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2.38: Democritus' Idea of the Atom

Difficulty Level: At Grade Created by: CK-12
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The man shown above has been called the “laughing philosopher” because of his cheerful disposition. He certainly looks cheerful in this picture. Why is a philosopher featured in a science text? He made an amazing contribution to science, although it was ridiculed by others and then ignored for more than 2000 years. His name was Democritus, and he introduced the idea of the atom as the basic building block of all matter. You can learn about Democritus’ place in the history of the atom at this URL:

http://www.youtube.com/watch?v=BhWgv0STLZs (9:03)

Cutting the Cheese

Democritus lived in Greece from about 460 to 370 B.C.E. Like many other ancient Greek philosophers, he spent a lot of time wondering about the natural world. Democritus wondered, for example, what would happen if you cut a chunk of matter—such as a piece of cheese like the one in the Figure below—into smaller and smaller pieces. He thought that a point would be reached at which the cheese could not be cut into still smaller pieces. He called these pieces atomos, which means “uncuttable” in Greek. This is where the modern term atom comes from. In the video at the following URL, Bill Nye the Science Guy demonstrates how Democritus arrived at his idea of the atom.

http://www.youtube.com/watch?v=cnXV7Ph3WPk (6:37)

How many times could you cut this piece of cheese in half? How small would the smallest pieces be?

Just a Guess

Democritus’ idea of the atom has been called “the best guess in antiquity.” That’s because it was correct in many ways, yet it was based on pure speculation. It really was just a guess. Here’s what Democritus thought about the atom:

  • All matter consists of atoms, which cannot be further subdivided into smaller particles.
  • Atoms are extremely small—too small to see.
  • Atoms are solid particles that are indestructible.
  • Atoms are separated from one another by emptiness, or “void.”

Q: How are Democritus’ ideas about atoms similar to modern ideas about atoms?

A: Modern ideas agree that all matter is made up of extremely small building blocks called atoms.

Q: How are Democritus’ ideas different from modern ideas?

A: Although atoms are extremely small, it is now possible to see them with very powerful microscopes. Atoms also aren’t the solid, uncuttable particles Democritus thought. Instead, they consist of several kinds of smaller, simpler particles as well as a lot of empty space. In addition, atoms aren’t really indestructible because they can be changed to other forms of matter or energy.

Keep on Moving

Did you ever notice dust motes moving in still air where a beam of sunlight passes through it? You can see an example in the forest scene in the Figure below. This sort of observation gave Democritus the idea that atoms are in constant, random motion. If this were true, Democritus thought, then atoms must always be bumping into each other. When they do, he surmised, they either bounce apart or stick together to form clumps of atoms. Eventually, the clumps could grow big enough to be visible matter.

Dust motes dance in a beam of sunlight.

Q: Which modern theory of matter is similar to Democritus’ ideas about the motion of atoms?

A: The modern kinetic theory of matter is remarkably similar to Democritus’ ideas about the motion of atoms. According to this theory, atoms of matter are in constant random motion. This motion is greater in gases than in liquids, and it is greater in liquids than in solids. But even in solids, atoms are constantly vibrating in place.

Why Matter Varies

Democritus thought that different kinds of matter vary because of the size, shape, and arrangement of their atoms. For example, he suggested that sweet substances are made of smooth atoms and bitter substances are made of sharp atoms. He speculated that atoms of liquids are slippery, which allows them to slide over each other and liquids to flow. Atoms of solids, in contrast, stick together, so they cannot move apart. Differences in the weight of matter, he argued, could be explained by the closeness of atoms. Atoms of lighter matter, he thought, were more spread out and separated by more empty space.

Q: Democritus thought that different kinds of atoms make up different types of matter. How is this similar to modern ideas about atoms?

A: The modern view is that atoms of different elements differ in their numbers of protons and electrons and this gives them different physical and chemical properties.

That’s Ridiculous!

Democritus was an important philosopher, but he was less influential than another Greek philosopher named Aristotle, who lived about 100 years after Democritus. Aristotle rejected Democritus’ idea of the atom. In fact, Aristotle thought the idea was ridiculous. Unfortunately, Aristotle’s opinion was accepted for more than 2000 years, and Democritus’ idea was more or less forgotten. However, the idea of the atom was revived around 1800 by the English scientist John Dalton. Dalton developed an entire theory about the atom, much of which is still accepted today. He based his theory on experimental evidence, not on lucky guesses.


  • Around 400 B.C.E., the Greek philosopher Democritus introduced the idea of the atom as the basic building block matter.
  • Democritus thought that atoms are tiny, uncuttable, solid particles that are surrounded by empty space and constantly moving at random.
  • Democritus surmised that different kinds of matter consist of different types or arrangements of atoms.


Watch the video at the following URL, and then develop a hypothesis to explain the results of the two experiments. Relate your hypothesis to Democritus’ ideas about atoms.

http://www.youtube.com/watch?v=g7debF-bSj8 (3:32)


  1. Who was Democritus?
  2. How did Democritus reason for the existence of atoms?
  3. How did Democritus describe atoms?
  4. Discuss how well Democritus’ ideas about atoms have withstood the test of time.
  5. Iron and lead are both metals, but iron is much harder than lead. How do you think Democritus might have explained this difference?

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Difficulty Level:
At Grade
7 , 8
Date Created:
Oct 31, 2012
Last Modified:
Oct 31, 2016
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