To understand the development of modern ideas about the inner workings of atoms and the contributions of J.J. Thomson.
This lesson is the fourth of a five-part series that will broaden and enhance students’ understanding of the atom and the history of its discovery and development from ancient to modern times.
The History of the Atom 1: The Ancient Greeks examines the ancient Greeks’ theories about the atom. The History of the Atom 2: Dalton explores early milestones in atomic theory and the role of John Dalton. The History of the Atom 3: The Periodic Table reviews the early development of the periodic table and its impact on atomic thought. This lesson analyzes the evolution of modern ideas on the inner workings of atoms and J.J. Thomson’s contributions. The History of the Atom 5: The Modern Theory investigates the development of modern atomic theory.
Greek philosophers Leucippus and Democritus first developed the concept of the atom in the 5th century B.C.E. However, since Aristotle and other prominent thinkers of the time strongly opposed their idea of the atom, their theory was overlooked and essentially buried until the 16th and 17th centuries. In time, Lavoisier’s groundbreaking 18th-century experiments accurately measured all substances involved in the burning process, proving that “when substances burn, there is no net gain or loss of weight.” Lavoisier established the science of modern chemistry, which gained greater acceptance because of the efforts of John Dalton, who modernized the ancient Greek ideas of element, atom, compound, and molecule and provided a means of explaining chemical reactions in quantitative terms. (Science for All Americans, pp. 153–155.)
As this series of lessons explores further discoveries in the configuration, bonding, and inner structures of atoms, students will come to realize how much more refined, modernized, and scientific atomic theory has become since the critical breakthroughs of Lavoisier and Dalton three centuries ago.
It is important for students to understand that the study of matter continues to this day, and that humankind’s millenniums-old effort to identify, understand, and document the nature of matter eventually created modern sciences like chemistry and continues to lead to countless, purposeful technological advancements and inventions—like the TVs and computers that make the quality of life for humankind more and more fulfilling, convenient, and sometimes troubling. Students also should come to realize that, over time, the ancients’ ideas of matter were often proven inaccurate through modern science.
In middle school, students should have become familiar with the early theories of matter and how they led to the work of Lavoisier and the birth of modern chemistry. This awareness will help students better understand the importance of John Dalton’s work, and how he ultimately ushered in “the consistent use of language, scientific classification, and symbols in establishing the modern science of chemistry.” (Benchmarks for Science Literacy, pp. 250–251.)
Ideas in this lesson are also related to concepts found in these Common Core State Standards:
- CCSS.ELA-Literacy.RST.9-10.1 Cite specific textual evidence to support analysis of science and technical texts, attending to the precise details of explanations or descriptions.
- CCSS.ELA-Literacy.RST.9-10.2 Determine the central ideas or conclusions of a text; trace the text’s explanation or depiction of a complex process, phenomenon, or concept; provide an accurate summary of the text.
This lesson should take either a single 90-minute class period or two 45-minute class periods.
Briefly review what students learned in The History of the Atom 1 through 3. Students should take out their notes, charts, and timelines marking the evolution of atomic theory from the ancients to Dalton, including the “discovery” of the periodic table. Use these and other materials as a basis for an open review.
From the first lesson, students should recall that the origins of atomic thought date back to the ancient Greek atomists Leucippus and Democritus, who believed that all matter was composed of atoms that were infinitely small, indivisible, solid, homogeneous, and varied in their size, shape, and weight. They also theorized that atoms came with a void, an empty space between atoms that allowed them to change and move. Though their theories were remarkably insightful and partly accurate, these ideas were deemed controversial and “Godless” by Aristotle and, later, by the Catholic Church and were ultimately written off until the 17th century.
From the second lesson, challenge students to recall the early modern atomic and pre-chemistry ideas and findings of Galileo, Bacon, Boyle, and Newton—some of whom “resurrected” the theories of the ancient atomists (at the expense of Aristotle) to scrutinize or support their more modern and empirical studies and findings. From a historical standpoint, remind students of the long, natural, progressive, cumulative human process that has been involved—and is mostly responsible for—the development of the modern atomic theory. In reviewing Dalton, students should be able to (1) differentiate physical and chemical atomism; (2) articulate the circumstances, motivation, goals, and accomplishments that led to Dalton’s chemical atomic theory; and (3) list the four basic ideas behind his landmark theory, which helped to formally modernize, systematize, and legitimize the study of atomism.
From the third lesson, students need to be able to discuss the critical efforts and discoveries in the periodic table’s long history of development. Their notes and timelines will similarly help them recall the ongoing discovery of individual elements and triads, and how scientists like de Chancourtois and Newlands recognized similarities between the properties of elements and attempted to arrange them in a “natural” order. Students should be able to explain how Mendeleev became the first to succeed in creating an internationally accepted periodic table, and the kinds of extraordinary risks and measures he took to ensure its adaptability to future scientific discoveries. The work of Rutherford, A. van den Broek, Moseley, and Seaborg is also worth touching on to remind students of the considerable group-effort involved in fine-tuning the accuracy of today’s periodic table. Emphasize how this concrete, systematic organization of elements established a stronger framework from which modern scientists have been able to further explore the nature, bonding, and inner workings of atoms in the past, present, and for centuries to come.
After this full, general review, students should use their J.J. Thomson student esheet to visit the Atomic Structure Timeline. Briefly describe the site, pointing out the key figures from Democritus to Dalton to Enrico Fermi in the development of atomic theory. Students should use this as a general resource to review, reinforce, and learn more about the long and eventful history of atomic discovery.
After this helpful review, students should be well prepared to begin this lesson and read about science’s exploration of the inner structure of the atom through the work of J.J. Thomson in the late 19th century.
Students should use their esheet to go to and read A Look Inside the Atom and 3 Experiments, 1 Big Idea. Inform students that they will be responsible for the content on these two pages, but encourage them to explore the other interesting sections, links, and pictures to enhance their learning. Allow students 15 minutes to do this reading activity and answer the questions on the J.J. Thomson student sheet.
Once students are finished, discuss what they have learned about J.J. Thomson, his experiments on the inner structures of atoms, and how he came to discover the electron. Go over the questions from the student sheet to guide your discussion. You can find answers to the questions on the J.J. Thomson teacher sheet. NOTE: Depending on your goals and the level of the class, you may wish to ask more specific questions on the technical points of Thomson’s experiments.
In the second part of this lesson, students should read and explore through each section of the interactive Discovery, 1897 Web resource. After students briefly review the “Background to the Discovery,” they should watch the Reconstruction of Thomson’s Experiment video presenting a modern portrayal of Thomson’s discovery of the electron.
After students watch this very short clip a few times, ask them these and other questions to gauge their comprehension:
- What do you see?
- (Accept all reasonable answers.)
- What is happening in the experiment?
- (Accept all reasonable answers.)
- What is remarkable about the experiment itself and what you just viewed?
- (Accept all reasonable answers. Elicit comments about the unusual apparatus; Thomson’s scientific genius; and the use of rays, tubes, and magnetic and electrical fields to discover something that could advance the study of atomism and ultimately change the world.)
Now students should view an animation of Thomson’s Experiment measuring the e/m ratio. Students should first read the orientation on the e/m experiment at the Interactive Animation of the E/M Experiment page from Discovery, 1897 so they understand in better detail what the e/m ratio represents and how it came to determine that an electron’s mass is about 2000 times smaller than a simple hydrogen atom. Please note that the animation linked to from this page does not appear to work. Students should instead go to this animation of Thomson’s Experiment.
Encourage students to have fun playing with this innovative graphic apparatus by changing the different variables and adjusting the direction of the beam by moving the magnetic and electric field sliders.
Ask and answer students’ questions about what is happening. Have them try to recreate Thomson’s experiment. They will notice that as they increase the magnetic field intensity, the dot (or beam of electrons) will deflect upward. Then, when they increase the electric field intensity, the dot will move back to its original position, just like Thomson’s experiment. Conversely, when the electric strength is maximized and the magnet is zero, the beam will deflect downward.
Now, students should listen to the archive recording of J.J. Thomson. [NOTE: the recording is from 1934 and can be difficult to understand, partly due to Thomson’s English accent! For this reason, have students listen to the recording a few times before they address these and other related questions.]
- What is Thomson talking about?
- (In brief, Thomson is talking about the significance of the discovery of the electron, and he marvels over how something with such a small mass could change science and directly affect the lives of people through the creation of “new industry.”)
- What connection does Thomson make between the discovery of the electron and unemployment?
- (He is asserting that continued electron-related research and development could have a considerable effect in creating jobs and healing social ills in Britain and elsewhere.)
- In your opinion, how has the work of Thomson advanced the study of atomic theory and the role of science in society?
- (Accept all reasonable answers. The discovery of the electron was the first significant discovery milestone on the internal structure of atoms. From the research and industry that his discovery triggered, he helped to take science out of the laboratory and applied it to help and alleviate social ills through the kind of “scientific enterprises” Francis Bacon emphasized in the early 17th century.)
Finally, students should read through the last three sections of the site:
- Biography of J.J. Thomson
- Physics around 1900
- Other significant scientific and technological anniversaries in 1997
Emphasize how the “new” science and research of physics continues to be “carried out in laboratories supported by government, university, and industry” for the benefit of greater society. Like the discovery of the electron, the anniversaries highlighted on the last page will provide students with further examples of how scientific discovery leads to technological growth (which creates jobs, enhances lives, and yet sometimes creates new challenges for humankind).
After reading, discussing, and applying what they have learned in this lesson, students should now be able to explain (1) the significance of Thomson’s experiments, and (2) what they contributed to our modern theory of the atom. Depending on your preferences, students can address these points in an essay or report. Encourage students to consider and reference in the assignment how Thomson’s discovery complements what they have learned—and recently reviewed—about the history of atomic theory in past lessons.
Follow this lesson with the final lesson in the history of the atom series: The History of the Atom 5: The Modern Theory.
Discovery of Electrons: Legacy for Today is the final section of the “Inside the Atom” resource that continues to enlighten students about the future of the electron beyond Thomson’s initial discovery. It examines the work of Einstein and George Paget Thomson, J.J.’s Nobel Prize-winning son. It also features links to exhibits on electron-inspired transistors, telecommunications, and medical imaging.