In 400 B.C., the Greek thinker Democritus proposed that all matter was made of tiny indivisible particles, which he named atomos.
But scientific investigation as we know it was not commonly practiced back
then, and Democritus never carried out any experiments that could prove his
theory. He had his supporters, and the Democritus University of Thrace was
named in his honor, but lacking any solid evidence, his theory was rejected
by Aristotle; and therefore the idea of atoms was likewise rejected by nearly
everyone else for the next two thousand years.
Then, in the early 1800s, an English school teacher named
John Dalton made the theory of atoms a cornerstone of modern science. So
what happened that changed everyone's minds? What did Dalton have that
Democritus didn't? He had evidence.
The fundamental principle behind what we call science today is the idea that we gain knowledge
from evidence. We know something scientifically if we can point to some experience that proves
it. Just what evidence did John Dalton have? That's what we're going to find out in this
Your task is to search for the evidence that persuaded Dalton of the existence of atoms and for
the evidence that helped shape his ideas about atoms. In order to do this, you must know the four main
principles of Dalton's atomic theory, which are listed below.
- All matter is made of tiny particles, called atoms.
- Atoms are neither created nor destroyed in chemical reactions.
- Atoms of different elements combine in whole number ratios, with more than one ratio being
possible for a given combination of elements.
- Each element is made of a different kind of atom, and the atoms of different elements have
Included on this CD is a WebQuest: Evidence for Atoms Question and Answer file.
Research to answer all of the questions on the sheet, type your answers
into the template and then print it out to bring to class. The point
values assigned to each question are given on the sheet.
Here are a few websites to help you begin your quest. Remember, these are just starting places.
Feel free to make use of any additional resources, electronic or print, in your search for
answers. Keep in mind that general references, such as your library's encyclopedia, can be
useful as well.
Your report should answer all the questions listed in the Process
section. To make things fair, everyone's report will be graded using a standard grading system,
applying the point values listed next to each question.
After completing the WebQuest, we hope you will understand just how Dalton arrived at each of
the four principles of his atomic theory. More importantly, you should learn how
scientific theories are developed by observing and drawing conclusions based on observation.
All theories, not just atomic theory, must be based on observation and must account for all observed
facts. If not, they can't really be accepted as scientific theories. This role of observed
evidence as the final judge of a theory's validity is the fundamental basis of science.
This CHF Chemistry WebQuest was created by Mark Michalovic.
Chemical Heritage Foundation