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.
Aristotle and Plato criticized many of the details of the ideas presented by Democritus and his school of thought. Primarily, the ideas that a human soul also has atomic structure was unacceptable for them as too mechanical.
After a long gap in philosophical and scientific development related to a structure of matter, experimental science made its first steps with such famous people as Galileo in 17th century.
In 18th century Sir Isaac Newton published his views on atoms similar to that of Democritus.
In the 19th century John Dalton has formulated his view on matter as consisting of individual indivisible atoms. Atoms are specific for each element, all atoms of a particular element are identical to each other and, according to Dalton, any chemical combinations of atoms preserve the total mass. He also suggested that any molecule, however big and complex, consists of certain (not very big) number of atoms of individual elements.
The latter was very significant, because the multitude of thousands of different chemical compounds was reduced to combinations of relatively small number of elements, each of which consists of identical atoms.
At the end of 19th century it became obvious that atoms are not indivisible. Sophisticated experiments conducted by Thomson allowed him to discover a small negatively charged electron as a particle inside an atom.
Knowing the electric neutrality of the atom, Thomson suggested later on that negatively charged electrons are distributed within a positively charged substance - the "plum pudding" model of an atom with electrons resemble plums inside a positively charged "pudding".
Some experiments with radioactive uranium compounds showed that so-called α-particles emitted by this compound go through a thin metallic foil. That brought Ernest Rutherford to an idea of planetary model of an atom, based on Coulomb's Law and Newton's Law, with central positive nucleus and circling around it negative electrons, thus leaving a lot of free space in-between for α-particle to penetrate the space occupied by atoms.
However, the planetary model had a few very important problems. Electrons moving along a circular trajectory around a nucleus move with centripetal acceleration and, as was explained in the part "Waves" of this course, emit energy in the form of electromagnetic oscillations. That would necessitate the gradual fall of these electrons onto a nucleus, that is a destruction of atom, which was not observed.
Another problem surfaced when physicists examined light emitted by gas between electric contacts. This light always had certain spectral lines specific for a particular gas, regardless of the intensity of electricity used to force the emitting of light.
This needed some theoretical explanation, and the planetary model did not provide it.
A few years late, in the beginning of 20th century Albert Einstein suggested that light delivers its energy in units called photons that we spoke about in the previous part of the course, "Waves".
The amount of energy E carried by one photon is proportional to a frequency f of electromagnetic waves:
E=h·f
where h is Planck's constant.
Ernest Rutherford discovered a positively charged particle inside an atom called protons. The positive protons neutralized negative electrons to make an atom electrically neutral. That was the time when physicists realized an important role of electricity in holding an atom together.
This was further developed by Niels Bohr into an atom model (called Bohr's model) with certain stable (not emitting energy) fixed shells around a nucleus specific for each element and having fixed energy levels. Electrons jumping from one such shell to another emit an energy specific for this particular jump, so each element has only specific energy amounts it can emit, thus specific colors of a spectrum of emitted light.
Jumping from a shell with higher energy level Ehi to a shell with lower energy level Elo, electron emits an energy Ehi−Elo, which causes to emit light of frequency f that satisfies the equation
h·f = Ehi−Elo
that is, with frequency equal to
f = (Ehi−Elo) / h
Next step was accomplished by James Chadwick discovering neutron - another particle in the atom's nucleus composition.
All the discoveries made by that time allowed to build a consistent model of an atom that we will be analyzing in this chapter of the part "Atoms" of the course.
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