History of Atom Model: UNIZOR.COM - Physics4Teens - Atoms - Building Blocks of Matter
Channel: Zor Shekhtman on UNIZOR Education
Date: 2023-01-10

UNIZOR.COM - Physics through a Looking Glass of Mathematician
Notes to a video lecture on http://www.unizor.com

Early History of Atom Model

Let me mention one important fact about Physics in general.
All our statements about Nature and its inner working only represent our model, the fruit of our mind. This model is based on our experience at a particular time and space and seems to correspond this experience to a high degree of precision.

As time passes, we enrich our experience with more facts and observations, which will necessitate updating our model.
No model, no physical law we came up with are absolute and final. Everything is a subject of constant improvement, update, even complete rewrite.

In this chapter we will present a simplified view on what atoms are, which, to a certain degree, can be confirmed experimentally.
By no means this view is the true picture of what atoms really are. It's an approximation to an as high degree as possible at current level of our knowledge, and it will change, with every iteration being closer and closer to what atoms and their components really are.

A drop of water is water.
If we split a drop of water into two smaller drops, each small drop is water.
How small a drop of water can be to retain the properties of water?
Can we divide a drop infinitely, and the smaller and smaller part of it would still be water?
No, there is a limit.

The smallest part of a drop of water that retains the properties of water exists. It has a name - molecule. It has a size and a weight. We cannot divide it further into smaller parts that still retain the characteristics of water.

How about some other kind of matter, like salt?
The same thing. We can divide a salt into smaller and smaller pieces until we reach a limit - a molecule of salt.
Dividing it further will produce something which is not salt anymore.

Every kind of matter that has certain properties is divisible into smaller pieces until we reach the smallest one that retains the original properties of this kind of matter - a molecule of this particular kind of matter.

A small complication to this picture is dealing with a combination of different kinds of matter.
Consider salt dissolved in water - a salty water.
This substance consists of two kinds of molecules mixed together. If we start dividing it, we will, for awhile, have smaller and smaller amounts of salty water. Eventually, we will reach a point that some small amount of substance we deal with is either a molecule of salt or a molecule of water.
Still, we reach the level of molecules, but in this case two different kinds of molecules have been mixed together. The principle of the smallest possible amount of substance that retains its properties is molecule.

How many different molecules exist? Too many to really count with certainty. Some big, some small, some heavy, some light. With all different properties, solid, liquid, gaseous, red, white, transparent etc.

The fundamental question is: "Are there much smaller particles than molecules of a very limited number of types, from which all the molecules consist, differing only in what kinds of these smaller particle participate in a construction of a molecule, what is their number and configuration, as they are organized in a molecule?"

The first, as we know, people who attempted to answer this question positively were Greek philosophers Leuccipus, Democritus and those close to them, who lived about 2400 years ago.
They have proposed that all kinds of matter consist of small indivisible particles. It was Democritus, who named these particles a tomos, literally "indivisible".
From our perspective their concept of atomos would correspond to our current concept of a molecule.