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Key Objectives 4.2.1 IDENTIFY three types of subatomic particles. 4.2.2 DESCRIBE the structure of atoms according to the Rutherford atomic model. CHEMISTRY Y U YO &YOU Additional Resources Q: You can X-ray a person’s hand to see inside it—but how can you see inside an atom? You may have seen X-rays like the one of the hand shown here. Doctors often use X-rays to see bones and other structures that cannot be seen through the skin. Scientists tried to figure out what was inside an atom without being able to see inside the atom. In this lesson, you will learn the methods scientists used to “see” inside an atom. Reading and Study Workbook, Lesson 4.2 Available Online or on Digital Media: • Teaching Resources, Lesson 4.2 Review • Laboratory Manual, Lab 5 • Virtual Chemistry Laboratory Manual, Labs 4-6 Subatomic Particles What are three kinds of subatomic particles? Key Questions What are three kinds of subatomic particles? How can you describe the structure of the nuclear atom? Vocabulary telectron tcathode ray tproton tneutron tnucleus Much of Dalton’s atomic theory is accepted today. One important change, however, is that atoms are now known to be divisible. They can be broken down into even smaller, more fundamental particles, called subatomic Three kinds of subatomic particles are electrons, protons, particles. and neutrons. Electrons In 1897, the English physicist J. J. Thomson (1856Ľ1940) discovered the electron. Electrons are negatively charged subatomic particles. Thomson performed experiments that involved passing electric current through gases at low pressure. He sealed the gases in glass tubes fitted at both ends with metal disks called electrodes. The electrodes were connected to a source of electricity, as shown in Figure 4.4. One electrode, the anode, became positively charged. The other electrode, the cathode, became negatively charged. The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode. Gas at very low pressure á ź Metal disk (cathode) Cathode ray (electrons) & CHEMISTRY Y YO YOU U Have students study the opening photo and read the accompanying text. Ask How could you determine what your hand looks like under the skin without dissecting it? (X-rays, CT scans, or MRI scans) Explain that scientists use technology to “see” inside atoms, just as doctors use it to “see” inside the body. Build Background High voltage Figure 4.4 Cathode-Ray Tube In a cathode-ray tube, electrons travel as a ray from the cathode (Ź) to the anode (à). Televisions used to be made with a specialized type of cathode-ray tube. Engage Ask Do you have an electronic item in your home that uses a CRT? (Sample answers: a television or a computer monitor) Explain that television and computer CRTs have a source of rays at the small end. The rays are projected on the large receiving end to create the display. Metal disk (anode) Vacuum pump Atomic Structure 105 Focus on ELL National Science Education Standards A-1, A-2, B-1, B-4, E-2, G-2, G-3 1 CONTENT AND LANGUAGE Write the term subatomic on the board. Explain that the prefix sub- means “below” or “a part of”. Have students brainstorm the literal meaning of subatomic. (below atomic) Now write the words electron, proton, and neutron on the board. Explain that in these three words, the suffix –on means “subatomic particle”. 2 FRONTLOAD THE LESSON Have students preview Figure 4.4 and identify any similarities and differences between the parts of a cathode ray tube and parts of a battery. Explain that, just as a battery generates a stream of electrons that flow through an electrical circuit, a cathode ray generates a stream of electrons that travel as a beam. 3 COMPREHENSIBLE INPUT Draw Table 4.1 on the board, using a contrasting color to write the mathematical signs in the Symbols column. Use the same color to write the values of the relative charges. Point out the relative mass of an electron, and ask students to locate the text where this figure is referenced. Atomic Structure 105 LESSON 4.2 4.2 Structure of the Nuclear Atom LESSON 4.2 Thomson found that a cathode ray is deflected by electrically charged metal plates, as in Figure 4.5a. A positively charged plate attracts the cathode ray, while a negatively charged plate repels it. Thomson knew that opposite charges attract and like charges repel, so he hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed. Thomson called these particles corpuscles; later they were named electrons. To test his hypothesis, Thomson set up an experiment to measure the ratio of an electron’s charge to its mass. He found this ratio to be constant. Also, the charge-to-mass ratio of electrons did not depend on the kind of gas in the cathode-ray tube or the type of metal used for the electrodes. Thomson concluded that electrons are a component of the atoms of all elements. The U.S. physicist Robert A. Millikan (1868Ľ1953) carried out experiments to find the quantity of an electron’s charge. In his oil-drop experiment, Millikan suspended negatively charged oil droplets between two charged plates. He then changed the voltage on the plates to see how this affected the droplets’ rate of fall. From his data, he found that the charge on each oil droplet was a multiple of 1.60 × 10Ź19 coulomb, meaning this must be the charge of an electron. Using this charge value and Thomson’s charge-to-mass ratio of an electron, Millikan calculated an electron’s mass. Millikan’s values for electron charge and mass are similar to those accepted today. An electron has one unit of negative charge, and its mass is 1/1840 the mass of a hydrogen atom. Foundations for Reading BUILD VOCABULARY Have students think of words that start with the same word parts as the three subatomic particles described in this section: neutron, electron, and proton. (Sample answer: neutral, electric, protein) Words that students already know may help them remember the meaning of new terms. Explore Teacher Demo PURPOSE To demonstrate a cathode-ray tube and observe properties of cathode rays MATERIALS cathode-ray tube, magnet PROCEDURE Demonstrate a cathode-ray tube in class. Use a magnet to deflect the beam of particles. Review the components of a cathoderay tube, and discuss the connection to television picture tubes and computer monitors. EXPECTED OUTCOME Students should be able to explain how the CRT works and see how the cathode ray is deflected by a magnetic field. Figure 4.5 Thomson’s Experiment a. Thomson found that cathode rays are attracted to metal plates that have a positive electrical charge. b. A cathode ray can also be deflected by a magnet. Infer If a cathode ray is attracted to a positively charged plate, what can you infer about the charge of the particles that make up the cathode ray? ET KIN IC ART High voltage a Positive plate Slit – + See cathode-ray ttubes animated online. Explain Cathode Vacuum pump Negative plate Anode Subatomic Particles b USE VISUALS Have students study Table 4.1, and compare the masses and charges of the three elementary particles. Students should recognize from this comparison that the mass of an atom is due mainly to the number of protons and neutrons that it has. Point out that the assigned charges for protons and electrons are relative charges. Ask What particles make up most of the mass of an atom? (protons and neutrons) Ask Why are relative charges and mass useful in talking about subatomic particles? (The actual values are unwieldy numbers.) The absolute charge on an electron is 1.602177 ⫻ 10⫺19 coulombs. 106 $IBQUFSt-FTTPO Differentiated Instruction LPR LESS PROFICIENT READERS Direct students to use Figures 4.4 and 4.5 to describe J.J. Thomson’s experiment with cathode-ray tubes in their own words. L1 SPECIAL STUDENTS Prior to explaining subatomic particles, divide students into small groups. Give each group pair of inflated rubber balloons (be aware of any latex allergies in the class) and a swatch of wool cloth. Have students rub the balloons vigorously with the wool. Have students test the balloons against different objects and against each other to discover how opposite charges attract and like charges repel. Use this activity as a lead in to Thompson’s cathode ray experiment. L3 ADVANCED STUDENTS Have students use the Internet or library to find the original papers for the discoveries described in this chapter and write a report on what they have learned. 106 Chapter 4 • Lesson 2 Properties of Subatomic Particles Particle Electron Symbol Relative charge Relative mass (mass of proton â 1) Actual mass (g) eŹ 1Ź 1/1840 9.11 ñ 10Ź28 à Proton p 1à 1 1.67 ñ 10Ź24 Neutron n0 0 1 1.67 ñ 10Ź24 Explore Teacher Demo PURPOSE To trace the history of atomic models, and examine the role of the scientific method in the development of such models MATERIALS Library or Internet access PROCEDURE Have students create a timeline that traces the development of the atomic model. Have them note the data that led to an existing model being changed. EXPECTED OUTCOME Students’ timelines should list at least some of the atomic models shown on page 133. Students should be able to explain how certain scientific discoveries (e.g., Rutherford’s gold foil experiment) resulted in the revision of the prevailing atomic model at the time. Protons and Neutrons If cathode rays are electrons given off by atoms, what remains of the atoms that have lost the electrons? For example, after a hydrogen atom (the lightest kind of atom) loses an electron, what is left? You can think through this problem using four simple ideas about matter and electric charges. First, atoms have no net electric charge; they are electrically neutral. (One important piece of evidence for electrical neutrality is that you do not receive an electric shock every time you touch something!) Second, electric charges are carried by particles of matter. Third, electric charges always exist in whole-number multiples of a single basic unit; that is, there are no fractions of charges. Fourth, when a given number of negatively charged particles combines with an equal number of positively charged particles, an electrically neutral particle is formed. Considering all of this information, it follows that a particle with one unit of positive charge should remain when a typical hydrogen atom loses an electron. Evidence for such a positively charged particle was found in 1886, when Eugen Goldstein (1850Ľ1930) observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He called these rays canal rays and concluded that they were composed of positive particles. Such positively charged subatomic particles are called protons. Each proton has a mass about 1840 times that of an electron. In 1932, the English physicist James Chadwick (1891Ľ1974) confirmed the existence of yet another subatomic particle: the neutron. Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton. Table 4.1 summarizes the properties of these subatomic particles. Although protons and neutrons are exceedingly small, theoretical physicists believe that they are composed of yet smaller subnuclear particles called quarks. Extend Explain to students that by 1887, the British scientist William Crookes knew that metal atoms contained negatively charged particles. He used a cathode ray tube containing hydrogen gas at low pressure, and discovered that hydrogen contains positive charges. Ask students to research the following: What conclusion about the mass of the atom and the mass of its protons was derived from Crookes’s findings? (The mass of an atom is greater than its proton content.) Which subatomic particle accounted for this additional mass? (the neutron) The Atomic Nucleus How can you describe the structure of the nuclear atom? When subatomic particles were discovered, scientists wondered how the particles were put together in an atom. This question was difficult to answer, given how tiny atoms are. Most scientistsĿincluding J. J. Thomson, discoverer of the electronĿthought it likely that electrons were evenly distributed throughout an atom filled uniformly with positively charged material. In Thomson’s atomic model, known as the “plum-pudding model,” electrons were stuck into a lump of positive charge, similar to raisins stuck in dough. This model of the atom turned out to be short-lived, however, due to the work of a former student of Thomson, Ernest Rutherford (1871Ľ1937), shown in Figure 4.6. Figure 4.6 Ernest Rutherford Born in New Zealand, Rutherford was awarded the Nobel Prize in Chemistry in 1908. His portrait appears on the New Zealand $100 bill. Atomic Structure 107 Check for Understanding The Essential Question What structures make up an atom? Give each student an index card. Assess students’ knowledge of subatomic particles by reading out loud the description of each of the three kinds of particles. Students should write the name of the particle on the card in the order in which they were stated. Then ask students to indicate where each particle is located within the atom. (electron, proton, and neutron; protons and neutrons are located inside the nucleus, electrons are located outside) ADJUST INSTRUCTION If students are having difficulty remembering which particles are located inside the nucleus, have them review Rutherford’s atomic model by drawing and labeling an atom. Answers FIGURE 4.5 The particles are negative. A negatively charged plate repels a cathode ray. Atomic Structure 107 LESSON 4.2 Table 4.1 Explain &YYOU Q: How did scientists “see” inside an atom to determine the structures that are inside an atom? USE AN ANALOGY Explain to students that the ratio of the size of the nucleus to the size of an atom is about 10–5. Discuss how small the nucleus is compared to the entire atom. Explain that if a housefly sitting on second base in a baseball stadium represented the nucleus of an atom, the rest of the atom would be the size of the stadium. Evaluate Have each student come to your desk and orally state in one minute or less one of the discoveries by Thomson, Millikan, or Rutherford. Then ask them to describe how the discovery led to the current understanding of atomic structure. Then have students complete the 4.2 Lesson Check. U IRT A Have students review Table 4.1. Ask them to create a table or diagram to compare the characteristics of electrons, protons, and neutrons. CHEMISTRY L Reteach LAB Figure 4.7 Rutherford’s Experiment Rutherford’s gold-foil experiment yielded evidence of the atomic nucleus. a. Rutherford and his co-workers aimed a beam of alpha particles at a sheet of gold foil surrounded by a fluorescent screen. Most of the particles passed through the foil with no deflection at all. A few particles were greatly deflected. Y YOU U & YO Scientists observed the behavior of atoms in different experimental conditions. This gave them clues as to what is inside an atom without actual “seeing” inside an atom. Rutherford’s Gold-Foil Experiment In 1911, Rutherford and his co-workers at the University of Manchester, England, wanted to test the existing plum-pudding model of atomic structure. So, they devised the gold-foil experiment. Their test used alpha particles, which are helium atoms that have lost their two electrons and have a double positive charge because of the two remaining protons. In the experiment, illustrated in Figure 4.7, a narrow beam of alpha particles was directed at a very thin sheet of gold foil. According to the prevailing theory, the alpha particles should have passed easily through the gold, with only a slight deflection due to the positive charge thought to be spread out in the gold atoms. Rutherford’s results were that most alpha particles went straight through the gold foil, or were slightly deflected. However, what was surprising is that a small fraction of the alpha particles bounced off the gold foil at very large angles. Some even bounced straight back toward the source. Rutherford later recollected, “This is almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” The Rutherford Atomic Model Based on his experimental results, Rutherford suggested a new theory of the atom. He proposed that the atom is mostly empty space, thus explaining the lack of deflection of most of the alpha particles. He concluded that all the positive charge and almost all the mass are concentrated in a small region that has enough positive charge to account for the great deflection of some of the alpha particles. He called this region the nucleus. The nucleus is the tiny central core of an atom and is composed of protons and neutrons. Informal Assessment V LESSON 4.2 CHEMISTRY a Fluorescent screen Gold foil b. Rutherford concluded that most of the alpha particles pass through the gold foil because the atom is mostly empty space. The mass and positive charge are concentrated in a small region of the atom. Rutherford called this region the nucleus. Particles that approach the nucleus closely are greatly deflected. b Nucleus Lead shield Alpha particles Source of alpha particles Beam of alpha particles Atoms of gold foil See Rutherford’s gold-foil experiment animated online. 108 $IBQUFSt-FTTPO Focus on ELL 4 LANGUAGE PRODUCTION Have students work in partners to complete the Quick Lab. Make sure each pairing has ELLs of varied language proficiencies, so that a more proficient student can help a less proficient one. Have students work according to their proficiency level. BEGINNING: LOW/HIGH Model this lab for these students. Show them your data, sketch, and measurements. Explain your method for guessing what is in the box. INTERMEDIATE: LOW/HIGH Ask students to brainstorm what they think they may observe, prior to performing each step. Allow students to orally present their results. ADVANCED: LOW/HIGH Ask students to present to the class their sketch, data, and interpretation of the object in one of the boxes. 108 Chapter 4 • Lesson 2 ET KIN IC ART Explore Quick Lab PURPOSE Th The students d will determine the shape of Quick Lab Purpose To determine the shape of a fixed object inside a sealed box without opening the box Materials rbox containing a regularly shaped object fixed in place and a loose marble Using Inference: The Black Box Procedure 1. Do not open the box. 2. Manipulate the box so that the marble moves around the fixed object. 3. Gather data (clues) that describe the movement of the marble. 4. Sketch a picture of the object in the box, showing its shape, size, and location within the box. 5. Repeat this activity with a different box containing a different object. Analyze and Conclude 1. Compare Find a classmate who had a box with the same letter as yours, and compare your findings. 2. Apply Concepts Think about the experiments that have contributed to a better understanding of the atom. Which experiment does this activity remind you of? the hidden object by analyzing the rebound paths of a marble rolled at the object. SKILLS FOCUS Observing, inferring PREP TIME 5 minutes CLASS TIME 10 minutes MATERIALS box containing a regularly shaped object fixed in place and a loose marble ADVANCE PREP Cut geometric shapes—such as a triangle, circle, or L, from a sheet of 1-inch plastic foam. EXPECTED OUTCOME Students’ inferences may or may not be different for the same object. ANALYZE AND CONCLUDE 1. 2. See Expected Outcome. The activity simulates the strategy that Rutherford used to probe the structure of metal atoms. Like the students, Rutherford and his coworkers were also faced with the problem of identifying properties of an object not visible to the eye. NLIN PR S E O FOR ENRICHMENT Make a more challenging black 9. 10. M OBLE box for students who have an easy time with the simple boxes. Put two single objects in one box, or a single object with a complex shape. 4.2 Lesso LessonCheck Review What are three types of subatomic particles? Explain How does the Rutherford model describe the structure of atoms? 11. Review What are the charges and relative masses of the three main subatomic particles? 12. Explain Describe Thomson’s and Millikan’s contributions to atomic theory. 13. Compare and Contrast Compare Rutherford’s expected outcome of the gold-foil experiment with the actual outcome. 14. Analyze Data What experimental evidence led Rutherford to conclude that an atom is mostly empty space? 15. Compare and Contrast How did Rutherford’s model of the atom differ from Thomson’s? Atomic Structure 109 Lesson Check Answers 9. protons, neutrons, and electrons 10. A positively charged nucleus surrounded by electrons, which occupy most of the volume. 11. proton, positive charge, relative mass = 1; electron, negative charge, relative mass = 1/1840; neutron, no charge, relative mass = 1 12. Thomson passed an electric current through sealed glass tubes filled with gases. The resulting glowing beam consisted of tiny negatively charged particles moving at high speed. Thomson concluded that electrons must be parts of the atoms of all elements. Millikan determined the charge and mass of the electron. 13. Rutherford expected all the alpha particles to pass straight through with little deflection. He found that most alpha particles passed straight through, but some particles were deflected at very large angles—and some even bounced straight back. 14. The great majority of the alpha particles passed straight through the gold foil. 15. Rutherford’s atomic model described the atom as having a positively charged, dense nucleus that is tiny compared to the atom as a whole. In Thomson’s plum-pudding model, electrons were stuck in a chunk of positive charge. Atomic Structure 109 LESSON 4.2 The Rutherford atomic model is known as the nuclear atom. In the nuclear atom, the protons and neutrons are located in the positively charged nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atom. According to this model, the nucleus is tiny and densely packed compared with the atom as a whole. If an atom were the size of a football stadium, the nucleus would be about the size of a marble. Although it was an improvement over Thomson’s model of the atom, Rutherford’s model turned out to be incomplete. In Chapter 5, you will learn how the Rutherford atomic model had to be revised in order to explain the chemical properties of elements. CHEMISTRY & YOU CHEMISTRY Y Electron Microscopy & CHEMISTRY Y YO YOU U Electron microscopes are capable of much greater levels of magnification than light microscopes, but their real power lies in their resolution. Resolution refers to the degree of detail that is revealed by a microscope. The greater a microscope’s resolution, the more detail that can be seen in the images it transmits. Light microscopes rely on the natural reflective, refractive, and absorbent behaviors of light to make objects visible under magnification. However, these same behaviors can inhibit the degree of resolution, regardless of the magnitude of magnification. Thus, the resolution of a typical compound light microscope is about 200 nm. In contrast, some SEMs can achieve a resolution of 5 nm at high magnification. Pose the following question to students: How does the behavior of light inhibit the ability of a light microscope to deliver images with high resolution? You may need to assist students in the following ways: • Waves of visible light occur in wavelengths from 400 nm to 700 nm. • The light provided to a light microscope by its mirror or electric light source are unfocused and spread out from the light source, while that electron beam in an SEM is highly focused and narrow. • Light waves reflected by and transmitted through the object are subject to interference from each other and from incident light from both the source and from the microscope’s surroundings. • Some light waves transmitted through the sample may be directed away from the lens of the microscope. Y U: TECHNOLOGY YO T &YOU: Electron Microscopy Within 30 years of J. J. Thomson’s discovery of the electron, scientists were studying how to produce images of objects by using an electron beam. In 1931, German scientists Ernst Ruska and Max Knoll built the first electron microscope. There are two types of electron microscopes, scanning electron microscopes (SEM) and transmission electron microscopes (TEM). The images shown here are from SEMs. In an SEM, a beam of electrons is focused down to a very small diameter and scanned across the sample. Most materials eject electrons when the electron beam hits them. The location of the ejected electrons is detected and used to produce an image. A typical light microscope is capable of magnifying an object 1000 times. An electron microscope can magnify an object over 100,000 times. Another advantage of electron microscopes is their higher resolution. Resolution is the ability to differentiate two objects that are very close to each other. So, an electron microscope has the ability to produce a clearer image than a light microscope at the same magnification. Electron microscopes do not produce color images. The color images shown here have false color that has been added to the images. Electron microscopes are useful in chemistry, but also in other fields, such as archeolology, pharmacology and quality assurance testing. SEM This microscope is a scanning electron microscope. The image from the microscope is seen by using a computer screen. BIOLOGY This diatom is a single-celled organism that lives in the water. The image shown above is a dust mite on a piece of fabric. 110 110 Chapter 4 • Chemistry & You 21st Century Learning To be successful in the 21st century, students need skills and learning experiences that extend beyond subject matter mastery. This project helps students build the following 21st Century Learning: Creativity and Innovation; Communication and Collaboration; Information, Media, and Technology Literacy; Initiative and Self-Direction; and Social Skills. Pose the following challenge to your students. Your school has invited four different specialists in electron microscope technology to participate in a roundtable discussion during a college fair. The specialists are there to answer questions and convey information about the uses of electron microscopes and the types of careers that utilize this type of technology. Ask for four student volunteers to be the team of four “experts” in electron microscopy in the following fields: biology, forensics, biochemistry, and food science. The remaining students will form four interview panels. Each panel will select one of the four specialists to interview. The interview panels should prepare a set of questions and their expected answers on the specific uses of electron microscopy in the expert’s field, as well as pros and cons of its use. The team of “experts” will prepare a set of expected questions and their answers on which to base their preparation for their roles. USE VISUALS Have students examine the image of the scientists working with the electron microscope. Point out how a person sees an image made by the electron microscope; the scientists in the photo are not looking into eyepieces, but see a projection on a computer screen. Explain that the type of electron microscope a person uses is typically dependent on the type of information he or she needs to know. A microscope is typically selected based on its resolution, magnification, depth of field, field of view, illumination method, degree of automation, and type of image produced. BIOCHEMISTRY A chromosome from the nucleus of a eukaryotic cell FORENSICS The image on the left is from a light microscope, the image on the right from an SEM. The SEM image shows a clearer image of the fingerprint left on the page because oil from our fingers produces a different intensity of ejected electrons than paper or the ink. Extend Connect to TECHNOLOGY Emphasize the time period that the electron microscope was invented and the technology associated with the apparatus. Tell students that cathode ray tubes, magnets, electron beams, and vacuum chambers are all integral parts of the electron microscope. In pairs, have students research other technological advances that came about using the same or similar components. (Televisions, hand held radios, X-ray machines and lasers all were invented around the same time as the electron microscope and as a result of the same technology.) MATERIALS SCIENCE A tip of a pencil is pointing to a tiny pressure sensor (mauve square) used in a car’s air bag. When the sensors detect rapid deceleration, they trigger the inflation of the air bag. Take It Further 1. Infer Why would a forensic investigator want to analyze gunshot residue using an electron microscope? 2. Compare Research the differences between an SEM and a TEM. Chemistry & You 111 Differentiated Instruction Answers L1 STRUGGLING STUDENTS Have students draw pictures of an ordinary object as they imagine it would appear under an ordinary microscope and then as they imagine it would appear under an electron microscope. TAKE IT FURTHER ELL ENGLISH LANGUAGE LEARNERS Have magnifying glasses available in class, as well as a compound microscope. Allow students time to examine and use both tools. L3 ADVANCED STUDENTS Assign students to write a summary explaining how an electron microscope provides a far greater degree of magnification and a better resolution than an ordinary compound microscope. Summary should include how Thomson’s research on cathode rays propelled electron microscope technology. 1. An electron microscope would allow scientists to see details in the gunshot residue that might not be visible by other microscopes. 2. An SEM is a scanning electron microscope. In an SEM, an electron beam is passed over the surface of an object. An SEM produces an image of the surface of on an object. A TEM is a transmission electron microscope. In a TEM, an electron beam is directed through an object, producing an image of what is inside an object. Chemistry & You 111 CHEMISTRY & YOU Explain