30.1 Discovery of the Atom

From the air we breathe to the leaves on the forest trail, we learn that atoms are a substructure.

The existence and properties of atoms are used to explain many phenomena in this text.
We apply quantum mechanics to the description of atoms and their properties in this chapter.
New concepts emerge with applications far beyond the boundaries of atomic physics, just like the scientists who made the original discoveries.

A brief account of the progression from the proposal of atoms by the Greeks to the first direct evidence of their existence follows.

The philosophers Leucippus and Democritus are some of the earliest significant ideas to survive.
The question of whether a substance can be divided into smaller pieces was considered.
There are a few possible answers.
It is possible that infinitesimally small subdivisions are possible.
According to Democritus, there is a smallest unit that cannot be further divided.
The Greeks were correct when they said that atoms can be divided, but their identity is destroyed in the process.
The Greeks believed that atoms were moving in a constant motion.

The proposal that the basic elements were earth, air, fire, and water was incorrect.

The basic elements were not the most common examples of the four states of matter identified by the Greeks.
It took more than 2000 years for equipment capable of revealing the true nature of atoms to be available.

Efforts to transmute common and rare elements resulted in the recognition of certain systematic features.
Secrecy was a problem.
Many facts were rediscovered but were not made broadly available.
The science of chemistry came about as the Middle Ages ended.
It was no longer possible to keep discoveries secret.
By the beginning of the 19th century, an important fact was established--the mass of reactants in specific chemical reactions always have a particular mass ratio.
There are basic units that have the same mass ratios.
The English chemist John Dalton did a lot of this work, as did the Italian physicist Amedeo Avogadro.
Avogadro's number is named after him because he developed the idea of a fixed number of atoms and molecules in a mole.
The Austrian physicist was the first to measure the value of the constant using the theory of gases.

We have been able to make many discoveries because of the recognition and appreciation of patterns.

The proposed periodic table of elements was an organized summary of the known elements that led to many other discoveries.
Patterns in the properties of particles lead to the idea of quarks as their underlying structure, an idea that is still bearing fruit.

The development of the periodic table of the elements was the culmination of knowledge of elements and compounds.

The periodic nature of elements was highlighted by the array proposed by Mendeleev.
He predicted the existence of unknown elements to complete the periodic table.
The periodic table became universally accepted once these elements were discovered.

The existence of atoms and molecules in random thermal motion is the basis for the theory of the gas laws, heat transfer, and the Gas Laws and y.
It is still indirect evidence that individual atoms and molecules have not been observed.
Before direct evidence of atoms was obtained, there were heated debates about the validity of the theory.

He noticed that the tiny pollen grains were moving in complex paths.
A microscope can be used to observe small particles in a fluid.
Statistical fluctuations in the number of molecules hitting the sides of a visible particle cause it to move first.
The effects of the molecule on the particle can be seen.
The size of Molecules can be calculated by examining Brownian motion.
The smaller and more numerous they are, the smaller the fluctuations in the numbers are.

Brownian motion is caused by fluctuations in the number of atoms and molecules colliding with a small mass.

A satisfactory alternative explanation for the existence of atoms cannot be found.

Albert Einstein published several papers in 1905 explaining how Brownian motion could be used to measure the size of atoms.

He worked days as a patent examiner, so he did all of this in his spare time.
Their sizes were only known to be based on a comparison of surface tension and heat created by Thomas Young of double-slit fame and Simon Laplace.

Einstein's ideas were used by the French physicist Jean-Baptiste Perrin to confirm his theory of Brownian motion.

Knowing atomic and molecular sizes allowed a precise value for Avogadro's number to be obtained.
The ideas that Perrin used to explain atomic andmolecular agitation effects in sedimentation were used to win the 1926 Nobel Prize.
The accurate observation and analysis of Brownian motion was the first direct evidence of the existence of atoms.

It has become possible to measure the mass of an individual ion by using a mass spectrometer, similar to how electrons are accelerated in cathode-ray tubes.
The scanning tunneling electron microscope is one of the devices that can observe individual atoms.
Our understanding of the properties of matter is based on the atom.
The atom's substructures, such as electron shells and the nucleus, are both important.
The particles of which the nucleus is composed have a substructure.
The question of whether there is a smallest basic structure to matter will be explored in later parts of the text.

The scanning tunneling electron microscope can be used to detect individual gold atoms.