You will workwith a partner for this investigation.You and your classmate shouldeach construct an 8-by-10 grid ofsquares as shown on the followingpage. You can label the columns ofthe grid with letters A, B, C Labelthe rows of the grid 1, 2, 3 Without letting your classmate seeyour grid, color in a section of10 squares. The squares must toucheach other. To make the simulationrelatively simple, begin with acompact design. This shape (coloredregion) represents your target.Safety gogglesand an apronmust be wornat all times in achemistry lab.Always usethe minimumvoltage andcurrentnecessary tooperate theCrooke’s tubeto limit thehazard of X-rayproduction.In addition,maximizedistance andminimizetime for thedemonstration.You and your partner will try toguess the shape of each other’starget by sending “missiles” ontoany of the 80 squares in this array.For the purpose of this description,designate one person to be Player Xand the other person to be Player Y.To begin, Player X will tell Player Ythe destination (number and letter)of the missile being sent. Player Ywill respond, indicating that themissile “hit” or “missed” the targetshape. Player X will make noteof the response. Then Player Y121Active Chemistry
Fun with the Periodic Tablesends the next missile, noting theresponse. Continue this processuntil one player identifies the otherplayer’s target.a) Record the number of turns takento complete the game.b) Repeat the game with atarget of only two adjacentsquares. Record the numberof turns taken.4. Now do a thought experiment.The same-size game grid is dividedinto smaller squares. Supposethere are 100 squares across and100 squares down. There are now10,000 squares in the same sizeboard as before. A target of onlyone square is chosen.a) Record an estimate of how manyturns will be required to identifythe target square amongst the10,000 squares in the game grid.5. Now modify the thoughtexperiment. The same-size grid isnow 1000 rows across and 1000squares down. That is 1,000,000squares.ABDEFG12345678In Rutherford’s experiment, he wasrequired to send an incredibly largenumber of “missiles” to get a “hit.”He concluded that the grid of theatom must be composed of very tinycells and only one cell contains allof the positive charge of the atom.a) In your Active Chemistry log,explain why you thinkhe concluded this.a) Record an estimate of how manyturns will be required to identifythe target square among the1,000,000 squares in the grid.6. In 1911, Lord Ernest Rutherfordconducted an experiment similar toyour game of Battleship. Rutherfordsought to learn something aboutthe structure of the atom bybombarding gold atoms withenergetic particles given off bycertain atoms.122Active ChemistryCHIJ
Section 4 Parts of the Atom: Electrons and the NucleusChem TalkModels in ScienceIn science, it often is necessary to develop models to investigateand explain phenomena. A model is a representation of somethingin the real world. Scientists use models to investigate things thatare too big, too distant, too small, too unwieldy, or too unsafe toobserve or test directly. To use a model, the model needs to besimilar to the real world in ways that are important for what thescientist is investigating.In this section, you will begin to read about the different types ofmodels of an atom that scientists have developed over the years.Models are extremely important when trying to understand thestructure of an atom because atoms are much too small to see.It is important to remember, however, that models have theirlimitations. They are not able to duplicate the real world. As youwork your way through Active Chemistry, look for ways that modelscan help you understand and explain phenomena. Also, be aware ofthe each model’s limitations.THE CHANGING MODEL OF AN ATOMJ. J. Thomson’s Model of an AtomChem Wordssubatomic particles: thethree kinds of particlesthat make up atoms:protons, neutrons, andelectrons.electron: the negativelycharged subatomicparticles of an atom.As was noted in this investigation, in the late 1800s J. J. Thomson, anEnglish physicist, found evidence for the existence of negatively chargedparticles that could be removed from atoms. He called these subatomicparticles with negative charges electrons. Using this new information,Thomson proposed a model of an atom. This model was a positivesphere, with electrons evenly distributed and embedded in it, as shownin diagram (a). Using the same evidence, Hantaro Nagaoka, a Japanesescientist, modeled the atom as a large, positively charged spheresurrounded by a ring of negative electrons, as shown in diagram (b).These models show that scientists agreed that atoms contain electrons.They also agreed that atoms were electrically neutral. To maintain thiselectric neutrality, an atom must contain an equal number of positiveand negative charges.Millikan Determines the Mass and Charge of an ElectronThomson was able to measure only the ratio of mass-to-charge for anelectron, not the mass itself. The American physicist Robert A. Millikandevised a method for determining the mass of an electron. His famousexperiment is called the Millikan oil-drop experiment.123Active Chemistry
Fun with the Periodic TableFirst, Millikan produced smalldroplets of oil by spraying themfrom a nozzle into a box. The tinymist of negatively charged dropletswas allowed to fall between twocharged metal plates. It was possibleto stop the drops from falling byhaving the plates exert an upwardelectrical force on the drops.Millikan was able to calculatethe charge of an electron as1.6 10–19 C (coulombs). He thenused Thomson’s charge-to-massratio to determine the mass of an–28electron to be 9.10 10 g. This is about 1/2000 of the mass of aproton. Thomson remarked that these electrons were very, very tiny.Rutherford’s Discovery of the NucleusChem Wordsalpha particle: heliumions that are positivelycharged.For several years there was no evidence tocontradict either Thomson’s or Nagaoka’s atomicmodels. However, in the early 1900s, ErnestRutherford, a New Zealand–born scientist,designed experiments to test the currentmodel of an atom. In Rutherford’s experiment,positively charged alpha particles were sentas “missiles” toward a thin sheet of gold. Goldwas used because it is malleable and could beErnest Rutherfordhammered into a very thin sheet. Most of thepositively charged alpha particles went through the sheet and were notdeflected. It is as if they missed the target. This was expected sinceit was assumed that the atom’s charge and mass was spread evenlythroughout the gold. Some of the alpha particles were deflected slightly.However, most interesting to Rutherford was that occasionally one ofthe alpha particles “hit” the gold sheet and bounced straight back at thesource. This was unexpected. The conclusion was that there must betiny places containing lots of charge and mass. Since the bouncing backwas so unusual, it was assumed that the places where all the charge andmass were concentrated were only 1/100,000 of the area of the gold.Rutherford concluded that almost all the mass and all of the positive124Active Chemistry
Section 4 Parts of the Atom: Electrons and the Nucleuscharge of the atom is concentrated in an extremely small part at thecenter. He called this center part the nucleus. He used the termproton to name the smallest unit of positive charge in the nucleus.The story of Rutherford’sdiscovery of the atomic nucleus isbest told by Rutherford himself.Examining the deflection of highspeed alpha particles as theypassed through sheets of goldfoil, Rutherford and his studentHans Geiger noticed that someparticles were scattered throughlarger angles than predicted bythe existing theory of atomicstructure. Fascinated, Rutherfordasked Geiger’s research studentErnest Marsden to search for morelarge-angle alpha scattering. Rutherford did not think that any of thealpha particles in his experiment would actually bounce backward. “ThenI remember two or three days later Geiger coming to me in greatexcitement and saying, ‘We have been able to get some of the alphaparticles coming backwards . . .’ It was quite the most incredible eventthat has ever happened to me in my life. It was almost as incredible as ifyou fired a 15-inch shell (a missile) at a piece of tissue paper and it cameback and hit you.”A Physics ConnectionWhat was responsible for the wide-angle scattering of the alpha particlesand their bouncing back? Well, the major forces involved in chemistryare electrical. They are based on the physics associated with Coulomb’slaw of electrostatics.Chem Wordsnucleus: the very smalldense region in thecenter of an atom thatcontains all the positivecharge and most of themass.proton: the positivelycharged subatomicparticle contained in thenucleus of an atom.The force between two charged particles (q1 and q2) is inverselyproportional to the square of the distance (d) between them.dq1q2where k is a constant 9.0 109N i m2/C2kq1q2F d2125Active Chemistry
Fun with the Periodic TableIf the particles are of opposite charge, then the force is attractive. If theparticles are of the same charge, then the force is repulsive. Oppositecharges attract, and like charges repel. The closer the positively chargedalpha particle gets to the positively charged nucleus, the larger the force.This causes a larger deflection of the alpha particle.In this investigation, you learned about experimental evidence for theexistence of electrons and a nucleus. Think back to when you first heardabout electrons and the nucleus. Was it in elementary school? Often,theories about the structure of the atom are presented as facts withoutexplaining any of the evidence that led to the theory. Science involvesthe accumulation of evidence and the building of a model and theory. Itis not memorizing a set of facts.What Is the Difference Between a Law and a Theory?Sometimes a word’s scientific meaning is different from its everydaymeaning. For example, you may say that you have a theory aboutsomething when you mean you have a hunch or a guess. You alsomay say that you obey the law, referring to laws that are writtenand passed by a government. However, in science, the words theoryand law have very different, very precise meanings.In their observations over long periods of time, scientists oftennotice certain patterns that occur over and over again. Fromthese observations, they develop a scientific law to describe thepatterns they have observed. For example, over time, scientistsobserved that the force between two charged particles is inverselyproportional to the square of the distance between them. Coulombstated this as the law of electrostatics. This law does not explainwhy all charged particles experience this force, but provides a wayof describing what has been observed.Checking Up1. What is an electron?2. Why was Rutherfordsurprised that somealpha particlesbounced back from thegold foil?A scientific theory provides an explanation for those observationsand can be used to predict new phenomena that have not beenobserved. For example, the atomic theory explains the nature ofmatter. This theory is not a hunch or a guess. It is the result of yearsof experiments, observations, measurements, and mathematicalapplications. The work of many people over many years culminatesin a theory and the subsequent work of many people and manyyears is required for acceptance of that theory in the sciencecommunity. With more research scientists may develop newertheories to replace older theories. As you explore more laws andtheories in Active Chemistry, be aware of the difference betweena law and a theory. Notice the process by which the community ofscientists develop and accept new theories. Understand that lawsdo not become theories and theories do not become laws.3. What is the nucleus ofan atom?126Active Chemistry
Section 4 Parts of the Atom: Electrons and the NucleusWhat Do You Think Now?At the beginning of the section you were asked the following: If you could observe a single atom of gold and a single atom of lead, how do youthink they would be different? What might they have in common?Compare and contrast an atom of gold and an atom of lead using what you learnedin this section.ChemEssential QuestionsWhat does it mean?Chemistry explains the macroscopic phenomenon (what you observe)with an explanation of what happens at the nanoscopic level (atoms andmolecules) using symbolic structures as a way to communicate. Completethe chart below in your Active Chemistry log.MACRONANOSYMBOLICDescribe yourobservations of thecathode rays and theirinteraction with amagnet.Chemistry uses the structure of the atomto describe reactions of one materialwith another. What explanations at theatomic level did Thomson and Rutherfordeach make based on their evidence?In chemistry, you create symbolicstructures. How could you build a modelthat shows a tiny positively chargednucleus that takes up only 1/100,000 ofthe total space?How do you know?What evidence do you have for the existence of a nucleus in the atom?Why do you believe?The electrons in a cathode ray are also used to create a picture on yourTV screen and your computer monitor. How could you test to see ifthose electrons behave similarly to those in the cathode-ray tube?Why should you care?Do not carryout your ideawithout yourteacher’spermission.You can damagethe TV.The periodic table will be a symbolic structure that summarizes muchof what you know about atoms. What three things would you want theplayers of your periodic table game to know at the end of the game?Reflecting on the Section and the ChallengeIn this section, you learned of evidence that atoms are made of a positively charged nucleusand negatively charged electrons. Mendeleev’s periodic table of the elements providedinsights into the structure of matter and atoms. Your periodic table game will also revealinformation about the structure of the atom and the role of evidence in creating models.Chem to Go1. Since the electron has a negative electric charge and the nucleus has a positiveelectric charge, where would you expect to find electrons in atoms?2. Are atoms indivisible? Support your answer using information from this section.3. Construct a chart or diagram to summarize what you have learned in thissection about the particles that make up an atom. Include electric charge andlocation of the particles.127Active Chemistry
Fun with the Periodic Table4. Lead has an atomic number (the number of protons) of 82; iron has an atomicnumber of 26; and copper has an atomic number of 29. How do the charges ofthe nuclei of these three elements compare?5. An atom is neutral and an electron has a charge of –1.6 10 –19 C.What is the charge of a proton? Explain why you chose this value.6. Sketch the outside outline of three grids. Pretend that each grid has100,000 squares.a) If the target was 50,000 squares, draw the target.b) If the target was 25,000 squares, draw the target.c) If the target was only 1 square, draw the target.d) Which grid most closely relates to the nucleus found in Rutherford’sexperiment? Explain your answer.7. Cathode rays (electrons) originate at the cathode (negative) terminal.They move in a straight line.a) What will happen when you bring a magnet near the cathode ray?b) How can you get it to deflect it in the opposite direction?c) If the cathode-ray beam travels between a positive and a negative plate,which plate will the cathode rays be attracted toward?8. In Millikan’s oil-drop experiment, you wish to suspend the negatively chargedoil drop. Since gravity pulls the oil drop down, should the negative plate beplaced above or below the negatively charged oil drop?Inquiring Further1. An atomic timelineConstruct a timeline that reflects how scientists’ views of the atom have changedthrough the ages. Identify significant scientists, their beliefs, and experimentalfindings as mentioned in this chapter. You may also wish to consult other resources.Add information to your timeline as you continue to work through this chapter.2. Avogadro’s number and a moleChemists are interested in keeping track of quantities of particles. However, theparticles are very small so chemists use a particular quantity that is convenient forcounting particles. The quantity is called a mole. The quantity of particles in a moleis 602,000,000,000,000,000,000,000. The mole can be represented more easily inscientific notation as 6.02 1023. This is a very large number because many, manysmall particles (atoms or molecules) make up a mole. The number 6.02 1023 issometimes called Avogadro’s number.Research to find the significance of Avogadro’s number and a mole. Record yourfindings in your Active Chemistry log.Then, to appreciate how huge a mole is, answer the following question:Imagine there are 7 billion people (7 109 people, which is approximately the humanpopulation of the world) and they are given the task of dropping 1 bills once everysecond into a large hole. How long will it take 7 109 people to drop one mole ofdollar bills into the hole? How old will you be when they complete this task?128Active Chemistry
2. For the particles that make up the cathode rays to change direction, the magnet must be exerting a force on these electrically charged particles. In 1897, Joseph John (J. J.) Thomson showed that the cathode rays were made of electrons. Electrons are negatively charged, tiny particles.
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Le genou de Lucy. Odile Jacob. 1999. Coppens Y. Pré-textes. L’homme préhistorique en morceaux. Eds Odile Jacob. 2011. Costentin J., Delaveau P. Café, thé, chocolat, les bons effets sur le cerveau et pour le corps. Editions Odile Jacob. 2010. 3 Crawford M., Marsh D. The driving force : food in human evolution and the future.
