Michael Fowler
University of Virginia
Physics 252 Home Page
Link to Previous Lecture
The first "atomic theorists" we have any record of were two
fifth-century BC Greeks, Leucippus of Miletus (a town now in Turkey) and
Democritus of Abdera. Their theories were naturally more philosophical
than experimental in origin. The basic idea was that if you could look
at matter on smaller and smaller scales (which they of course couldn't)
ultimately you would see individual atoms - objects that could not
be divided further (that was the definition of atom). Everything was made
up of these atoms, which moved around in a void (a vacuum). The different
physical properties -- color, taste, and so on -- of materials came about
because atoms in them had different shapes and/or arrangements and orientations
with respect to each other.
This was all pure conjecture, but the physical pictures they described
sometimes seem uncannily accurate. For example, here is a quote from Lucretius,
a contemporary of Julius Caesar, on the ideas of Epicurus, who was a follower
of Democritus:
…look closely, whenever rays are let in and pour the sun's light
through the dark places in houses … you will see many particles there stirred
by unseen blows change their course and turn back, driven backwards on
their path, now this way, now that, in every direction everywhere. You
may know that this shifting movement comes to them all from the atoms*.
For first the atoms of things move of themselves; then those bodies which
are formed of a tiny union, and are, as it were, nearest to the powers
of the atoms, are smitten and stirred by their unseen blows, and they,
in their turn, rouse up bodies a little larger. And so the movement passes
upwards from the atoms, and little by little comes forth to our senses,
so that those bodies move too, which we can descry in the sun's light;
yet it is not clearly seen by what blows they do it.
(*called "first-beginnings" by Lucretius - we'll
put "atoms", he meant the same thing.)
Is it possible some young Greeks had acute enough eyesight to see Brownian
motion?
These Greek philosophers believed that atoms were in constant motion,
and always had been, at least in gases and liquids. Sometimes, however,
as a result of their close-locking shapes, they joined in close-packed
unions, forming materials such as rock or iron. Basically, Democritus and
his followers had a very mechanical picture of the universe. They thought
all natural phenomena could in principle be understood in terms of interacting,
usually moving, atoms. This left no room for gods to intervene. Their atomic
picture included the mind and even the soul, which therefore did not survive
death. This was in fact a cheerful alternative to the popular religions
of the day, in which the gods constantly intervened, often in unpleasant
ways, and death was to be dreaded because punishments would surely follow.
Little conceptual progress in atomic theory was made over the next two thousand years, in large part because Aristotle discredited it, and his views held sway through the Middle Ages.
Things began to look up with the Renaissance. Galileo believed in atoms,
although, like the early Greeks, he seemed to confuse the idea of physical
indivisibility with that of having zero spatial extent, i.e. being a mathematical
point. Nevertheless, his ideas in this area apparently got him into theological
hot water. The Church felt that the doctrine of transubstantiation - the
belief that the bread and wine literally became the body and blood of Christ
- was difficult to believe if everything was made up of atoms. This was
an echo of the tension between atoms and religion two thousand years earlier.
Galileo's theory of atoms was not very well developed. He gives the impression in some places they were infinitely small (Two New Sciences, pages 51, 52), and in view of his excellent grasp of dimensional scaling arguments, he may have thought that vacuum suction between infinitesimally small surfaces would suffice to hold solids together, since smaller objects have proportionately more surface. Of course, this was on the wrong track. (Ironically, shortly after Galileo's death, his pupil Torricelli was the first to realize that suction forces were really a result of air pressure from the weight of the atmosphere.)
A much more modern perspective on atoms and interatomic forces was set
out later in the seventeenth century by Isaac Newton, who wrote (Opticks,
Book 3, Part 1):
Quest. 31. Have not the small Particles of Bodies certain Powers, Virtues, or Forces, by which they act at a distance, not only upon the Rays of Light for reflecting, refracting and inflecting them, but also upon one another for producing a great Part of the Phenomena of Nature? For it's well known, that Bodies act upon one another by the Attractions of Gravity, Magnetism, and Electricity; and these Instances show the Tenor and Course of Nature, and make it not improbable that there be more attractive Powers than these… . For we must learn from the Phenomena of Nature what Bodies attract one another, and what are the Laws and Properties of the Attraction, before we enquire the Cause by which the attraction is perform'd. The Attractions of Gravity, Magnetism and Electricity, reach to very sensible distances, and so have been observed by vulgar eyes, and there may be others which reach to so small distances as hitherto escape Observation, and perhaps electrical Attraction may reach to such small distances, even without being excited by friction.
In fact, although the forces binding atoms together in molecules cannot
be properly understood without quantum mechanics, many of these forces
are "short range" electrical forces - forces between bodies
having overall electrical neutrality, but distorted charge distributions.
These forces could definitely be categorized as "electrical Attraction
reaching to small distances". Notice that Newton also leaves the door
open for other short range forces, which were finally discovered
in the 1930's!
Newton goes on to argue that assuming the existence of forces of attraction between particles suggests very natural explanations for various physical chemistry-type phenomena, such as deliquescence, ease of distillation and heat of mixing:
For when Salt of Tartar runs per Deliquium, is not this done by an
Attraction between the Particles of the Salt of Tartar, and the Particles
of the Water which float in the Air in the form of Vapors? … And whence
is it but from this attractive Power that Water which alone distills with
a gentle lukewarm heat, will not distill from Salt of Tartar without a
great heat? … And when Water and Oil of Vitriol poured successively into
the same Vessel grow very hot in the mixing, does not this heat argue a
great Motion in the Parts of the Liquors? And does not this Motion argue,
that the Parts of the two Liquors in mixing coalesce with Violence, and
by consequence rush towards one another with an accelerated Motion?
Evidently, Newton had already realized that heat is molecular motion,
and how such heat is generated when dissimilar molecules that attract each
other are mixed, so their potential energy translated into kinetic energy
as they move towards each other.
Finally, I can't resist the following quote from Sir Isaac:
Quest. 30. Are not gross Bodies and Light convertible into one another,
and may not Bodies receive much of their Activity from the Particles of
Light which enter into their Composition?… The changing of Bodies into
Light, and Light into Bodies, is very conformable to the Course of Nature,
which seems delighted with Transmutations…
(Of course, despite all this Newtonian insight, we mustn't get carried away: Newton didn't believe in the kinetic theory of gases - he thought the atoms in a gas were more or less static, the pressure arising from mutual repulsion between neighboring atoms. He also firmly believed light was made up of particles, not waves, although in retrospect that wasn't maybe so wrong.)
The impressive quotes from Newton above, which sound like he's on the right
track, do not tell the whole story. Newton thought that part of
chemistry (especially the physical part) could be explained in terms
of the mechanics of corpuscles, but that there was something more important
- a harder-to-pin-down vital spirit, which was the basis of life (and also
somehow connected with mercury and other elements). He also felt this was
the key to the way God ran the universe -- the merely mechanical interaction
of corpuscles could not, in his opinion, generate the rich variety of life.
And Newton wanted to understand just how God did run the universe.
Newton probably spent more time studying alchemy than he did working on
his laws, gravitation and calculus combined! In fact, Newton probed "the
whole vast literature of the older alchemy as it has never been probed
before or since" according to a recent historical study (see Never
at Rest, Richard Westfall, page 290). He also used quite precise quantitative
measures in many of his investigations. This did not provide the insight
into mass conservation that Lavoisier's work did a century later, probably
because Newton didn't count the various gases absorbed or emitted, these
were still considered incidental and not really important to the reactions.
Also, maybe they didn't smell too great -- a recipe for preparing phosphorus
Newton copied from Boyle begins "Take of Urin one Barrel". Enough
already. (Never at Rest, page 285).
Not that this matters too much as far as developing the atomic concept
is concerned. On the positive side, the alchemists, in their fruitless
quest to turn lead into gold (and find the elixir of life, etc.) did get
very skillful at managing a great variety of chemical reactions, and so
learned the properties of many substances.
The alchemists' point of view was based on Aristotle's four elements, earth, air, fire and water, but they added what they called principles. For example, there was an active principle in air important in respiration and combustion. There was an acidic principle, and others. And then there was phlogiston. Looking at something in flames, it seems pretty clear that something is escaping the material. That they called phlogiston. After Boyle discovered that metals become heavier on combustion, it was decided that phlogiston had negative weight.
The first major step towards modern quantitative chemistry was taken
by Lavoisier towards the end of the eighteenth century. He realized that
combustion was a chemical reaction between the material being burned and
a component of the air. He carried out reactions in closed vessels
so that he could keep track of the amounts of the various reagents involved.
One of his great discoveries was that in reactions, the total final
weight of all the materials involved is exactly equal to the total initial
weight. This was the first step on the road to thinking about chemistry
in terms of atoms. He also established that pure water was not transmuted
to earth by heating, as had long been believed - the residue left on boiling
dry came from the container if the water itself was pure.
Lavoisier discovered oxygen. He was the first to realize that air has two (major) components, only one of which supports respiration, meaning life, and combustion. In 1783, working with the mathematician Laplace, and a guinea pig in a mask, he checked out quantitatively that the animal used breathed-in oxygen to form what we now term carbon dioxide (this is the origin of the "guinea pig" as experimental subject).
Lavoisier tightened up the very loose terminology in use at that time:
there were no generally agreed on definitions of elements, principles or
atoms, although a century earlier Boyle had suggested that element
be reserved for substances that could not be further separated chemically.
In his Elements of Chemistry (1789) Lavoisier writes:
…if, by the term elements we mean to express those simple
and indivisible atoms of which matter is composed, it is extremely probable
that we know nothing about them; but if we apply the term elements,
or principles of bodies, to express our idea of the last point which
analysis is capable of reaching, we must admit as elements all the substances
into which we are capable, by any means, to reduce bodies by decomposition.
Not that we are entitled to affirm that these substances we consider as
simple may not be compounded of two, or even of a greater number of principles;
but since these principles cannot be separated, or rather since we have
not hitherto discovered the means of separating them, they act with regard
to us as simple substances, and we ought never to suppose them compounded
until experiment and observation have proved them to be so.
In sum, Lavoisier began the modern study of chemistry: he insisted on
precise terminology and on precise measurement, and suggested as part of
the agenda the classification of substances into elements
and compounds. Once this program was truly underway, the atomic interpretation
soon appeared.
Unfortunately for chemistry, five years after this book appeared Lavoisier went to the guillotine. In pre-revolutionary France, government tax collection was privatized, and Lavoisier was one of the very unpopular "tax-farmers". Few of them survived the revolution. Lavoisier was also accused of anti-French activities, in that he corresponded with foreigners. The fact that all the correspondence was exchange of scientific papers did not impress the revolutionaries, who remarked that "the Republic has no need of savants" as they sent him to the guillotine.
John Dalton (1766-1844) was born into a poor family near Manchester,
England. He supported himself to some extent by teaching from the age of
twelve, when he started his own small Quaker school. Dalton wrote A
New System of Chemical Philosophy, from which the following quotes
are taken:
Matter, though divisible in an extreme degree, is nevertheless
not infinitely divisible. That is, there must be some point beyond
which we cannot go in the division of matter. The existence of these ultimate
particles of matter can scarcely be doubted, though they are probably much
too small ever to be exhibited by microscopic improvements. I have chosen
the word atom to signify these ultimate particles … .
He assumed that all atoms of an element were identical, and atoms of
one element could not be changed into atoms of another element "by
any power we can control". He assumed further that compounds of
elements had compound atoms:
I call an ultimate particle of carbonic acid a compound
atom. Now, though this atom may be divided, yet it ceases to be carbonic
acid, being resolved by such division into charcoal and oxygen.
He also asserted that all compound atoms (molecules, as we would say)
for a particular compound were identical, and, furthermore: "Chemical
analysis and synthesis go no farther than to the separation of particles
one from another, and to their reunion. No creation or destruction of matter
is within reach of chemical agency".
By Dalton's time it had become clear that when elements combine to form a particular compound, they always do in precisely the same ratio by weight. For example, when hydrogen burns in oxygen to form water, one gram of hydrogen combines with eight grams of oxygen. This constancy is to be expected in Dalton's theory, presumably the compound atom, or molecule, of water has a fixed number of hydrogen atoms and a fixed number of oxygen atoms. Of course, the weight ratio doesn't tell us the numbers, since we don't know the relative weights of the hydrogen atom and the oxygen atom. To make any progress, some assumptions are necessary. Dalton suggested a rule of greatest simplicity: if two elements form only one compound, assume the compound atom has only one atom of each element. Since H2O2 had not been discovered, he assumed water was HO. (He actually used symbols to represent the elements, H was a circle with a dot in the center. However, just as we do, he used strings of such symbols to represent an actual molecule, not a macroscopic mixture.) On putting together data on many different reactions, it became apparent to Dalton that the rule of greatest simplicity wasn't necessarily correct, by 1810 he was suggesting that the water molecule perhaps contained three atoms.
One of the strongest arguments for Dalton's atomic theory of chemistry was the Law of Multiple Proportions. For example, he found that when carbon combined with oxygen to form a gas, there were two possible outcomes, depending on the conditions - and in one outcome each gram of carbon combined with precisely twice as much oxygen as in the other. He correctly interpreted this as the formation of CO2 and CO respectively.
Meanwhile in Paris, Joseph Louis Gay-Lussac investigated carefully the
ratio of the volume of hydrogen gas that combined with a given volume of
oxygen gas to form water. He found the oxygen could combine with exactly
twice its own volume of hydrogen. There were similar simple volumetric
ratios for other reactions between gases, and furthermore, if the product
of the reaction was also a gas, it filled a volume simply related to those
of the combining gases - so two volumes of hydrogen combined with one volume
of oxygen to give two volumes of steam (assuming of course the temperature
is not allowed to cool below the boiling point of water).
Unfortunately, Dalton didn't believe Gay-Lussac's results. Dalton was convinced that Newton had proved the atoms of a gas were large, elastic objects, essentially filling space, and of different sizes for different atoms. This was hard to reconcile with the simple volume ratios.
Gay-Lussac had guts. Since it was now clear that nitrogen was a little lighter than oxygen, he thought there might be proportionately less oxygen in the air at higher elevations. To find out, in 1802 he went up in a balloon to 23,000 feet! He found the mix to be pretty much the same.
In 1811, the Italian physicist Amedeo Avogadro suggested that Dalton's
picture of atoms and molecules could be reconciled with Gay-Lussac's results
on volumes if one assumed that equal volumes of all gases, elements
or compounds, contain equal numbers of molecules. Of course, he had
no idea what the number might be, but the hypothesis made many predictions
without knowing the number.
Dalton didn't buy this one either. For one thing, if one volume of oxygen combined with hydrogen to form two volumes of water, it looked to Dalton as if the water molecules each had half an oxygen atom, if we believe Avogadro's hypothesis. Dalton did not believe that the original oxygen gas could consist of diatomic molecules, because in his picture the large oxygen atoms repelled each other, that's why a gas resisted compression. So how could they attract each other to form molecules?
Although Dalton's work had set the agenda for chemistry and led to many fruitful investigations, his picture of a gas was a roadblock to a real understanding of gas reactions. The large molecules he envisioned had small centers surrounded by an atmosphere of caloric, which was a fluid of heat, the same stuff you feel seeping into your fingers when you touch something hot. The reason a gas expanded on heating, in this theory, was that the caloric moved in and attached itself to the atmospheres around the molecules (Lavoisier also believed in caloric). However, the work of Rumford and later physicists established that heat was best understood as molecular motion, and there was no fluid - the caloric was just an illusion. This made the whole Newton/Dalton picture difficult to believe. In spite of this, Bernoulli's 1738 kinetic model, in which tiny gas molecules were shooting around in otherwise empty space, was not widely debated until half a century after Avogadro's hypothesis. At that point, our modern picture of gases began to emerge - the results of Dalton, Gay-Lussac and Avogadro could be put together in a simple, consistent way.
Volta invented the electric battery in 1800, and it was used within weeks to electrolyze water into its constituent elements, hydrogen and oxygen. More surprisingly, when an electric current was passed through soda or potash - two substances previously thought to be elements - metallic substances appeared at the cathodes. New elements-sodium and potassium - had been discovered. Evidently, electrolysis could break up compounds that were impervious to chemical attack. Chlorine gas was discovered by Sir Humphry Davy, on electrolyzing muriatic acid. His assistant Michael Faraday went on to analyze electrolysis quantitatively. He found that when electrolysis liberated elements at an electrode, it took always the same total amount of electric current (or some small integer multiple) to liberate one mole of the element (that is, Avogadro's number of atoms). The real significance of this result was not fully realized until 1881, when Helmholtz, in a Faraday Memorial Lecture, said: "If we accept the hypothesis that elementary substances are composed of atoms, we cannot well avoid concluding that electricity also is divided into elementary portions which behave like atoms of electricity".
In the year 1800, 31 elements were known. By 1860, that number had almost
doubled, to 60, and the relative atomic weights, as well as many of the
chemical properties, were known. In particular, in analyzing molecule formation,
a valuable emerging concept was that of valence - the idea that
each atom had a particular number of little hooks on it to attach itself
to similar hooks on other atoms. Some atoms exhibited different valencies
in different compounds, but many didn't, so it was a useful guide. In 1865,
the English chemist J. A. R. Newlands looked for correlations between chemical
properties, including valence, and atomic weight.
The lightest elements known at the time, in order of increasing atomic
weight, were:
H, Li, Be, B, C, N, O, F, Na,
Mg, Al, Si, P, S, Cl, K, Ca, Ti, …
.
Newlands suggested there was a law of octaves - elements 1, 8,
15 were a lot alike, as were 2, 9, 16, and so on. (Hydrogen is sufficiently
anomalous that it does not fit in the pattern as convincingly as the other
light elements.) Newlands' colleagues were unconvinced. For one thing,
the pattern did not seem to extend much further up the list. One colleague
suggested to Newland that he search for patterns by putting the list in
alphabetical order, rather than by atomic weight.
A few years later, in 1872, the Russian physicist Mendeleev drew up
his periodic Table of the Elements, with eight columns reflecting Newland's
octaves. Mendeleev had the courage to leave gaps where he believed new
elements would be discovered - so that the periodic patterns of chemical
behavior recurred throughout the table. For a specific example, he predicted
an element of atomic weight about 72, to follow silicon in his table. He
gave a list of expected properties of its compounds, such as a tetrachloride
of density 1.9 and boiling point about 90 degrees, etc. This prediction
was made in 1871, and in 1887 germanium was discovered, atomic weight 72.5,
the tetrachloride had density 1.9 and boiled at 83 degrees. His predictions
for other compounds of germanium were similarly accurate. (Holton, page
341)
These discoveries led to acceptance of the validity of Mendeleev's work towards the end of the century. The one big surprise was the inert gases He, Ne, Ar, Kr, Xe, Rn - a whole new column was needed! Mendeleev was pretty upset by this development, but accepted it in the end. (Glashow, page 263)
It is worth bearing in mind that even as late as the 1890's some of
the most eminent German chemists did not accept the atomic idea. Here is
an 1895 quote from Ostwald (Chemistry Nobel Laureate in 1909): "The
proposition that all natural phenomena can ultimately be reduced to mechanical
ones cannot even be taken as a useful working hypothesis: it is simply
a mistake. This mistake is most clearly revealed by the following fact.
All the equations of mechanics have the property that they admit of sign
inversion in the temporal quantities. That is to say, mechanical processes
can develop equally well forwards or backwards in time. Thus, in a purely
mechanical world there could not be a before and an after as we have in
our world: the tree could become a shoot and a seed again … . The actual
irreversibility of natural phenomena thus proves the existence of processes
that cannot be described by mechanical equations … .(Pais, Subtle is
the Lord, page 83)
Of course, this argument harks back to the kinetic theory, entropy and
Boltzmann, and as we mentioned at the end of the lecture on that subject,
Ostwald was one of the reasons Boltzmann committed suicide. Nevertheless,
within few years, Boltzmann's ideas were widely accepted. The atom had
finally come to stay.
Copyright ©1997 Michael Fowler
Books I used in preparing this lecture:
Introduction to Concepts and Theories in Physical Science, Gerald
Holton and Stephen Brush, Princeton, 1985.
The Project Physics Course: Text, Holt, Rinehart, Winston, 1972.
From Alchemy to Quarks, Sheldon Glashow, Brooks/Cole, 1993
Subtle is the Lord … , Abraham Pais, Oxford, 1982.