DOCSLIB.ORG

Atoms, Nuclei and Radioactivity

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Atoms, Nuclei and Radioactivity 1 Atoms, Nuclei and Radioactivity Elizabeth M. Parvin CHAPTER OUTLINE Introduction Transverse and Longitudinal Waves Atomic Structure Electromagnetic Radiation Particles Continuous Spectra and Characteristic Radiation The Atom and the Nucleus Radioactive Decay The Forces Stable and Unstable Isotopes Electron Energy Levels Half-life Band Theory of Solids Alpha Decay Impurity Bands Beta Decay Particles in Electric and Magnetic Fields Gamma Decay Electric Fields Electron Capture and Internal Conversion Magnetic Fields Radioactive Decay Series The Lorentz Equation Radionuclides of Medical Interest Waves INTRODUCTION fundamental particles; the protons and neutrons are composed of quarks. Charges are, as is customary in physics, given as multiples of À The aim of this first chapter is to lay some of the foundations of the the electronic charge, e, which is 1.602  10 19 C. The proton and pos- physics of radiotherapy. It starts, in the section titled Atomic Structure, itron have charges of +e and the electron has a charge of –e; all the other by looking at the main subatomic particles and the forces that hold particles listed are neutral. The fourth column gives the masses in kilo- them together in the atom. This leads on to an examination of the dif- grams, but in nuclear physics, it is common practice to express the mass ferent types of nuclei, with an emphasis on some of the important ones of a particle not in kilograms but in terms of its rest mass energy. This is used in medical physics. The behaviour of charged particles in electric based on Einstein’s famous equation, which gives the equivalence of and magnetic fields, central to much of the physics of radiotherapy, is mass and energy: covered in section titled Particles in Electric and Magnetic Fields. E ¼ mc2 Waves, including the electromagnetic spectrum, and the basics of 1.1 radioactive decay are introduced in the following sections. where m is the mass of the particle, c is the speed of light in a vacuum À For some readers, this chapter will be a reminder of previous knowl- (2.998  108 ms 1) and E is the energy. For an electron, the rest mass À À edge, for others it will be new territory. For the latter, the references energy associated with a mass of 9.109  10 31 kg is 8.187  10 14 should provide some more in-depth material that it has not been pos- joules (J). It is more convenient to express this very small magnitude sible to include here. For convenience, SI units are listed in the Physical of energy in units of the electron-volt (eV), where Units and Constants Section. 1eV¼ 1:602  10À19 J 1.2 ATOMIC STRUCTURE The electron volt is the amount of energy acquired by an electron when Particles it is accelerated through a voltage of 1 volt (see the section titled Electric Most readers will be familiar with the idea that molecules are composed Fields), hence the name electron volt. Using this conversion, we arrive at of atoms chemically bonded together. Perhaps the most familiar exam- the values given in column 5 of Table 1.1. Note that the proton and neu- ple is the water molecule, which consists of two hydrogen atoms tron (known collectively as nucleons) are very much more massive than — bonded to one oxygen atom to give the well-known molecular formula the electron and positron, and that the neutrino has almost zero mass the exact value is still the subject of experiment. H2O. In radiotherapy, we are often more interested in the particles that make up the atom—these are known as subatomic particles. The positron is the antiparticle of the electron, having the same β+ Table 1.1 lists the properties of the sub-atomic particles of most rel- mass but the opposite charge; it is emitted during decay (see the sec- evance to radiotherapy; the proton, neutron, electron, positron, neutrino tion titled Beta Decay) and is important in positron emission tomog- β and antineutrino. Strictly, only the electron, positron and neutrinos are raphy (PET) (see Chapter 6). Neutrinos play a role in decay (see 2 CHAPTER 1 Atoms, Nuclei and Radioactivity 3 the section titled Beta Decay). The photon is the particle associated with respectively and are known as isotopes of carbon. For many elements, electromagnetic radiation (see the section titled Waves). some of the isotopes are radioactive (see the section titled Radioactive Decay) and this fact can be very useful in clinical investigations because The Atom and the Nucleus the chemical behaviour of all the isotopes is the same. For example, 15 The atom is the smallest identifiable amount of an element. Each atom radioactive 8 O is taken up by the body in the same way as the stable 16 consists of a central nucleus, made up of protons and neutrons, which is (i.e. nonradioactive) isotope, 8 O, and can be used in PET; the radioac- ‘ ’ 131 surrounded by a cloud of electrons. The diameter of an atom and tive iodine isotope 58 I is taken up by the thyroid gland in the same way À10 À14 127 nucleus are typically 10 m and 10 m, respectively. To put these as the stable isotope 58 I, so can be used to treat thyroid cancer. dimensions into a more accessible perspective, if the atomic nucleus is represented by the point of a pencil (diameter approximately 0.5 mm) The Forces held in the centre of a medium-sized room (say 5 m  5 m), then the The next point to address is the question of what holds the atoms electron cloud surrounding the nucleus would extend to the walls of together. The protons in the nucleus are positively charged, and the the room. electrons surrounding the nucleus are negatively charged, so there is It is the number of protons in a nucleus that determines the type of an attractive force between them. This electrostatic or Coulomb force element. Because the protons in the nucleus are positively charged and depends on the product of the charges and is inversely proportional the electrons are negatively charged, a neutral atom must contain equal to the square of the distance between them. For one electron numbers of protons and electrons. It is the electrons, which surround (charge –e) and a nucleus (charge Z), the magnitude of the force the nucleus and are often described as orbiting it, that interact with (Fel) is given by the equation electrons from other atoms, thereby determining the chemical behav- Ze2 iour of the atom. Fel ¼ k 1.4 For example, a hydrogen atom has one proton in the nucleus, helium r 2 has two, carbon has six and so on. This number is known as the atomic where k is a constant and r is the distance between the electron and the number, Z, of the element. The elements listed in order of increasing nucleus. This inverse-square relationship is analogous to the gravita- atomic number form the periodic table of the elements [1]. tional force between two masses and we could use the rules of classical As shown in Table 1.1, the neutrons in the nucleus carry no charge physics to calculate the orbits of the electrons around the nucleus (anal- but do have a similar mass to the protons. The electrons have a very ogous to the orbits of the planets around the Sun). However, there is small mass, so the mass of an atom is almost entirely due to from one big difference between the planetary orbits and the orbits of the the mass of the protons and neutrons. The sum of the number of neu- electrons around the nucleus; in the planetary case, it is possible to have trons (N) and protons in a nucleus is known as the atomic mass num- any value of the radius (and therefore energy), whereas, in the atomic ¼ ber, A and A Z + N. Because both A and Z are needed to identify a case, quantum theory only allows certain permitted orbits. This gives nucleus, the notation used is of the form rise to electron energy levels (or shells), which are the subject of the A section titled Electron Energy Levels. Z X 1.3 For like charges, the Coulomb force is repulsive, so, because the pro- The symbol shown here as X is the chemical symbol for the element—H tons in the nucleus are all positively charged, it might be expected that for hydrogen, He for helium, C for carbon and so on—and A and Z the Coulomb force would cause the nucleus to fly apart. However, there are the mass and atomic numbers. Because Z determines the chemistry is another force that acts on both protons and neutrons: the strong force. and therefore the element, strictly speaking, it is not necessary to have This force acts on protons and neutrons and other heavy particles called 12 the value of Z shown. For example, 6 C represents a carbon nucleus with hadrons; it is independent of charge and is always attractive, but only at six protons and six neutrons, but it could be written simply as 12C, or even very short range. Fig. 1.1 shows the way in which the energy of a proton as carbon-12 because carbon always has six protons. However, to avoid varies depending on how far away from the nucleus it is. As a proton confusion, it is often easier to include both atomic and mass numbers. approaches the nucleus, it experiences a repulsive force but, if it has For any one element, the number of protons is always the same, but enough energy to overcome this ‘Coulomb barrier’ and gets within the number of neutrons, and hence A, can vary.
Load more

Recommended publications
  • Neutron Stars
    Chandra X-Ray Observatory X-Ray Astronomy Field Guide Neutron Stars Ordinary matter, or the stuff we and everything around us is made of, consists largely of empty space. Even a rock is mostly empty space. This is because matter is made of atoms. An atom is a cloud of electrons orbiting around a nucleus composed of protons and neutrons. The nucleus contains more than 99.9 percent of the mass of an atom, yet it has a diameter of only 1/100,000 that of the electron cloud. The electrons themselves take up little space, but the pattern of their orbit defines the size of the atom, which is therefore 99.9999999999999% Chandra Image of Vela Pulsar open space! (NASA/PSU/G.Pavlov et al. What we perceive as painfully solid when we bump against a rock is really a hurly-burly of electrons moving through empty space so fast that we can't see—or feel—the emptiness. What would matter look like if it weren't empty, if we could crush the electron cloud down to the size of the nucleus? Suppose we could generate a force strong enough to crush all the emptiness out of a rock roughly the size of a football stadium. The rock would be squeezed down to the size of a grain of sand and would still weigh 4 million tons! Such extreme forces occur in nature when the central part of a massive star collapses to form a neutron star. The atoms are crushed completely, and the electrons are jammed inside the protons to form a star composed almost entirely of neutrons.
    [Show full text]
  • TEK 8.5C: Periodic Table
    Name: Teacher: Pd. Date: TEK 8.5C: Periodic Table TEK 8.5C: Interpret the arrangement of the Periodic Table, including groups and periods, to explain how properties are used to classify elements. Elements and the Periodic Table An element is a substance that cannot be separated into simpler substances by physical or chemical means. An element is already in its simplest form. The smallest piece of an element that still has the properties of that element is called an atom. An element is a pure substance, containing only one kind of atom. The Periodic Table of Elements is a list of all the elements that have been discovered and named, with each element listed in its own element square. Elements are represented on the Periodic Table by a one or two letter symbol, and its name, atomic number and atomic mass. The Periodic Table & Atomic Structure The elements are listed on the Periodic Table in atomic number order, starting at the upper left corner and then moving from the left to right and top to bottom, just as the words of a paragraph are read. The element’s atomic number is based on the number of protons in each atom of that element. In electrically neutral atoms, the atomic number also represents the number of electrons in each atom of that element. For example, the atomic number for neon (Ne) is 10, which means that each atom of neon has 10 protons and 10 electrons. Magnesium (Mg) has an atomic number of 12, which means it has 12 protons and 12 electrons.
    [Show full text]
  • Models of the Atomic Nucleus
    Models of the Atomic Nucleus Second Edition Norman D. Cook Models of the Atomic Nucleus Unification Through a Lattice of Nucleons Second Edition 123 Prof. Norman D. Cook Kansai University Dept. Informatics 569 Takatsuki, Osaka Japan [email protected] Additional material to this book can be downloaded from http://extras.springer.com. ISBN 978-3-642-14736-4 e-ISBN 978-3-642-14737-1 DOI 10.1007/978-3-642-14737-1 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2010936431 © Springer-Verlag Berlin Heidelberg 2006, 2010 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: WMXDesign GmbH, Heidelberg Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Preface to the Second Edition Already by the 1970s, some theorists had declared that nuclear structure physics was a “closed chapter” in science, but since then it has repeatedly been found necessary to re-open this closed chapter to address old problems and to explain new phenom- ena.
    [Show full text]
  • A Study on the Wireless Power Transfer Efficiency of Electrically Small, Perfectly Conducting Electric and Magnetic Dipoles
    A study on the wireless power transfer efficiency of electrically small, perfectly conducting electric and magnetic dipoles Article Published Version Open access Moorey, C. L. and Holderbaum, W. (2017) A study on the wireless power transfer efficiency of electrically small, perfectly conducting electric and magnetic dipoles. Progress In Electromagnetics Research C, 77. pp. 111-121. ISSN 1937- 8718 doi: https://doi.org/10.2528/PIERC17062304 Available at http://centaur.reading.ac.uk/75906/ It is advisable to refer to the publisher’s version if you intend to cite from the work. See Guidance on citing . Published version at: http://dx.doi.org/10.2528/PIERC17062304 To link to this article DOI: http://dx.doi.org/10.2528/PIERC17062304 Publisher: EMW Publishing All outputs in CentAUR are protected by Intellectual Property Rights law, including copyright law. Copyright and IPR is retained by the creators or other copyright holders. Terms and conditions for use of this material are defined in the End User Agreement . www.reading.ac.uk/centaur CentAUR Central Archive at the University of Reading Reading’s research outputs online Progress In Electromagnetics Research C, Vol. 77, 111–121, 2017 A Study on the Wireless Power Transfer Efficiency of Electrically Small, Perfectly Conducting Electric and Magnetic Dipoles Charles L. Moorey1 and William Holderbaum2, * Abstract—This paper presents a general theoretical analysis of the Wireless Power Transfer (WPT) efficiency that exists between electrically short, Perfect Electric Conductor (PEC) electric and magnetic dipoles, with particular relevance to near-field applications. The figure of merit for the dipoles is derived in closed-form, and used to study the WPT efficiency as the criteria of interest.
    [Show full text]
  • Centripetal Force Is Balanced by the Circular Motion of the Elctron Causing the Centrifugal Force
    STANDARD SC1 b. Construct an argument to support the claim that the proton (and not the neutron or electron) defines the element’s identity. c. Construct an explanation based on scientific evidence of the production of elements heavier than hydrogen by nuclear fusion. d. Construct an explanation that relates the relative abundance of isotopes of a particular element to the atomic mass of the element. First, we quickly review pre-requisite concepts One of the most curious observations with atoms is the fact that there are charged particles inside the atom and there is also constant spinning and Warm-up 1: List the name, charge, mass, and location of the three subatomic circling. How does atom remain stable under these conditions? Remember particles Opposite charges attract each other; Like charges repel each other. Your Particle Location Charge Mass in a.m.u. Task: Read the following information and consult with your teacher as STABILITY OF ATOMS needed, answer Warm-Up tasks 2 and 3 on Page 2. (3) Death spiral does not occur at all! This is because the centripetal force is balanced by the circular motion of the elctron causing the centrifugal force. The centrifugal force is the outward force from the center to the circumference of the circle. Electrons not only spin on their own axis, they are also in a constant circular motion around the nucleus. Despite this terrific movement, electrons are very stable. The stability of electrons mainly comes from the electrostatic forces of attraction between the nucleus and the electrons. The electrostatic forces are also known as Coulombic Forces of Attraction.
    [Show full text]
  • Power Waveforming: Wireless Power Transfer Beyond Time Reversal
    IEEE TRANSACTIONS ON SIGNAL PROCESSING, VOL. 64, NO. 22, NOVEMBER 15, 2016 5819 Power Waveforming: Wireless Power Transfer Beyond Time Reversal Meng-Lin Ku, Member, IEEE, Yi Han, Hung-Quoc Lai, Member, IEEE, Yan Chen, Senior Member, IEEE,andK.J.RayLiu, Fellow, IEEE Abstract—This paper explores the idea of time-reversal (TR) services [1]. In particular, such an embarrassment is unavoid- technology in wireless power transfer to propose a new wireless able when wireless devices are untethered to a power grid and power transfer paradigm termed power waveforming (PW), where can only be supplied by batteries with limited capacity [2]. In a transmitter engages in delivering wireless power to an intended order to prolong the network lifetime, one immediate solution receiver by fully utilizing all the available multipaths that serve is to frequently replace batteries before the battery is exhausted, as virtual antennas. Two power transfer-oriented waveforms, en- ergy waveform and single-tone waveform, are proposed for PW but unfortunately, this strategy is inconvenient, costly and dan- power transfer systems, both of which are no longer TR in essence. gerous for some emerging wireless applications, e.g., sensor The former is designed to maximize the received power, while the networks in monitoring toxic substance. latter is a low-complexity alternative with small or even no perfor- Recently, energy harvesting has attracted a lot of attention in mance degradation. We theoretically analyze the power transfer realizing self-sustainable wireless communications with perpet- gain of the proposed power transfer system over the direct trans- ual power supplies [3], [4]. Being equipped with a rechargeable mission scheme, which can achieve about 6 dB gain, under various battery, a wireless device is solely powered by the scavenged en- channel power delay profiles and show that the PW is an ideal ergy from the natural environment such as solar, wind, motion, paradigm for wireless power transfer because of its inherent abil- vibration and radio waves.
    [Show full text]
  • Of the Periodic Table
    of the Periodic Table teacher notes Give your students a visual introduction to the families of the periodic table! This product includes eight mini- posters, one for each of the element families on the main group of the periodic table: Alkali Metals, Alkaline Earth Metals, Boron/Aluminum Group (Icosagens), Carbon Group (Crystallogens), Nitrogen Group (Pnictogens), Oxygen Group (Chalcogens), Halogens, and Noble Gases. The mini-posters give overview information about the family as well as a visual of where on the periodic table the family is located and a diagram of an atom of that family highlighting the number of valence electrons. Also included is the student packet, which is broken into the eight families and asks for specific information that students will find on the mini-posters. The students are also directed to color each family with a specific color on the blank graphic organizer at the end of their packet and they go to the fantastic interactive table at www.periodictable.com to learn even more about the elements in each family. Furthermore, there is a section for students to conduct their own research on the element of hydrogen, which does not belong to a family. When I use this activity, I print two of each mini-poster in color (pages 8 through 15 of this file), laminate them, and lay them on a big table. I have students work in partners to read about each family, one at a time, and complete that section of the student packet (pages 16 through 21 of this file). When they finish, they bring the mini-poster back to the table for another group to use.
    [Show full text]
  • The Exponential Function
    University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln MAT Exam Expository Papers Math in the Middle Institute Partnership 5-2006 The Exponential Function Shawn A. Mousel University of Nebraska-Lincoln Follow this and additional works at: https://digitalcommons.unl.edu/mathmidexppap Part of the Science and Mathematics Education Commons Mousel, Shawn A., "The Exponential Function" (2006). MAT Exam Expository Papers. 26. https://digitalcommons.unl.edu/mathmidexppap/26 This Article is brought to you for free and open access by the Math in the Middle Institute Partnership at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in MAT Exam Expository Papers by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. The Exponential Function Expository Paper Shawn A. Mousel In partial fulfillment of the requirements for the Masters of Arts in Teaching with a Specialization in the Teaching of Middle Level Mathematics in the Department of Mathematics. Jim Lewis, Advisor May 2006 Mousel – MAT Expository Paper - 1 One of the basic principles studied in mathematics is the observation of relationships between two connected quantities. A function is this connecting relationship, typically expressed in a formula that describes how one element from the domain is related to exactly one element located in the range (Lial & Miller, 1975). An exponential function is a function with the basic form f (x) = ax , where a (a fixed base that is a real, positive number) is greater than zero and not equal to 1. The exponential function is not to be confused with the polynomial functions, such as x 2. One way to recognize the difference between the two functions is by the name of the function.
    [Show full text]
  • Using Odes to Model Drug Concentrations Within the Field of Pharmacokinetics Andrea Mcnally Augustana College, Rock Island Illinois
    CORE Metadata, citation and similar papers at core.ac.uk Provided by Augustana College, Illinois: Augustana Digital Commons Augustana College Augustana Digital Commons Mathematics: Student Scholarship & Creative Mathematics Works Spring 2016 Using ODEs to Model Drug Concentrations within the Field of Pharmacokinetics Andrea McNally Augustana College, Rock Island Illinois Follow this and additional works at: http://digitalcommons.augustana.edu/mathstudent Part of the Applied Mathematics Commons Augustana Digital Commons Citation McNally, Andrea. "Using ODEs to Model Drug Concentrations within the Field of Pharmacokinetics" (2016). Mathematics: Student Scholarship & Creative Works. http://digitalcommons.augustana.edu/mathstudent/2 This Student Paper is brought to you for free and open access by the Mathematics at Augustana Digital Commons. It has been accepted for inclusion in Mathematics: Student Scholarship & Creative Works by an authorized administrator of Augustana Digital Commons. For more information, please contact [email protected]. Andrea McNally MATH-478 4/4/16 In 2009, the Food and Drug Administration discovered that individuals taking over-the- counter pain medications containing acetaminophen were at risk of unintentional overdose because these patients would supplement the painkillers with other medications containing acetaminophen. This resulted in an increase in liver failures and death over the years. To combat this problem, the FDA decided to lower the dosage from 1000 mg to 650 mg every four hours, thus reducing this risk (U.S. Food and Drug Administration). The measures taken in this example demonstrate the concepts behind the field of Pharmacology. Pharmacology is known as the study of the uses, effects, and mode of action of drugs (“Definition of Pharmacology”).
    [Show full text]
  • Introduction to Chemistry
    Introduction to Chemistry Author: Tracy Poulsen Digital Proofer Supported by CK-12 Foundation CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook Introduction to Chem... materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based Authored by Tracy Poulsen collaborative model termed the “FlexBook,” CK-12 intends to pioneer the generation and 8.5" x 11.0" (21.59 x 27.94 cm) distribution of high-quality educational content that will serve both as core text as well as provide Black & White on White paper an adaptive environment for learning. 250 pages ISBN-13: 9781478298601 Copyright © 2010, CK-12 Foundation, www.ck12.org ISBN-10: 147829860X Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made Please carefully review your Digital Proof download for formatting, available to Users in accordance with the Creative Commons Attribution/Non-Commercial/Share grammar, and design issues that may need to be corrected. Alike 3.0 Unported (CC-by-NC-SA) License (http://creativecommons.org/licenses/by-nc- sa/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), We recommend that you review your book three times, with each time focusing on a different aspect. which is incorporated herein by this reference. Specific details can be found at http://about.ck12.org/terms. Check the format, including headers, footers, page 1 numbers, spacing, table of contents, and index. 2 Review any images or graphics and captions if applicable.
    [Show full text]
  • Appendix: Key Concepts and Vocabulary for Nuclear Energy
    Appendix: Key Concepts and Vocabulary for Nuclear Energy The likelihood of fission depends on, among other Power Plant things, the energy of the incoming neutron. Some In most power plants around the world, heat, usually nuclei can undergo fission even when hit by a low- produced in the form of steam, is converted to energy neutron. Such elements are called fissile. electricity. The heat could come through the burning The most important fissile nuclides are the uranium of coal or natural gas, in the case of fossil-fueled isotopes, uranium-235 and uranium-233, and the power plants, or the fission of uranium or plutonium plutonium isotope, plutonium-239. Isotopes are nuclei. The rate of electrical power production in variants of the same chemical element that have these power plants is usually measured in megawatts the same number of protons and electrons, but or millions of watts, and a typical large coal or nuclear differ in the number of neutrons. Of these, only power plant today produces electricity at a rate of uranium-235 is found in nature, and it is found about 1,000 megawatts. A much smaller physical only in very low concentrations. Uranium in nature unit, the kilowatt, is a thousand watts, and large contains 0.7 percent uranium-235 and 99.3 percent household appliances use electricity at a rate of a few uranium-238. This more abundant variety is an kilowatts when they are running. The reader will have important example of a nucleus that can be split only heard about the “kilowatt-hour,” which is the amount by a high-energy neutron.
    [Show full text]
  • Elements Make up the Periodic Table
    Page 1 of 7 KEY CONCEPT Elements make up the periodic table. BEFORE, you learned NOW, you will learn • Atoms have a structure • How the periodic table is • Every element is made from organized a different type of atom • How properties of elements are shown by the periodic table VOCABULARY EXPLORE Similarities and Differences of Objects atomic mass p. 17 How can different objects be organized? periodic table p. 18 group p. 22 PROCEDURE MATERIALS period p. 22 buttons 1 With several classmates, organize the buttons into three or more groups. 2 Compare your team’s organization of the buttons with another team’s organization. WHAT DO YOU THINK? • What characteristics did you use to organize the buttons? • In what other ways could you have organized the buttons? Elements can be organized by similarities. One way of organizing elements is by the masses of their atoms. Finding the masses of atoms was a difficult task for the chemists of the past. They could not place an atom on a pan balance. All they could do was find the mass of a very large number of atoms of a certain element and then infer the mass of a single one of them. Remember that not all the atoms of an element have the same atomic mass number. Elements have isotopes. When chemists attempt to measure the mass of an atom, therefore, they are actually finding the average mass of all its isotopes. The atomic mass of the atoms of an element is the average mass of all the element’s isotopes.
    [Show full text]