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ZingPath: Atomic Structure

Atomic Model History: From Ancient Greece to Thomson

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Atomic Structure

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Lesson Focus

Atomic Model History: From Ancient Greece to Thomson

Physical Science

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You will learn about the evolution of the concept of the atom from philosophers of ancient Greece to early twentieth century ideas.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that matter is made up of atoms.
  • Explain that the word atom comes from the Greek atomos, meaning “cannot be cut into smaller pieces.”
  • Describe observations made by Crookes and Thomson using a cathode ray tube.

Everything You'll Have Covered

All substances are built of atoms. An atom consists of a dense nucleus of positively charged protons and neutral neutrons that is surrounded by a cloud of negatively charged electrons. We did not, however, always have this model. The Greek philosopher, Democritus, proposed the idea of an atom around 400 BC. He theorized that elements consisted of tiny, solid particles that could not be subdivided and called them atomos, which means "uncuttable." Aristotle disagreed with Democritus and said that matter was uniform throughout and was not composed of smaller particles. Aristotle's incorrect proposal was accepted as truth by many for about 2000 years until John Dalton, an English scientist, offered proof in the 1800s that atoms do exist. Dalton constructed models of atoms as wooden spheres. Dalton's wooden sphere atom models are on display in the Science Museum of London. Many of Dalton's contemporaries derided Dalton's idea of the atom, however. They said that atoms could not be seen and, therefore, did not exist.

In the early 1900s, scientists discovered through experimentation that atoms were composed of even smaller particles. In 1904, English physicist Joseph John Thomson concluded from his experiments that atoms contain small negatively charged particles which are electrons, and that they were evenly spaced and embedded throughout a positively charged sphere. He was incorrect but was closer to the truth than ever before. Another English physicist, Ernest Rutherford, suggested in 1911 that nearly all of the mass of an atom, which included all of its positively charged protons, was located in a nucleus of the atom and were surrounded by empty space containing electrons. Niels Bohr, a Danish physicist, theorized in 1913 that electrons orbit the nucleus of an atom in fixed patterns. It was later discovered that the atom's nucleus contained positive protons and neutral neutrons.

Our current model remains much the same as Bohr's, except that it is now believed that electrons do not travel around the nucleus of the atom in fixed patterns. Werner Heisenberg demonstrated in 1927 that it is impossible to know exactly at any given moment both an electron's position and momentum. The electron's mass is so small and is moving so quickly around the nucleus that we cannot describe its exact location. Imagine a wheel in motion. The spokes are moving so quickly around the center that you cannot pinpoint the exact location of a single spoke at any one moment. You just see the blur of all the spokes moving around the center. This is like the electrons moving around the nucleus of an atom. Currently we describe the location of electrons in an atom as being part of an electron cloud.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should be familiar with the concept of negative charges attracting positive charges and repelling negative charges.
Course Physical Science
Type of Tutorial Concept Development
Key Vocabulary ancient Greece, atoms, atomic models