Topic 2 – Dalton’s Atomic Theory

The Evolution of the Atom

This shows the evolution of the model of the atom. It begins at top left with Dalton’s idea of an indivisible particle and moves to the left until Bohr’s model appears in the lower left. Bottom right is a modern representation.

Last time, I introduced the atomic nature of the physical universe. This time we will look a little deeper at its roots.

The history of chemistry goes back thousands of years, probably starting with the alchemists, and it’s interesting. The alchemists might be among the greatest con men of all time. That period in the development of science is a great story for another time.

When thinking about atoms, we start much later in history, with the work of John Dalton who contributed the major portion of his work around 1800. His model of the structure of matter marks the beginning of modern chemistry and while today, we know he missed the mark on a couple of his assertions, his basic model still serves as a simple way to understand much of the physical universe. Just as important, we can focus on the building of a physical model. In simplest terms, in science, a physical model gives us a means of understanding observations and experiments. A scientific model lists what is thought to be true in simple physical terms. It is a way to carry in our head a collection of ideas and concepts that are easy to apply and understand.

Dalton’s model of atomic structure consists of four basic assertions. He assumes these four ideas to be true and then uses the statements to understand and plan experiments where they either work to explain results or they fail.

Let’s consider these assertions one at a time.

1. Matter is composed of small indivisible particles called atoms. That word indivisible is important – it means that the atom cannot be cut into pieces or broken down into smaller components. Today we know that is not true. Today we know there is definite structure within an atom – it can be divided into protons, electrons and neutrons. And beyond that, high energy physics shows us that those pieces can be divided further – quarks, charms, leptons, and so on. But as a model in chemistry – Assertion 1 allows us to begin to understand that all objects in our physical world are made of atoms linked together in some way, and that molecules are relatively easily deconstructed down into atomic pieces. So while incorrect, as a model, Assertion 1 is still very useful. That’s an important lesson in science. As long as we understand a model’s shortcomings, it can still be useful in many applications.

2. All atoms of the same element are identical and have the same properties. Again, modern understanding shows us that a collection of atoms of the same element can contain a variety of isotopes of that element – strictly speaking, all of the atoms are not identical. They differ only in their mass, and even with that, differ only slightly. But again, this is a detail that usually does not make the model useless. Unless one is considering the chemical property of radiation or paying very close detail to mass, Assertion 2 of Dalton’s model is very useful for understanding the structure of matter.

3. Chemical compounds are composed of atoms of different elements combined in small whole number ratios. This is the major success of the model. It follows from Assertion 1 – the ratios must be small whole numbers but the important concept is that atoms group together to form larger more varied compounds. Atoms are building blocks. This idea still serves us well today.

4. Chemical reactions are merely the rearrangement of atoms into different combinations. This is another way of saying that matter is neither created nor destroyed, but also, the way atoms are linked together is dynamic – the links and the ratios can be changed in a chemical reaction.

This physical model, Dalton’s atomic theory as it has come to be known, is simple to understand and remember, which makes it easy to apply. It is an example of the way science is done. Models are constructed and then applied and then modified if needed and refined.

Physical models provide a means to understand the things we cannot see because they allows us to construct pictures in our head.


5 thoughts on “Topic 2 – Dalton’s Atomic Theory

  1. Sam, thanks for your comment and suggestions. The names you mention are not tied closely to the development of the model of the atom but I will consider them as I move forward. I need a little clarification on your comments about textual support. Are you asking for suggestions for further reading, or are you asking for references that show I am correct in my writing? I’m trying to keep this as informal as possible, so anyone with an interest, from 10 years old to 100 years old, can learn a little something. I’m not trying to be a textbook – there are plenty of those already available. I will naturally provide more links with some posts than others, but if you can help me understand more clearly what you hope for, I will try to provide what I can. Thanks again.

    • Thanks for reply and question.
      What I meant by textual support is that I would like more idea of what assertions Dalton made and when he made them. Though you wrote it quite clearly in this blog, I felt like I would’ve liked to read the primary source myself.

  2. If you’re going to look at primary sources, could you look at Gay-Lussac, Avogadro, and Dulong? I’d like to learn more about these three.

    Also, which treatise are you using in this post? I really enjoyed the blog post, but I wish you had textual support for your interpretation. Thanks.

I appreciate comments

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s