School Science Lessons
2008-12-17
Please send comments to: J.Elfick@uq.edu.au
Images of Atoms
by Tony Wright, School of Education, The University of Queensland, Brisbane, Australia
Contact details
References
Abstract
A simple image, the fuzzy ball atom, is recommended to help students develop a useful understanding of our molecular world. It is argued that the image helps students grasp ideas about atoms and molecules readily and leads naturally to more advanced ideas of atomic structure, chemical bonding and later, quantum mechanics. Everyday contexts and analogies for introducing the concepts are considered. Alternative images are reviewed and it is argued
that the widely used planetary images of atoms should be abandoned because of the misconceptions they introduce. Increasing evidence suggests that it is appropriate to introduce the ideas early in the middle years of schooling.
Images of Atoms
What image springs to mind when the word "atom" is mentioned? The question may seem of minor importance, but having a useful image in mind is critical for students learning about the atomic nature of our world. The atomic hypothesis lies at the heart of our scientific knowledge. When students learn about atoms, they are given the key to unlocking many of the doors across the sciences, in physics, chemistry biology and earth science. Since our understanding of the gross features of the atoms have not changed in the past 50+ years it might be expected that the introduction of the concept would follow a well-established path. Nothing could be further from the truth! A survey of introductory texts at all levels reveals an array of introductory strategies. In addition, a confusing variety of images are used to represent the atom. Recent chemical education research shows that even the best students who are exposed to this range of images struggle to learn a scientifically acceptable version of the concept (Birk, 1999, Harrison, 2000, Nicoll, 2001). The task of the science teacher to facilitate this learning is made doubly difficult because the information is scattered around the literature. In this article I have combined a review of the science education research literature with a survey of current practice. The aim is to reflect on the teaching of the concepts, canvassing ideas that need to be addressed and providing a list of readily available sources of information. It is my belief that there is a simple solution to the question of what image of atoms to use to introduce the topic. The answer is the image that comes from the quantum mechanical model of the atom. The image is simple but it needs to be given a name that is meaningful for the student rather than the expert chemist.
The article is about the how, when and why they should be used. The initial part of the article contains a discussion of the image:
  • Probing why it is useful for the student and matches current chemical understanding.
  • Examining the relationship of the image to the properties of the underlying model of the atom.
  • Canvassing contexts for introducing the concept of the atom.
  • Surveying the conceptual steps commonly used to elaborate the model.
  • The later sections give:
  • A review of the common models used by students.
  • A survey of some important misconceptions held by students.
  • A reflection on the timing of the introduction of students to atomic theory.
    • A useful image
    The image of the atom as a sticky, fuzzy ball has the virtue of simplicity (See Figure 2). The image is not new. It has been recommended for use in chemical education resources (Orta, 994) and used in Australia (Bucat 1983) and the United States (Moore 2002). Part of the reason that the image has not been widely accepted probably derives from the lack of a useful name. I have been using the term "fuzzy" for more than 20 years, having picked it up
    from physics Nobel laureate and distinguished educator, Richard Feynman (Feynman 1995).
    In my experience students find the image (and term) memorable and useful. Features favouring its use include:
  • The idea that atoms are like a soft ball with an ill-defined surface is a simple idea that can be illustrated with a range of balls bought from toy stores.
  • Stickiness of atoms leads naturally to ideas about the formation of molecules and the different states of matter.
  • Further, thinking about stickiness leads to consideration of the structure of atoms and the introduction of electrons and nuclei.
  • The concepts that underpin the image are In line with the quantum mechanical picture of the atom.
  • The image is easily illustrated using drawing software.
    • When elaborated to include sub-atomic structure, the image can lead to a couple of misconceptions to which students need to be alerted:
  • Including the dot that represents the nucleus in the image inevitably gives a misleading idea about the size of the nucleus because the dot would be too small to see if drawn to scale.
  • If the electron "cloud" is very grainy in the image, students may conclude the electrons are the grainy dots, like the water droplets in a cloud. In fact we do not know how big the electron is, just where it is likely to be found. The cloud represents the region where the electron is likely to be.
    • Elaborating the model of the atom
    An important feature of any image is that it can be elaborated further without criticism that early forms are incorrect. This is true for the fuzzy ball atom because the image is based on the quantum mechanical model, one of the most-tested models in the history of science. Table 3 shows how this elaboration can take place, highlighting some important landmarks and underlying constructs.
    Table 3 Three steps that are commonly used to help students elaborate their model of the atom
    Atoms are minute particles (Fuzzy, sticky ball) Atoms, molecules, elements and compounds. States of matter and conversions. Solutions and introduction to concentrations. . .
    . Atoms have structure (Electrons, Nuclei, Protons, Energy levels, Neutrons) Ions and ionic compounds. Molecular shape, Lewis structures and VSEPR theory. .
    . . Atoms have quantun mechanical properties (Energy levels, Orbitals, Quantum numbers, Electron configurations)
    Middle School Senior high school University
    Features of the elaboration process include.
  • An early step from atoms to molecules is important if the concepts are to be introduced in context because most contexts involve molecules. This can be done before the introduction to sub-atomic structure since molecules can be considered simply as atoms stuck together.
    • The idea of chemical bonding can be elaborated once electron clouds are introduced since a merging of electron clouds is readily grasped and can be elaborated later into orbital overlap.
    See Fig. 4: Atoms stick together by overlapping electron cloud
  • There are clear steps in the elaboration process from ideas about atoms to electrons, and then subsequently from counting electrons to considering electronic properties.
    • Can we see atoms? Yes and no
    See Fig. 1: Iron atoms being moved individually on a surface to form a ring
    (Reproduced with permission from IBM, Almaden Research Center)
    Atoms are much too small to be seen using visible light, but if shorter wavelength light is used, then images of atoms can be created (using X ray crystallography). The more recent development of scanning tunnelling microscopes has provided an alternative way of imaging atoms. These images can be used to track the manipulation of individual atoms as is shown in this image from IBM. These images show iron atoms being moved individually on a surface to form a ring.
    The fuzzy ball atom (Quantum Mechanical Model)
    See Fig. 2: Fuzzy ball atom
    Atoms are tiny, almost spherical, particles that are sticky and do not have a defined surface (they are fuzzy). They are made up of a much tinier, dense nucleus that surrounded by electrons. The electrons form a "cloud" of electron density that tapers off moving away from the nucleus so there is no sharp boundary to an atom.
    Contexts and analogies
    Everyday Contexts
    Making the concept useful is a key to facilitating successful learning. In the case of atoms, this is easy in the sense that everything we touch or touch with, is made of atoms, but traditional approaches often ignore the obvious and take an historical perspective. Given that we cannot directly see atoms, making use of our ability to feel them is a powerful potential context. It Is with gases that we can illustrate this most easily when we feel the movement of air while the atoms remain invisible. Breathing, smelling, or simply moving about provides a stepping off point to a wide range of applications that allow drawings or stories that describe the role of the atoms (and molecules). The stickiness of atoms and molecules can then be invoked to develop ideas about liquids and solids (although care must be taken to tread carefully around ions and ionic solids until electrons and other subatomic particles have been introduced). There are good resources on the Web to support this approach such as those developed by the Operation Primary Physical Science project (Kirwan 2002). This approach allows the mastery of the first step in the development of the atomic concept before launching into the second phase. This is the approach recommended by the influential Project 2061 drawn from the American Association for the Advancement of Science and the American
    Science Teachers Association (AAAS Project 2061, 2001).
    Historical Context
    The other common approach involves an examination of the historical development of the concepts. This story is one of the major triumphs of the physical sciences and therefore deserves a place in the curriculum. However the timing for the use of the story needs to be carefully judged because it is complex and steps along the way can provide students with misleading images if the later developments are not appreciated.
    The (planetary) Bohr model is a particular case in point because it is often the most sophisticated model given to students who do not move on to the quantum mechanical model. Students may be left with this model that is misleading and hinders the development of ideas about chemical bonding.
    Alphabet Analogy
    See Fig. 5: Cat shape formed from letters
    A language analogy is powerful for helping students to grasp the relationship between atoms and molecules. The atoms are like letters that stick together to form molecules (words). In the analogy the shape of the cat bears no relationship to the shape of the letters or words of which it is composed. This is a point worth careful attention because students commonly hold ideas that the properties of atoms and individual molecules reflect the collective properties of the macroscopic object of which they are part. The point that molecules always contain multiple atoms while single letter words are possible shows that the analogy is not perfect.
    Sports Team Analogy
    An analogy of a sports team being like a molecule is an alternative way of helping students to grasp the concept of atoms sticking together to form molecules at a very simple level. Chemical reactions can come alive for students the members of teams are rearranged to play different sports. The analogy is also good for emphasizing that atoms and molecules are in constant motion (a deficiency in the alphabet analogy.)
    Rival images
    Recent research into the mental images that students carry of atoms shows that students often have several alternatives to the fuzzy ball image (Nicoll 2001, Harrison 2000).
    Hard-sphere atom
    See Fig. 6: Ball-and-stick and space-filling images of methane
    This image is common because most pictures of atoms involve the drawing of a sphere. The obvious shortcoming of the image is that the atom appears to have a clearly defined surface rather then petering out to nothing. Since students are bound to be exposed to this image, it is critically important that they are shown how the common boundaries for atoms are chosen to suit the purpose of the representation. This occurs whenever instruments such as scanning tunnelling microscopes or X-ray diffractometers are used to generate images of atoms and molecules. Similar choices are made when space-filling and ball-and-stick images are generated using chemical drawing software. Exploring the shortcomings of these images helps students appreciate the slightly more elaborate fuzzy-ball model.
    Planetary Images
    See Fig. 7 A: A common image used in textbooks to represent a planetary model of the atom
    There are several images associated with the planetary model of the atom that need more careful consideration because they can lead to several common misconceptions that are likely to hinder students' deeper understanding of atomic and molecular structure. In its most commonly used form, in introductory chemistry and general science textbooks, the model is drawn as a series of concentric rings representing electronic orbits around a central nucleus
    The number of electrons in a single orbit is determined from the rules of valence. Individual rings are labelled electron shells or energy levels.
    See Fig. 8: An image of a planetary atom that students may pick up outside the classroom
    Planarity is implied in the common two-dimensional representation and may be reinforced by the use of the term "planetary" model
    This misconception is overcome in some everyday images in which the electrons are represented as forming a cage of orbits around the nucleus. However, most of the other shortcomings of planetary model are not addressed in this version.
    Images of cells
    Students sometimes confuse the model they are learning for atoms with the model they are learning for cells (Nicoll, 2001). This is an understandable confusion and can lead to students ascribing animate properties to atoms (such as the ability to reproduce). Explicit attention to how the models for atoms differ from those for cells and planetary systems is a useful solution.
    Misconceptions
    A number of misconceptions associated with learning about atoms have been identified.
  • Electrons are not found in atoms in planetary orbits
    • See Fig. 9: An unsuccessful attempt to rationalize chemical bonding using electrons in orbits
    When asked to draw an atom, junior high school students commonly use images of atoms with electrons in orbits about the nucleus, Many prefer this image despite being aware of ideas about electron clouds (Harrison, 1996). One of the major disadvantages of electron orbits is that orbits cannot be used to help students learn about molecules and chemical bonding. The result of such an attempt is an image that does not bear scrutiny. For example, in the figure below, taken from a senior high school textbook, an attempt is made to represent the formation of H2. However, the electrons are not longer in orbits that have any meaning, see figure.
  • Electrons are not paired spatially
    • See Fig. 10: An image of a planetary atom that combines a physical and symbolic model of the atom
    Images that are hybrids of physical models and symbolic models are common in textbooks and have been criticized because the attributes of the models are no longer clearly represented (Justi, 2000). In the example shown, a planetary model is combined with a symbolic representation used as a tool for assigning electron configurations and drawing Lewis structures. The figure places electrons in pairs around an orbit. Simple electrostatic arguments uncover that this is physical nonsense, but students who lack practice with electrostatic arguments are likely to accept the image uncritically.
  • Energy levels are misleadingly represented by spatial orbits
    • See Fig. 9: An unsuccessful attempt to rationalize chemical bonding using electrons in orbits
    This misconception is revealed in images such as the one in the figure in which successive orbits are labelled energy levels or electron shells. The misconception is serious because the quantization of electronic energies is a surprising and important experimental result reproduced by the quantum mechanical model. To represent the levels spatially is to imply that the spatial distribution is fundamental rather than the energy.
    See Fig. 11: An atomic energy level diagram for a neon atom
    A more useful approach is to introduce students to energy level diagrams. These diagrams emphasize that it is the energy of the electrons that is being represented. In these diagrams, the electrons are quite rightly paired (where possible) because they have degenerate energies. The other term, "electron shells", is often used as a synonym for energy levels and is more misleading than "energy level" because the word energy is not used. Students interviewed by Harrison and Treagust (1996) talked about images of seashells or egg shells when asked about electron shells revealing that the relationship to the electron energy had been lost.
    Reflections on timing
    When should students be introduced to the concepts of atoms and molecules?
    The central theme of this article is that we need to help students learn about the structure of matter by carefully considering the learning experiences we, as teachers use. This process is usefully informed by marrying the research in science education research with science content. One part of this process involves deciding when a particular topic should be taught. There is a surprising lack of agreement about when students should be introduced to the concepts of atoms and molecules.
    On the one hand there is a view that says the ideas should be left until near the end of the middle school years. This view is promoted in the National Science Education Standards published in the United States (National Research Council, 1996). Advising teachers of grades 5-8 that: "At this level few students can comprehend the idea of atomic and molecular particles." A number of the Australian States have followed this line of thought and left the introduction of atoms until the final curriculum level of compulsory schooling.
    On the other hand there are relatively recent research studies (Lee, 1993) that show that the quality instruction and curriculum materials have a very marked influence on students' learning and: "We would conclude that these ideas are not beyond the intellectual reach of most sixth-grade students." In line with this idea, the American Association for the Advancement of Science (AAAS Project 2061, 2001) has endorsed a recommendation that the concept of atoms and molecules should be introduced in grades 6-8 and that subatomic structure be treated later.
    There are many good reasons for seriously considering this latter approach.
  • The most exciting frontiers of science in molecular biology and nanomaterials and a host of the serious environmental and science and society issues become accessible once the term molecule is understood.
  • Much of the middle years schooling science curriculum in biology and earth science and also chemistry and physics can be understood at a different level when students understand the structure of the matter they are thinking about.
  • Giving students plenty of practice in the use of the concepts of atoms and molecules in a range of contexts at the simplest level is likely to help students grasp the ideas more thoroughly.
    • The former approach has the flaw that students are asked to master the ideas of atoms and molecules at roughly the same time that they address issues about the subatomic structure. There is potential for serious confusion because students have to master the relationships between atoms, molecules, elements and compounds at the same time as those of electrons, protons, neutrons, ions and ionic compounds. These arguments lead to the suggestion that the introduction of students to the concepts of atoms and molecules in the early middle school years is pedagogically sound. However more research evidence that considers both timing and introductory strategies is needed.
    Summary
    "If in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis. "
    (Feynman 1995)
    The image of atoms that people carry around in their heads is of such importance that it needs to be taught with great care. It needs to be simple, accurate and accessible to everyone because it is the foundation of our understanding of the molecular world. The approach advocated in this paper is based on a fuzzy ball image used for several decades, but not widely taken up for use when introducing students to the concepts of atoms and molecules. It is suggested that hard sphere images of atoms also needs to be considered because they are so commonly used both inside and outside the classroom. In contrast it is recommended that planetary images of atoms should not be used because of the inherent misconceptions that are likely to impede student learning. Finally it is suggested that there increasing evidence that students should be introduced to atoms and molecules early in their middle years of schooling.
    Contact details
    Tony Wright is a chemist and science educator at The University of Queensland. He has twenty years experience teaching in schools and universities in England, Australia and New Zealand and has a special research interest in students' understanding of molecular shape.
    Address: Dr Tony Wright, School of Education, The University of Queensland, Building 4, 11 Salisbury Rd., Ipswich, Old 4305
    Fax (07) 3381 1515
    Email: tony.wright@uq.edu.au
    References
    AAAS Project 2061 (2001) Atlas of Science Literacy American Association for the Advancement of Science.
    Bucat, R. B. (1983) Elements of Chemistry. Earth, Air Fire and Water, Australian Academy of Science, p 36.
    Feynman, R. P. (1995) Atoms in Motion, Six Easy Pieces Addison-Wesley Publishing Company, 1-22.
    Harrison, A. G. and Treagust, D. F. (1996) Secondary Students' Mental Models of Atoms and Molecules: Implications for Teaching Chemistry, Science Education 80(5) 509-534.
    Harrison, A. G. and Treagust, D. E (2000) Learning about atoms, molecules, and chemical bonds: A case study of multiple-model use in grade 11 chemistry, Science Education 84(3) 352-381.
    Justi, R. (2000) History and philosophy of science through models: some challenges in the case of "the atom" International Journal of Science Education 22(9) 993-1009.
    Lee, 0. Eichinger, D. C. Anderson, C W, Berkheimer, G. D. and Blakeslee, T. D. (1993) Changing Middle School Student's Conceptions of Matter and Molecules, Journal of Research in Science Teaching 30 (3) 249-270.
    Moore, J. W., Stanitski, C.L. and Jurs, P. C. (2002) Chemistry the Molecular Science, Harcourt College Publishers, Fort Worth.
    National Research Council (1996) National Science Education Standards, National Academy Press, 149.
    Nicoll, G. (2001) A report of undergraduates' bonding misconceptions, International Journal of Science Education 23(7) 707-730.
    Orta, M. V. (1994) Atomic Structure, SourceBook, Vol. 1, Version 1.0 ChemSource Inc.
    Sewell, A. (2002) Cells and Atoms -Are They Related? Australian Science Teachers Journal 48(2) 26-30.