ATOMS AND MOLECULES
One of the most powerful developments in the history of science is the atomic/molecular model of matter, which can be used to explain and predict a large variety of phenomena. Ideas about elements, atoms, and their combination in molecules or large arrays develop from two notions: that matter is made of invisibly tiny pieces and that the enormous variety of materials in the world is the result of different combinations of a relatively small number of basic ingredients.
In high school, these ideas extend to benchmarks about the relation of a material's properties to its atomic or molecular make-up, the structure of the atom itself, and the existence of isotopes and radioactivity. The sections Understanding Fire and Splitting the Atom in Science for All Americans and Benchmarks Chapter 10: HISTORICAL PERSPECTIVES could provide context in instruction for the study of atomic/molecular theory.
The 3-5 benchmark "Air is a substance..." may seem an odd addition (the other 3-5 benchmarks talk about materials in general), but it helps prepare students for the idea of particles too small to see. More generally, to understand the structure of matter, students must eventually recognize gases as material substances.
Benchmarks about scale could help prepare students for the idea of atoms.The actual scale of atoms and molecules, however, is quite beyond most everyone's conception and "too tiny to see through a microscope" is adequate for most purposes.
The notion that roughly 100 "elements" make up all of the material world is introduced in 6-8. This benchmark, and the 6-8 benchmark that follows from it, "There are groups of elements...," are not intended to provide a fully developed concept of elements. The atomic definition comes later.
The 9-12 benchmark "When elements are listed in order..." conveys the empirical fact that patterns of properties recur as atoms get heavier. This statement is clearly incomplete and is meant mostly to set up study of formal periodicity for those students who will go beyond literacy.
Two 9-12 benchmarks discuss the relationship between the structure and properties of matter. One relates the properties of an atom to its arrangement of electrons, and the other relates the properties of a molecule to its arrangement of atoms. A double-headed arrow between the two benchmarks indicates that they are closely related, but that neither is conceptually dependent on the other.
The benchmarks about isotopes and radioactivity may seem extraneous, but they clearly build from the earlier benchmarks laid out here. Also, radioactive dating of materials provides an important connection to the extreme age of the earth in the Changes in the Earth's Surface map—and hence indirectly to the Natural Selection map (in Chapter 5).
Research in Benchmarks
Elementary- and middle-school students may think everything that exists is matter, including heat, light, and electricity (Stavy, 1991; Lee et al., 1993). Alternatively, they may believe that matter does not include liquids and gases or that they are weightless materials (Stavy, 1991; Mas, Perez, & Harris, 1987). With specially designed instruction, some middle-school students can learn the scientific notion of matter (Lee et al., 1993).
Middle-school and high-school students are deeply committed to a theory of continuous matter (Nussbaum, 1985b). Although some students may think that substances can be divided up into small particles, they do not recognize the particles as building blocks, but as formed of basically continuous substances under certain conditions (Pfundt, 1981).
Students at the end of elementary school and beginning of middle school may be at different points in their conceptualization of a "theory" of matter (Carey, 1991; Smith et al., 1985; Smith, Snir, & Grosslight, 1987). Although some 3rd graders may start seeing weight as a fundamental property of all matter, many students in 6th and 7th grade still appear to think of weight simply as "felt weight"—something whose weight they can't feel is considered to have no weight at all. Accordingly, some students believe that if one keeps dividing a piece of styrofoam, one would soon obtain a piece that weighed nothing (Carey, 1991).
Students of all ages show a wide range of beliefs about the nature and behavior of particles. They lack an appreciation of the very small size of particles; attribute macroscopic properties to particles; believe there must be something in the space between particles; have difficulty in appreciating the intrinsic motion of particles in solids, liquids and gases; and have problems in conceptualizing forces between particles (Children's Learning in Science, 1987). Despite these difficulties, there is some evidence that carefully designed instruction carried out over a long period of time may help middle-school students develop correct ideas about particles (Lee et al., 1993).