History of the Atom

Lesson Plan - Get It!

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As we have improved the tools we use to study the world, we have learned more about its structure and makeup.

The atom is the foundation of all matter on our planet. It is a tiny particle, made up of subatomic particles that include the proton, electron, and neutron. Our understanding of the atom has changed a great deal over time, because we were able to invent and discover new ways to study the chemical world.

Before continuing, if you missed, or want a refresher on, the four previous Atoms Related Lessons, find them in the right-hand sidebar.

The history of the atom (or its study, actually) starts with Democritus in Greece around 450 B.C.

Just think about the limited tools he had to study our world. He used observation and his senses to make hypotheses about the world. Our modern word, atom, is based on his term, atomos. Atomos describes pieces of matter that are unable to be divided into smaller components.

Think about what would happen if you took a sheet of paper and tore it in half, then repeated that process until you ended up with tiny pieces that were unable to be torn. Those pieces represent what we know as atoms.

Sadly, Democritus's work sat unused for a great amount of time, and was even refuted by Aristotle (another important Greek philosopher).

John Dalton, a chemist working in the early 1800s, reviewed Democritus's ideas and embarked on experiments that would prove the early theories.

Dalton carried out many experiments, including those on gases, during his career. His fascination with the atom started with his study of meteorology, the science of weather phenomena.

This passion pushed him to explore the role of gases in our atmosphere and discover that gas particles are constantly moving. He used mathematic principles to prove ratios in chemical compounds that led to the assumption that elements bond in similar ways.

He developed a model of the atom that stated that every substance on the earth is made of atoms that cannot be subdivided into smaller units. Based on his work with ratios, he assumed that atoms bond together in similar ways to ensure similar ratios. Finally, he stated that atoms of one element are different from those of another element.

While his model of the atom as small solid particles was incorrect, he proposed the theory of conservation of matter, which states that matter cannot be created or destroyed. This theory is the foundation of many scientific principles and laws we understand today.

In 1897, J.J. Thomson discovered electrons using the apparatus pictured below. He used a cathode ray experiment, flowing electric current through a vacuum to determine changes in the flow near the positive end of the tube. This indicated the presence of a difference in charge and size. He found that electrons are very small and carry a negative charge.

_{Image by Sharon Bewick, via Wikimedia Commons, is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.}

Based on his work, Thomson developed the Plum Pudding model of the atom, where electrons are scattered throughout the entire atom, as shown in the image below:

_{Image by Kurzon, via Wikimedia Commons, is licensed under the Creative Commons Attribution-Share Alike 4.0 International license.}

We now know that electrons surround the nucleus in a layered cloud.

The positive subatomic particle is called the proton. It was discovered by shooting a positive beam of positive particles through gold foil and counting the number of particles deflected back. This experiment showed that atoms have a dense positive core, indicating that atoms are mostly empty space.

James Chadwick found the neutron in 1932 by studying radioactive reactions. All of these discoveries influenced the development of the modern model of the atom. Rutherford proposed the planetary model, the precursor to the modern model.

In the planetary model, electrons orbit a central positive nucleus as shown in the image below:

_{Image by Bensteele1995, via Wikimedia Commons, is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.}

The modern model of the atom has a positive nucleus comprised of protons and neutrons. Electrons are found outside of the nucleus in the electron cloud. They occupy different shells on the outside of the atom. This structure allows the atom to bond with additional atoms or remain stable.

Our understanding of the atom has changed throughout history as scientists have conducted experiments and as technology has improved. We are continuing to push the limits of scientific understanding to learn more about how particles interact in different circumstances.

Summarize the changes in atomic theory by creating a diagram that shows how the model of the atom has changed. Within your diagram, you will need to include at least three sketches showing how the model changed.

In the Got It? section, you will test your understanding of these changes with an online quiz.