NAME PER DATE DUE ACTIVE LEARNING IN CHEMISTRY EDUCATION CHAPTER 28 NUCLEAR CHEMISTRY. (Part 1) , A.J. Girondi

Transcription

1 NAME PER DATE DUE ACTIVE LEARNING IN CHEMISTRY EDUCATION CHAPTER 8 NUCLEAR CHEMISTRY (Part ) 8-997, A.J. Girondi

2 NOTICE OF RIGHTS All rights reserved. No part of this document may be reproduced or transmitted in any form by any means, electronic, mechanical, photocopying, or otherwise, without the prior written permission of the author. Copies of this document may be made free of charge for use in public or nonprofit private educational institutions provided that permission is obtained from the author. Please indicate the name and address of the institution where use is anticipated. 997 A.J. Girondi, Ph.D. 55 Latshmere Drive Harrisburg, PA 79 Website: , A.J. Girondi

3 SECTION 8. Nuclear Notation and Isotopes Nuclear chemistry involves changes that occur in the nucleus of an atom. These changes in a nucleus often result in the release of great amounts of energy much greater than the amount of energy released in any chemical reactions. You will recall that chemical reactions involve the formation and breaking of bonds between atoms. In addition to the release of energy, certain types of particles are emitted from a nucleus during nuclear reactions. Before going on, there are a few basic facts which you should know:. Most of the mass of an atom is found in the nucleus. This is a result of the relatively close "packing" of the protons and neutrons in it.. All protons carry a positive charge. 3. Because they all carry a positive charge, the protons in a nucleus repel each other with a strong force; yet, the nucleus of a stable atom does not fall apart. You may recall that it is the number of protons in the nucleus of an atom (the atomic number) that determines what element the nucleus represents. A nuclear change sometimes involves a change in the number of protons in the nucleus. When this happens, a nucleus of one element is changed into a nucleus of a different element. This is called a transmutation. In a previous chapter, you were introduced to nuclear notation. Let's review it now to refresh your memory. The general form for nuclear notation can be represented by the expression shown below: mass number (sum of protons and neutrons) atomic number (number of protons) A Z X symbol of the element What would the expression A minus Z, or A - Z, represent? {} If the value of Z changes, will X change? {} If Z changes by a value of, what will happen to the value of A? {3} You learned previously that atoms of an element can exist in different forms known as isotopes. Isotopes are atoms of an element that contain different numbers of neutrons. Therefore, isotopes have different masses and different mass numbers although they have the same atomic number. Some elements have many isotopes, while others have only a few. In addition, some isotopes of elements are naturally-occurring while others are man-made. Some isotopes are unstable, meaning that they decompose or break apart on their own. Many elements possess both stable and unstable isotopes. Unstable isotopes are said to be radioactive. They give off energy and/or nuclear particles when they decompose. In Table 8., the mass numbers of isotopes of some selected elements are shown. If ALL of the isotopes of an element happen to be radioactive, then the element itself is categorized as being radioactive. With this in mind, which of the selected elements listed in Table 8. should be categorized as radioactive? {} How many radioactive isotopes does carbon (C) have? {5} How many nonradioactive isotopes does nitrogen (N) have? {6} How many man-made radioactive isotopes does helium (He) have? {7} All elements on the periodic table with an atomic number of 8 or greater are radioactive. These elements are shown as they occur on the periodic table in Table , A.J. Girondi

4 Table 8. Isotopes of Some Selected Elements In this table, mass numbers of naturally-occurring nonradioactive isotopes are given in plain type; mass numbers of naturally-occurring radioactive isotopes are double-underlined; mass numbers of any other isotopes are single-underlined. Naturally-occurring isotopes are listed in their order of abundance. All other isotopes are listed in order of decreasing half-life which is discussed later in this chapter. Element Mass numbers of isotopes H,, 3 He, 3, 6, 8 Be 9,, 7,, 6 B,, 8 C, 3,,,, 5, 6, 9 S 3, 3, 33, 36, 35, 38, 37, 3, 3, 9 N, 5, 3, 6, 7, 8 Ca,,, 8, 3, 6,, 5, 7, 9, 5, 39, 38, 37 Sn, 8, 6, 9, 7,,,,, 5, 6, 3, 3, 5,,, 7, 8,, 9, 8, 9, 3, 3, 3 U 38, 35, 36, 3, 33, 3, 3, 37, 3,, 9, 39, 8, 7 Lr 6, 56, 55, 5, 57, 56, 5, 5, 58 The simplest element, hydrogen, has three isotopes. The most common form of hydrogen (protium) has one proton and no neutrons in its nucleus. Its atomic number is, and its mass number is. The nuclear notation for protium is shown at right. In nature approximately % of all hydrogen atoms are protium. H The remaining.5% of hydrogen consists of deuterium atoms. Also known as heavy hydrogen, deuterium differs from protium in that it has one neutron in the nucleus in addition to one proton.using the letter D instead of H as the symbol, write the nuclear notation for deuterium: {8} Protium and deuterium are both stable, naturally-occurring isotopes. Water (HO) molecules which contain deuterium instead of protium are known as "heavy water" which is sometimes represented as DO. About two water molecules in every billion are "heavy." A third form of hydrogen is man-made and is radioactive. It is known as tritium, and it is a common by-product of the nuclear reactions that occur in a nuclear power plant. Tritium has two neutrons in its nucleus. Using the letter T instead of H as the symbol, write the nuclear notation for tritium. {9} 8-997, A.J. Girondi

5 A Table 8. The Radioactive Elements (All of their isotopes are radioactive) A 3A A 5A 6A 7A 8A 3 Tc 8 Po 85 At 86 Rn 87 Fr 88 Ra 89 Ac Unq 5 Unp 6 Unh 7 Uns 8 Uno 9 Une Uun Uuu 6 Pm 9 Th 9 Pa 9 U 93 Np 9 Pu 95 Am 96 Cm 97 Bk 98 Cf 99 Es Fm Md No 3 Lr It is common to identify which particular isotope of an element is being discussed by writing the mass number after the name of the element with a dash in between. For example, protium is hydrogen-, while deuterium is hydrogen-. Following this method, how would tritium be written? {} What is meant by mass number? {} SECTION 8. Four Types of Nuclear Reactions The equation at right represents a nuclear change. We will refer to it as a nuclear equation. More specifically, it depicts the change of an atom of carbon- into an atom of nitrogen-: 6 C -----> N + e 7 - Nuclear equations often include a special type of notation to represent subatomic particles such as electrons, protons, and neutrons. This notation looks similar to nuclear notation which represents a nucleus, but it is not the same. The notations describing an electron, a proton, and a neutron are shown below. Note that the superscripts represent the mass numbers of each particle. The mass number of an electron is zero. However, the subscripts represent the charge on the particle. Note that neutrons have no charge, so the subscript for them is zero. Thus, the difference between nuclear notation and the notation for these subatomic particles lies in the meaning of the subscript. electron: proton: neutron: - e p + n , A.J. Girondi

6 mass number atomic number mass number C p 6 charge + NUCLEAR NOTATION SUBATOMIC PARTICLE NOTATION According to the data in Table 8., is carbon- a radioactive isotope? {} How about nitrogen-? {3} Note that if the atomic number changes during a nuclear reaction, the identity of the resulting element changes, too. In the equation shown below, a nucleus of carbon becomes a nucleus of nitrogen as the atomic number changes from 6 to 7. An electron is also given off as a product. But hey! If the atomic number changes from 6 to 7, this means that one addition proton is now present. Where did it come from? Hmmmm. C -----> N + e Electrons are sometimes called beta particles (pronounced "bay-ta"). So, the giving off of an electron in a nuclear reaction is called a beta emission. In order for carbon- to change to nitrogen-, there was an increase in the number of {} in the nucleus. When a neutron decomposes, the products are a proton and an electron. The new proton causes the atomic number to increase by one, and the electron is given off. When the decomposition of a neutron produces a proton, the mass number remains unchanged. Since one element is changed into another in this reaction, this particular type of nuclear reaction is called a {5}. There are four types of nuclear reactions that release energy:. Natural Radioactive Decay Natural radioactive decay refers to the ability of a nucleus to decompose (decay) and give off energy spontaneously (without any external stimulation). As a result, the number of {6} (atomic number) in the nucleus may increase or decrease, depending on the type of radioactive decay. The equation below in which carbon- is converted to nitrogen- represents a natural radioactive decay. C -----> N + e Artificial Transmutation During artificial transmutation, a nucleus changes its identity as a result of some external stimulation created by man. For example, an external particle such as a neutron could be used to bombard the nucleus, causing it to decompose. This kind of nuclear disintegration results in the formation of an artificial (man-made) isotope of the element. The equation below shows the conversion of natural nonradioactive cobalt-59 to radioactive cobalt-6 by a process known as slow neutron bombardment. 59 Co + 7 n ----> 6 Co 7 Notice that since a neutron is being added to the nucleus, the mass number of the nucleus increases by one, from 59 to 6. The atomic number remains unchanged since the number of {7} is unchanged. Since the atomic number remains unchanged, the identity of the nucleus (cobalt) remains the same. What we have done here is to change one isotope of cobalt into a different isotope of cobalt , A.J. Girondi

7 3. Fission In fission, a nucleus with a large mass splits into two nuclei with smaller masses. To cause fission, man bombards certain nuclei with special particles. The fission process is used to generate heat in nuclear power plants, and is the kind of reaction which occurs during the explosion of an atomic bomb. Let's see where this energy comes from. Look at the equation below which represents the fission of uranium- 35. Find the total of the mass numbers of the two particles on the left side of the equation: {8}. 35 U + 9 n > Ba Kr n + energy Next, find the total of the mass numbers of the five particles on the right side: {9}. How do these totals compare? {} As a result, you would think that mass the amount of matter) is conserved (neither created nor destroyed). However, this is a bit misleading. Keep in mind that the mass number is the total number of the protons and neutrons in the nucleus, not their exact total mass. Remember that masses of atoms and subatomic particles are expressed in very tiny units called atomic mass units (amu). The mass of an atom of U-35 is actually a little greater than 35 amu, and the masses of Ba-38 and Kr-95 are actually a little less than 38 and 95, respectively. Therefore, in the equation above, there is a small loss of mass which appears as a great amount of energy. In other words, some mass is converted into energy. An atomic bomb gives off a tremendous amount of heat because some mass is converted into energy. A tiny amount of mass can produce a tremendous amount of energy. When the uranium nucleus splits into smaller nuclei, the energy which was needed to hold the whole thing together in the first place is no longer needed. This is the energy which is given off.. Fusion When fusion occurs, the nuclei of two lower mass elements are combined to form a nucleus with a greater mass representing a different element. Exceedingly high temperatures are needed to cause fusion to occur, since the two nuclei repel each other due to their similar positive charges. Fusion reactions are the source of the sun's energy where hydrogen nuclei combine to form helium nuclei. The equation below shows the fusion of deuterium nuclei to form one helium- nucleus (also called an alpha particle). H + H > He + energy Fusion reactions were used in weapons such as the hydrogen bomb. Scientists are experimenting with fusion reactions in devices known as breeder reactors which may someday replace fission reactors in nuclear power plants. Fusion, like fission, results in a loss of mass which is converted into a great amount of energy. However, fusion releases much more energy per gram of fuel than fission does. Problem. Let's practice writing nuclear notation. Keep in mind that the superscript is the mass number (sum of protons and neutrons) and the subscript is the atomic number (number of protons) if the particle is a nucleus. If the particle is a subatomic particle (proton, electron, or neutron,) then the subscript is the charge on the particle. Write the nuclear notation for each of the following: a. an isotope of carbon (C) which contains 6 protons and 8 neutrons b. an isotope of helium (He) which contains protons and neutrons c. an isotope of uranium (U) which contains 9 protons and has a mass number of 33 d. an isotope of tin (Sn) which contains 5 protons and 6 neutrons a. b. c. d , A.J. Girondi

8 Now, let's try working with some nuclear equations. Keep in mind that in a balanced nuclear equation, the total of the superscripts of all particles must be equal on both sides of the equation. The sum of the subscripts of all particles must also be equal on both sides. For example, consider the equation below. 6 Ra -----> Rn + He In this example, an isotope of radium (Ra) decomposes into an isotope of radon (Rn), and this decomposition is accompanied by the emission of a helium nucleus which is also called an alpha particle. What is the sum of the superscripts on the right side of the equation? {} How does this compare with the superscript on the left side? {} What is the sum of the subscripts on the right side? {3} How does this compare to the subscript on the left side? {} Is this nuclear equation balanced? {5} Problem. Complete the following transmutation reactions, indicating in each case, the nuclear notation of the element formed. What element is formed in the first equation below? Well, if you check it out, the atomic number of the missing particle will have to be 6. What element has an atomic number of 6? {6} Therefore, what element symbol will the missing particle have? {7} a. 9 Be + He -----> + n b. 8 Si + D -----> + n 7 c. Al + n -----> + 3 He 55 d. Mn + D -----> + n 5 e. Na -----> + - e Complete the following equations indicating in nuclear notation, in each case, what particle - if any - was ejected. Answers may include: electron: e - proton: p neutron: n alpha particle: He + f. N + n -----> + 7 B 5 g. 9 Be + D -----> + B , A.J. Girondi

9 7 h. Al 3 + He -----> + 3 P 5 i. 39 U > Np + 93 The radioactive elements with atomic numbers 8 through 9 (up to and including uranium) have some naturally-occurring radioactive isotopes. The elements beyond uranium (with atomic numbers greater than 9) do not have any naturally occurring isotopes. These elements beyond uranium are known as the transuranium elements. They are all synthetic elements since all of their isotopes are manmade. Most of the radioactive elements (with atomic numbers 8 and above) are too unstable to be assigned an atomic mass (atomic weight). If you look at a periodic table, you will notice that the atomic masses of these elements are given in parentheses. (Check this out on a periodic table now.) The number in the parentheses represents the atomic mass of the single most stable isotope. You will recall that atomic mass is defined as the average mass of the various naturally occurring isotopes of an element in the proportions in which they occur in nature. The radioactive isotopes of elements with atomic numbers 8 and above are constantly decomposing. These isotopes have different half-lives, which means that they are decomposing at different rates. Use this information to explain why these elements cannot have an atomic mass as defined above: {8} Most elements with atomic numbers smaller than 8 are stable because NONE of their naturallyoccurring isotopes are radioactive. There are some exceptions to this rule. For example, K- and Ca-6 are radioactive. Most of the elements below atomic number 8 are stable enough to be assigned an atomic mass. (Elements #3 and #6, Technetium and Promethium, are exceptions.) Man-made radioactive isotopes have been synthesized for many of these elements, but synthetic isotopes are not included in the calculation of atomic masses since they are not found in nature. Section 8.3 Early Studies of Radioactivity In896, a French scientist by the name of Henri Becquerel accidentally discovered natural radioactivity while conducting experiments with a uranium compound called potassium uranyl. In one of his experiments, Becquerel wrapped a photographic plate in black, lightproof paper and placed some of the uranium compound on top of the covered plate. He then placed this arrangement in the sunlight. Although the sunlight could not pass through the lightproof paper, the plate became exposed in the area of the uranium compound, as indicated by a dark area on the photograph. Becquerel thought that perhaps energy from the sun had been changed into some more penetrating form which was able to pass through the paper. He then attempted to repeat the experiment, but cloudy weather prevented him from doing so at that time. He decided to store his second set-up in a closed drawer. Later, on a sunny day, Becquerel repeated the experiment using a fresh photographic plate instead of the one he had stored in the closed drawer. He then developed both of the photographic plates. Since the stored plate had not been exposed to sunlight, Becquerel expected the developed photograph to be blank or almost blank. Instead, he found that it had a dark area like that of the fresh plate which had been exposed to sunlight. Becquerel reasoned that the uranium compound must have emitted some type of energy on its own without the stimulation of sunlight. This ability of a nucleus to emit energy spontaneously (without external stimulation) is called natural radioactivity. Uranium ore exhibits natural radioactivity with the greatest amount of energy coming from its most abundant naturally-occurring isotope, U , A.J. Girondi

10 Becquerel also discovered that as the energy is emitted from a radioactive nucleus and passes through molecules of oxygen and nitrogen in the air, it causes these molecules to lose electrons, forming positively charged ions. As a result, the air becomes ionized. The fact that radioactive nuclei can ionize gases is a principle used in the construction of equipment which can detect the presence of radioactivity. You probably have a smoke detector in your home. The most common form of smoke detector contains a small sample of a radioactive element (probably americium). The radiation emitted is capable of ionizing small particles in the air. When enough particles are present (as during a fire), the ions which are produced allow an electric current to form and the alarm goes off. An electroscope is a device which can detect and store an electric charge. See Figure 8.. A simple electroscope can be constructed by attaching two pieces of thin metal foil to a metal rod. This apparatus is then sealed inside a glass container such as a jar. When the electroscope is in its normal "uncharged" state, the two pieces of metal foil will hang beside each other. To convert the electroscope to its "charged" state, we have to supply it with an excess of electrons. How do you do this? Well, there are many ways. Even by combing your hair and then touching the comb to the metal rod on the electroscope will do it. The electrons on the comb (which came from your hair) will flow into the rod and into the two pieces of metal foil. At that point, both pieces of foil would carry a negative charge and they would repel each other. The greater the amount of charge they hold, the more they repel each other. So, an electroscope is a crude device for detecting and measuring an electrical charge. The air around the foil in the electroscope acts as an insulator, helping to prevent the electroscope from losing its stored charge right away. It is much harder for electrons to flow through air than through metal. If you touch the metal rod on the electroscope with any substance which is a good "acceptor" or conductor of electrons (such as a piece of metal), the excess electrons will flow out of the electroscope which will then lose its charge. discharged weakly charged highly charged Figure 8. An Electroscope When nuclear radiation ionizes the air forming positively-charged particles, these positive particles can draw negatively-charged electrons away from an electroscope in which they might be stored. It is possible to measure the rate at which radioactive emissions occur by measuring the rate at which an electroscope loses its charge. Marie Sklodowska, a student of Becquerel, used an electroscope to study the radioactivity of uranium and its various ores. She found that one uranium ore, pitchblende, gave off 8-997, A.J. Girondi

11 much more radioactivity than even pure uranium. After her marriage to the physicist Pierre Curie, they both studied the radioactivity of pitchblende. The Curies discovered that the increased radioactivity of pitchblende was due to the presence of two elements in the ore. Madame Curie called the first radioactive element which they discovered in the ore "polonium" after her native land, Poland. Find polonium (Po) on the periodic table. What is its atomic number? {9} On the periodic table, the mass number of polonium is (). What is so special about Po- and why is this mass number given in parentheses? It took the Curies four years to complete the processing of the ore from which they extracted only. gram of the second radioactive element, radium, in the form of radium chloride. Radium (Ra) has what atomic number on the periodic table? {3} Its mass number is given as (6). Both polonium and radium were found to be more radioactive than uranium. Although the use of the electroscope allowed the Curies to measure the rates at which radiation was emitted, it did not provide any indication as to the nature of the radiation. In other words, it did not indicate whether the radiation consisted of energy, or particles, or both. In 93, Ernest Rutherford performed an experiment which provided some new information about the properties of radiation. He placed a piece of pitchblende into a hole drilled deep into a block of lead. (See Figure 8.) Most of the radiation emitted by the pitchblende was absorbed by the lead. Only the radiation that was traveling in a straight line through the hole could escape. A photographic plate was positioned in the path of the escaping radiation. When the plate was developed, a small single spot appeared where it was struck by the radiation. Next, Rutherford placed the poles of a U-shaped magnet at right angles to the stream of radiation. This forced the radiation to pass through a magnetic field. Since a magnetic field deflects oppositely charged particles in opposite directions, it was possible to determine the charge of any particles in the radiation. Streams of radiation which do not contain particles would not be affected by the magnetic field. When the magnetic field was used, three distinct spots were produced. (See figure 6..) The three spots indicated that the magnet had separated the radiation into three distinct streams. Two streams were deflected in opposite directions, whereas one stream was not deflected at all. How many of these three streams contained particles? {3} Why were the two affected streams deflected in opposite directions? {3} The two deflected streams are called alpha ( ) and beta (ß) radiation in Figure 8.. The unaffected stream was called gamma ( ) radiation. What must be true about the stream of gamma radiation that was not deflected? {33} photographic plate photographic plate radiation single spot formed ( ) (+) 3 spots formed magnet pitchblende radiation pitchblende Lead Lead Figure 8. Rutherford's Study of Radiation from Pitchblende 8-997, A.J. Girondi

12 The particles which were deflected only slightly in a direction indicating a positive charge were called alpha particles. The Greek symbol for alpha is:. The fact that they were only slightly deflected indicated that they had a relatively large mass compared to beta particles. In later experiments, it was shown that alpha particles were actually bundles composed two protons and two neutrons. They have the same structure as helium nuclei. You can say that the term alpha particle is another name for a helium nucleus. Alpha particles are, therefore, designated by the same nuclear notation as is the most common isotope of helium which is helium-. Alpha particles travel at, to, miles per second, but can be stopped by a sheet of paper. They have a great ability to cause ionization by knocking electrons loose from atoms or molecules through which they pass. He Nuclear Notation for Helium- or for an Alpha Particle The very low mass particles were deflected much more than the alpha particles and in the opposite direction. Apparently, they were negatively charged. Rutherford called them beta particles. The Greek symbol for beta is: ß. They were later shown to be electrons which travel at a rate of up to, miles per second! Their ability to penetrate matter when they strike it is much greater than that of alpha particles; nevertheless, they still cannot penetrate more than a few inches of solid material. Beta particles cause much less ionization than alpha particles. The radiation emitted between the alpha and beta streams was not deflected at all by the magnetic field and, therefore, carries no electric charge. This stream was called gamma radiation. The Greek symbol for gamma is:. Gamma rays are similar to x-rays, but are higher in energy. Their penetrating power is much greater than either alpha or beta radiation, and they can penetrate almost one foot of solid lead! Gamma rays travel at the speed of light (86, miles per second). They cause practically no ionization at all when they interact with atoms or molecules. Table 8.3 summarizes some of the information presented about the three forms of radioactivity. Complete the column headed "Penetrating Power" by inserting the terms high, low, and moderate in the proper slots. Next, complete the column headed "Ionizing Power" by inserting the terms high, moderate, and almost none in the proper slots. Table 8.3 The Three Forms of Natural Radioactivity Penetrating Ionizing Decay Product Symbol Charge Power Power alpha particle He + {3} {37} beta particle - {35} {38} gamma rays none none {36} {39} In general, a radioactive isotope of an element emits alpha particles or beta particles, but not both. The emission of gamma rays generally accompanies both alpha emissions and beta emissions. Which of the three kinds of radioactive emissions is needed in order for a transmutation to occur? {} Explain: {} 8-997, A.J. Girondi

13 Name three radioactive elements found in pitchblende: {} SECTION 8. Methods of Detecting Radiation Electroscopes Radioactivity has an effect on matter as it passes through it. We can, therefore, study radioactivity by recording and measuring these effects. You already know that nuclear emissions can expose photographic plates and can ionize gases. Some measuring devices make use of the fact that gases will conduct electricity when they become ionized as a result of exposure to radiation. For example, the electrical charge stored in an electroscope can be lost when the air inside and around the electroscope becomes ionized. See Figure 8.3 below. molecules of air ions of air inside here incoming radiation ionizes the air charged foil strips charge lost Figure 8.3 Effect of Radiation on Stored Charge Ionization chambers In an ionization chamber, radiation passes through a gas. The radiation causes the gas particles to be split into pairs of ions which are then collected on the surfaces of oppositely charged electrodes. The number of pairs of ions produced can be measured. An example of a measuring instrument using this principle is the self-reading dosimeter. With such a device, radiation can be measured in units called Roentgens. This may sound a bit complicated, but a Roentgen is the amount of gamma radiation required to produce.6 X pairs of ions when it is absorbed by gram of air. Geiger Counter A Geiger counter (more accurately known as a Geiger Mueller counter) consists of a sealed tube containing argon gas at a low pressure. One end of the tube contains a thin glass window. There are two electrodes in the tube (see Figure 8.). The negative electrode is a metal cylinder located just inside the tube. The positive electrode is a wire which runs down the center of the cylindrical tube. A high voltage exists between these electrodes, but electric current does not flow, since the uncharged (un-ionized) argon gas atoms cannot carry the current from one electrode to the other. When radiation enters through the thin window, it ionizes some of the argon atoms, forming argon ions and free electrons. The argon ions become conductors of electric current between the electrodes. The electrical impulses are then sent into an amplifier. From there they may be sent to a counter or to an amplifier to be converted into sounds or flashes of light , A.J. Girondi

14 negative electrode positive electrode argon gas incoming radiation thin glass window Volts Figure 8. Geiger Counter To amplifier or counter Photographing Particle Trails As you know, fast moving charged particles such as those present in radioactive emissions can cause the formation of ions when they collide with molecules through which they pass. If this process occurs in a container which is saturated with water vapor, the water molecules can condense on ions forming tiny spots of fog. This fog forms along the paths of the radioactive emissions since that is where the ions form. These foggy paths are visible to the eye. They are called trails. Photographs of these particle trails enable scientists to study how certain decays occur. The device in which all this takes place is called a cloud chamber. In Figure 8.5, the curved vertical line represents the path of a subatomic particle passing through a thin sheet of lead. The path is curved due to the presence of a strong magnetic field in the cloud chamber. Figure 8.5 Particle Trails in a Cloud Chamber Scintillation Counter When radiation strikes fluorescent substances (known as phosphors) it causes flashes of light to be emitted. This is what happens in a fluorescent light bulb or on a television screen. There are instruments which can count these small flashes of light, and in this way, measure radiation. The process of producing light flashes is called scintillation. The devices are called scintillation counters , A.J. Girondi

15 Section 8.5 More Practice With Nuclear Equations Problem 3. Complete the equations below, and make sure that they are balanced. N 7 a. + He -----> 7 8 O + 9 b. Be + He -----> 6 C + c. 3 H -----> 3 He + 3 Na d. + He > H + e. + He > n + 3 N 7 Now, complete the equation below. Does anything appear strange? An electron with a positive charge! 3 f. P > + e + Yes, there is such a thing as an electron with a positive charge. It's call a positron. As you can imagine, there's a lot more to know about nuclear chemistry! SECTION 8.6 Learning Outcomes This is the end of Chapter 8. The subject of nuclear chemistry is continued in Chapter 6. Review the learning outcomes below. When you feel that you have mastered them, arrange to take the exam on Chapter 6, and then move on to Chapter 7.. Define and /or describe nuclear terms including: isotope, transmutation, alpha particle, beta particle, gamma rays, fission, fusion, radioactivity, Geiger counter, scintillation counter, and cloud chamber.. Write the nuclear notation of nuclear particles and of the nuclei of atoms given mass numbers, atomic numbers, or other relevant data. 3. Describe the historical contributions of Becquerel, Madame and Pierre Curie, and Rutherford.. Given sufficient information, complete and balance nuclear equations. 5. Be able to locate the radioactive elements on the periodic table , A.J. Girondi

16 SECTION 8.7 Answers to Questions and Problems Questions: {} number of neutrons; {} yes; {3} it will also change by a value of ; {} U and Lr; {5} six; {6} two; {7} none; {8} D; {9} 3 T; {} hydrogen 3; {} sum of protons and neutrons in nucleus; {} yes; {3} no; {} protons; {5} transmutation; {6} protons; {7} protons; {8} 36; {9} 36 (note that there are three neutrons represented); {} they are equal; {} 6; {} equal; {3} 88; {} equal; {5} yes; {6} carbon; {7} C; {8} since some isotopes are decomposing, the average mass of the isotopes is changing; {9} 8; {3} 88; {3} two; {3} they contained particles with opposite charges; {33} it contains no particles; {3} almost none; {35} moderate; {36} high; {37} high; {38} moderate; {39} almost none; {} alpha or beta; {} alpha emission results in loss of protons, while beta emission results in formation of one proton; {} polonium, radium, uranium Problems:. 6 C 6 He 33 a. b. c. U d. 9 Sn 5. a. 6 C 9 b. B Na 55 5 c. Fe d. Mg 6 e. f. He g. n h n i. e - a. P b. n c. e d. Mg e. B f. Si , A.J. Girondi