John Dalton- 1803
In 1803 a British fellow named John Dalton picked up the writings of Democritus and started
experimenting with his ideas. Dalton was not an alchemist. Dalton actually delivered a fatal blow to the
whole “philosopher’s stone” idea. He honored the role of alchemy in the creation of the science of
chemistry, but he also recognized that the science glasses and quantitative observations were necessary
to understand the true nature of matter. So he conducted many experiments and took many
measurements. Through his experiments he was able to come up with a set of 4 statements that are
now called “Dalton’s Atomic Theory”. With his atomic theory, Dalton effectively ended the practice of
alchemy.
1. What do you think of when you hear the word theory?
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In science, a THEORY is a set of statements that have been proven true through repeated
experimentation. We will look at a couple of theories this year. Theories can change if it turns out that
the theory is NOT true in a certain case, in fact, in the list below you will see that I have to qualify some
of Dalton’s statements because we have found them to be true in certain conditions, but not in every
case.
If the statements were true in every case, and if it could be quantified mathematically, then the
statement would be considered a scientific LAW…like gravity. The statement that every piece of matter
in the universe is attracted to every other piece of matter in the universe is always true and it can be
quantitatively measured in every case…that makes it a law.
2. What is a scientific theory?
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3. What is a scientific law?
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This list of the statements of Dalton’s atomic theory, was taken from the Royal Society of Chemistry
website. The statements in parenthesis were added by me to help with understanding.
Highlight Dalton’s Atomic theory in one color and Highlight my explanations, qualifications or
exceptions to the theory in a different color.
1. All matter is composed of atoms- (composed means ‘made’)
2. Atoms cannot be made or destroyed and chemical reactions occur when atoms are rearranged- (we
know that atoms can be split in NUCLEAR reactions, but Dalton didn’t have any knowledge of these
types of behaviors. His statement is true for any CHEMICAL reaction where substances are mixed
together and a chemical reaction or chemical change occurs. If you measure the mass of the elements
before the reaction and after the reaction, even if the substances have changed a lot, the total mass of
the elements is EXACTLY the same…Democritus got it right with the whole Lego Block idea!)
3. All atoms of the same element are identical and different elements have different types of atoms- (Dalton couldn’t ‘see’ that atoms were identical. He couldn’t ‘see’ atoms at all… we still can’t…but he
could measure their mass and he claimed that if an atom has the same mass as another atom, then it
must be made in the same way. From this he inferred that all atoms of gold look exactly the same as
any other atom of gold, but gold atoms don’t look like any other atom. Or that all atoms of oxygen look
like all other atoms of oxygen, but oxygen doesn’t look like any other element and so on… He
discovered that you could identify an element by weighing its mass…cool huh?)
4. Compounds are formed from atoms of the constituent elements- (a compound is what we have
when two or more elements combine together. For example, water is a compound because it is made
of two atoms of Hydrogen and one atom of Oxygen-H2O.)
4. Draw a picture of what each statement means in the boxes below.
1. All matter is made of atoms
2. Atoms are not created or destroy just rearranged in a reaction
3. All atoms of the same element are identical and each element is
different from other elements
4. Compounds are formed from combinations of different elements.
Dalton didn’t just come up with a theory about atoms, he also inferred a model of the atom based on
the evidence he had acquired. He experimented with Democritus’ ideas of hooks and barbs or locks and
keys and his results showed that there were no such structures on the atom. In fact, he inferred that
the atom was a round, smooth, indestructible sphere a lot like a marble, a pool ball.
5. Draw Dalton’s model of the atom.
This model of the atom raised more questions than it answered because now there was no explanation
for how atoms were holding onto each other or why chemical rearrangement of atoms occurs. Through
his experimentation, Dalton inferred that there was a force holding atoms together, not a
structure…something like magnetism, but not magnetism. This was a puzzle that still needed to be
solved and couldn’t be solved until the structure of the atom became clearer. Which brings us to our
next ‘giant’…but before that…let’s talk about magnetism.
6. What did Dalton learned that caused the model of the atom to change?
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7. When do models change?
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