The History of the Atom 5: The Modern Theory

What You Need

The History of the Atom 5: The Modern Theory


To explore the development of modern atomic theory.


This lesson is the fifth 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. The History of the Atom 4: J.J. Thomson analyzes the evolution of modern ideas on the inner workings of atoms and J.J. Thomson’s contributions. This lesson 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 Americanspp. 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 Literacypp. 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.
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Planning Ahead

This lesson should take two 45-minute class periods.


Briefly review and discuss what students learned in The History of the Atom 4. After reading and interacting with the work of J.J. Thomson, students were able to follow along on the first critical scientific adventure into the inner world of the atom and the discovery of the electron.

While reviewing Thomson’s purpose in experimenting with cathode rays and electrical currents inside glass tubes, point out that Thomson was the first scientist highlighted in this history series to use advanced instrumentation in the atomic discovery process. In this final lesson, they will learn of the greater role that bigger and more advanced instruments like accelerators have taken in advancing the study of modern atomic theory.

Students should be able to recall Thomson’s daring hypothesis—that cathode rays were “streams of particles much smaller than atoms.” Have them recount the innovative and mechanical details of the “3 experiments” he needed to isolate and identify “corpuscles” (electrons), or “the substance from which all the chemical elements are built up.”

While discussing Thomson’s bending rays and electromagnetic fields, take a moment to “step back” and have students consider and appreciate the truly amazing extent to which past and present scientists have had to go to identify the atom—the fundamental component from which all matter in life is created—and, now, the inner structure and workings of this basic, invisible, and mysterious entity.

In assessing the significance of the electron’s discovery, students might recall Thomson himself marveling over how something so miniscule could change science and address social ills and unemployment via research and technological development. It might be worth noting that Thomson and others had a critical hand in the kinds of computer technology, TVs, and other scientific enterprises that humanity enjoys and lives on today. As with other scientific discoveries, students should be reminded that Thomson had plenty of help and was able to make and confirm his discoveries through the work of countless other scientists (e.g., Perrin, Lenard, etc.).

Explain to students that, since the discovery of the electron in 1897, modern atomic theory has advanced considerably. Students will learn in this lesson how Thomson’s findings helped to kick off a widespread, ongoing exploration into the inner structure of atoms—an exploration that continues to uncover numerous kinds of particles, forces, patterns, and subatomic insights. These and other findings have led to a Standard Model, which scientists hope will one day explain in great detail the structure and stability of all matter.


Before starting, it is worth noting a number of misconceptions and difficulties students have had in the area of “particles.” For example, as noted in The History of the Atom 2, some middle- and high-school students “may think that substances can be divided up into small particles, [but] they do not recognize the particles as building blocks, but as formed of basically continuous substances under certain conditions” (Pfundt, 1981). Further research has uncovered the following:

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. (Benchmarks for Science Literacy, pp. 336–337.)

Class One
Students should use The Modern Theory student esheet to go to and read each section of Electrons in Atoms, a chronological timeline marking milestones from 1897 to the present in the advancement of atomism since the electron.

Depending on your preferences, you may either have students read on their own or as a class, where time is taken after each section to pose questions and discuss the significance of the electron-related discoveries and inventions. Once the material has been covered, assign the Electrons in Atoms student sheet, either as an assignment or quiz. Answers are provided on the Electrons in Atoms teacher sheet.

Class Two
This second class is based on the comprehensive Web resource The Particle Adventure.

To begin, students should read A Summary of Particle Physics, which briefly describes the evolution of particle physics from the early 1900s to the present. NOTE: This section covers both the history and science of particles. While it is important to keep the focus on the history, you may choose to emphasize the scientific particle basics, depending on your purpose and the level of your students. Use the questions on the Particle Physics teacher sheet to guide your discussion after students finish reading.

Next, direct students to read and review the Particle Physics Timeline section of the site, specifically the final two sections: 1900–1964 A.D.: Quantum Theory and 1964–Present: The Modern View (the Standard Model).

While these timeline sections cover the evolution of atomic theory since the Greeks, students should focus on these last two modern sections, which cover the advent of the quantized atom, leading to the recent theory of the Standard Model.

Before students begin reading the specific chronological events in each area, point out or paraphrase the helpful introductions in each section, which help to briefly summarize the history of subatomic development since J.J. Thomson. They are as follows:

Quantum Theory—1900 to 1964

At the start of the twentieth century, scientists believed that they understood the most fundamental principles of nature. Atoms were solid building blocks of nature; people trusted Newtonian laws of motion; most of the problems of physics seemed to be solved. However, starting with Einstein's theory of relativity which replaced Newtonian mechanics, scientists gradually realized that their knowledge was far from complete. Of particular interest was the growing field of quantum mechanics, which completely altered the fundamental precepts of physics.

Modern View—Standard Model—1964–present

By the mid-1960's, physicists realized that their previous understanding, where all matter is composed of the fundamental protons, neutrons, and electron, was insufficient to explain the myriad new particles being discovered. Gell-Mann's and Zweig's quark theory solved these problems. Over the last thirty years, the theory that is now called the Standard Model of particles and interactions has gradually grown and gained increasing acceptance with new evidence from new particle accelerators.

After students read and discuss these important events in the development of modern atomic theory, use the questions on the teacher sheet as a basis to review the material and pique their interest

Finally, depending on the time that remains, encourage students to go through the first three areas of the Particle Adventure: What Is Fundamental?, What Is the World Made Of?, and What Holds It Together? This will give students a simple, colorful, interactive, and fundamental review of what they have learned in this lesson and the other lessons in The History of the Atom series. If little or no time remains, encourage students to read through these and other sections as part of their Extensions.


Assign an essay where students explain in their own words how our modern understandings of the atom have evolved over the years due to the contributions of many people. You may have them begin with the ancient Greeks or with J.J. Thomson’s first successful journey inside the atom. While addressing key developments during this period, students should touch on the ongoing goals underlying this scientific pursuit, as well as the kinds of questions and issues that remain for present and future scientists in the area of modern atomic theory.


A Science NetLinks lesson that relates to this one is Splitting the Atom.

Students could explore other parts of The Particle Adventure website, including: Accelerators and Particle Detectors and Higgs Boson Discovered Fireworks on the 4th of July.

The Science of Matter, Space and Time attempts to answer the question of “What is the world made of?” by offering a clear, matter-of-fact explanation of building blocks, forces, antimatter, and the Standard Model theory. Other sections include current developments and the potential of future discoveries.

Seeing with Electrons is an extended exhibit that provides students with a broader understanding of the various types of instruments and applications in life that electrons have inspired.

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Lesson Details

Grades Themes Project 2061 Benchmarks National Science Standards