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Dalton's Atomic Theory

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Dalton's Atomic Theory
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You probably know what this sketch represents. It’s a model of an atom, one of the miniscule particles that make up all matter. The idea that matter consists of extremely tiny particles called atoms was first introduced about 2500 years ago by a Greek philosopher named Democritus. However, other philosophers considered Democritus’ idea ridiculous, and it was more or less forgotten for more than 2000 years.

Reintroducing the Atom

Around 1800, the English chemist John Dalton brought back Democritus’ ancient idea of the atom. You can see a picture of Dalton below . Dalton grew up in a working-class family. As an adult, he made a living by teaching and just did research in his spare time. Nonetheless, from his research he developed one of the most important theories in all of science. Based on his research results, he was able to demonstrate that atoms actually do exist, something that Democritus had only guessed.

Dalton’s Experiments

Dalton did many experiments that provided evidence for the existence of atoms. For example:

  • He investigated pressure and other properties of gases, from which he inferred that gases must consist of tiny, individual particles that are in constant, random motion.
  • He researched the properties of compounds, which are substances that consist of more than one element. He showed that a given compound is always comprised of the same elements in the same whole-number ratio and that different compounds consist of different elements or ratios. This can happen, Dalton reasoned, only if elements are made of separate, discrete particles that cannot be subdivided.

Atomic Theory

From his research, Dalton developed a theory about atoms. Dalton’s atomic theory consists of three basic ideas:

  • All substances are made of atoms. Atoms are the smallest particles of matter. They cannot be divided into smaller particles, created, or destroyed.
  • All atoms of the same element are alike and have the same mass. Atoms of different elements are different and have different masses.
  • Atoms join together to form compounds, and a given compound always consists of the same kinds of atoms in the same proportions.

Dalton’s atomic theory was accepted by many scientists almost immediately. Most of it is still accepted today. However, scientists now know that atoms are not the smallest particles of matter. Atoms consist of several types of smaller particles, including protons, neutrons, and electrons. For the back story on Dalton’s ideas about atoms and his place in the history of the atom, watch this video: http://www.youtube.com/watch?v=BhWgv0STLZs (9:03).

For a musical version of the history of the atom, listen to “Atom Song” at this URL: http://www.youtube.com/watch?v=0P3m_L-myog (3:29).

The Billiard Ball Model

Because Dalton thought atoms were the smallest particles of matter, he envisioned them as solid, hard spheres, like billiard (pool) balls, so he used wooden balls to model them. Three of his model atoms are pictured in the Figure below . Do you see the holes in the balls? Dalton added these so the model atoms could be joined together with hooks and used to model compounds.

Q: When scientists discovered smaller particles inside the atom, they realized that Dalton’s atomic models were too simple. How do modern atomic models differ from Dalton’s models?

A: Modern atomic models, like the one pictured at the top of this article, usually represent subatomic particles, including electrons, protons, and neutrons. For a fuller comparison of Dalton’s ideas and modern ideas, go to this URL:

http://www.youtube.com/watch?v=Qk1JdUyW-Xo (5:15)

Summary

  • Around 1800, the English chemist John Dalton reintroduced the idea of the atom, which was first introduced by the ancient Greek philosopher named Democritus.
  • Dalton did many experiments with gases and compounds that provided evidence for the existence of atoms.
  • Dalton developed an atomic theory that is still mostly accepted today. It is one of the most important theories in all of science.
  • Dalton thought individual atoms were solid, hard spheres, so he modeled them with wooden balls.

Vocabulary

  • Dalton’s atomic theory : Theory of John Dalton that all matter consists of indivisible particles called atoms that cannot be created or destroyed, that are the same for a given element, and that combine in fixed proportions to form compounds.

Practice

Read the article about Dalton’s atomic theory at the following URL. Then take the online quiz. Be sure to check your answers and read the explanations for any incorrect answers.

http://antoine.frostburg.edu/chem/senese/101/atoms/dalton.shtml

Review

  1. Who was John Dalton?
  2. What evidence did Dalton use to argue for the existence of atoms?
  3. State Dalton’s atomic theory.
  4. Explain how Dalton modeled atoms and compounds.

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