Students' Beliefs about the Role of Atoms in Radioactive Decay and Half-Life

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Contemporary science education research emphasizes the importance of considering students pre-instructional beliefs when designing effective, learner-centered instructional strategies. When scientists teach about dating geological events, most often the concepts of radioactive decay and half-life are presented. However, the research base on student understanding of radiation and radioactivity is currently quite limited. The principal research question used to focus this investigation asked: What are the common difficulties that students experience when trying to learn about radiation and radioactivity? Our research illustrates that students bring to the classroom many inaccurate ideas and reasoning difficulties on the topics of ionizing radiation, radioactivity, and radioactive decay that are well-poised to interfere with students' understanding of how half-life is used to determine geologic time. To uncover the range and frequency of the dominant student beliefs, we performed individual demonstration interviews and administered open-response and multiple-choice conceptual tests to students from a wide-range of science backgrounds. Our results show that students are often unable to differentiate between the ideas of irradiation and contamination, and that many of these students' reasoning difficulties about radioactive decay and half-life stem from their inaccurate mental models regarding the atom.


The topics of radioactivity, radioactive decay, and half-life are foundational concepts in geology, astrobiology, chemistry, physics, biology, paleontology, astronomy, mathematics and planetary science classes. Many laboratory activities (Mak, 1999; Hoeling et al., 1999; Peplow 1999; Russo, 1999; Lumb, 1989; Austen and Brouwer 1997), analogy-based teaching strategies (Wunderlich, 1978; Evans, 1974, Celnikier, 1980; Kowalski, 1981; Priest and Poth, 1983; McGeachy, 1988), and calculation or computer-based exercises (Weinberg, 1997; Caon, 1995; Ruddick, 1995; Shea, 2001; Huestis, 2002), are offered in the literature to help students better understand radioactive decay and half-life. However, the fundamental naive beliefs and reasoning difficulties held by many students are poised to interfere with the majority of these novel instruction methods. The underlying problem is that the majority of these materials and teaching strategies are not informed by recent research into student learning and reasoning difficulties on these topics and thus fail to effectively elicit or help students resolve the naive ideas that they hold prior to instruction. In fact, of the greatest potential concern is that many of the activities listed above are likely to enhance or reinforce the student difficulties described here.

The principal research question used to focus this investigation asked: What are the common difficulties that students experience when trying to learn about radiation and radioactivity? Prior research (see Part I) suggests that one student learning difficulty with these topics stems from their inability to correctly differentiate between irradiation and contamination, believing that an object exposed to ionizing radiation will become radioactive. In addition our research suggests that half of college students enrolled in introductory physics courses believe that orbital electrons are causally related to the radioactive state of atoms and to the decay process. Making matters worse, an equal percent of these students believe that the mass and volume of a radioactive substance will decrease in the period of a half-life.

We assert that the students' naive beliefs and reasoning difficulties related to a particular topic which are identified from an investigation conducted in one subject area (such as physics) are certainly also poised to interfere with instruction on the same topic while being covered in a different subject area (geology, chemistry, astronomy etc.) Furthermore the overall population of students who enroll in the introductory non-science major course through the introductory majors course in one subject area (such as geology) may well provide a representative sample of the general population of students that enroll in the same range of courses within any of the other subject areas (listed above) that teach about radiation and radioactivity. …


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