Worlds Within Worlds: The Story of Nuclear Energy, Volume 1 (of 3) - 3

molecule. It is quite hard to pull hydrogen molecules apart, and it is
even harder to pull oxygen molecules apart. You have to supply about 12%
more energy to pull an oxygen molecule apart than to pull a hydrogen
molecule apart. Naturally, if you let 2 oxygen atoms come together to
form an oxygen molecule, you get back 12% more energy than if you allow
2 hydrogen atoms to come together to form a hydrogen molecule.
It takes a considerably larger amount of energy to pull apart a water
molecule into separate atoms than to pull apart either hydrogen or
oxygen molecules. Naturally, that greater energy is also returned once
the hydrogen and oxygen atoms are allowed to come back together into
water molecules.
Next, imagine pulling apart hydrogen and oxygen molecules into hydrogen
and oxygen atoms and then having those atoms come together to form
_water_ molecules. A certain amount of energy is put into the system to
break up the hydrogen and oxygen molecules, but then a much greater
amount of energy is given off when the water molecules form.
It is for that reason that a great deal of energy (mostly in the form of
heat) is given off if a jet of hydrogen gas and a jet of oxygen gas are
allowed to mix in such a way as to form water.
Just mixing the hydrogen and oxygen isn’t enough. The molecules of
hydrogen and oxygen must be separated and that takes a little energy.
The energy in a match flame is enough to raise the temperature of the
mixture and to make the hydrogen and oxygen molecules move about more
rapidly and more energetically. This increases the chance that some
molecules will be broken up into separate atoms (though the actual
process is rather complicated). An oxygen atom might then strike a
hydrogen molecule to form water (O + H₂ → H₂O) and more energy is given
off than was absorbed from the match flame. The temperature goes up
still higher so that further breakup among the oxygen and hydrogen
molecules is encouraged.
[Illustration: _The formation of a sodium chloride molecule._]
This happens over and over again so that in very little time, the
temperature is very high and the hydrogen and oxygen are combining to
form water at an enormous rate. If a great deal of hydrogen and oxygen
are well-mixed to begin with, the rate of reaction is so great that an
explosion occurs.
Such a situation, in which each reacting bit of the system adds energy
to the system by its reaction and brings about more reactions like
itself, is called a “chain reaction”. Thus, a match flame put to one
corner of a large sheet of paper will set that corner burning. The heat
of the burning will ignite a neighboring portion of the sheet and so on
till the entire sheet is burned. For that matter a single smoldering
cigarette end can serve to burn down an entire forest in a vastly
destructive chain reaction.

Electrons and Energy
The discovery of the structure of the atom sharpened the understanding
of chemical energy.
In 1904 the German chemist Richard Abegg (1869-1910) first suggested
that atoms were held together through the transfer of electrons from one
atom to another.
To see how this worked, one began by noting that electrons in an atom
existed in a series of shells. The innermost shell could hold only 2
electrons, the next 8, the next 18 and so on. It turned out that some
electron arrangements were more stable than others. If only the
innermost shell contained electrons and it were filled with the 2
electrons that were all it could hold, then that was a stable
arrangement. If an atom contained electrons in more than one shell and
the outermost shell that held electrons held 8, that was a stable
arrangement, too.
Thus, the helium atom has 2 electrons only, filling the innermost shell,
and that is so stable an arrangement that helium undergoes no chemical
reactions at all. The neon atom has 10 electrons—2 in the innermost
shell, and 8 in the next—and it does not react. The argon atom has 18
electrons—2, 8, and 8—and it too is very stable.
But what if an atom did not have its electron shell so neatly filled.
The sodium atom has 11 electrons—2, 8, and 1—while the fluorine atom has
9 electrons—2 and 7. If the sodium atom passed one of its electrons to a
fluorine atom, both would have the stable configuration of neon—2 and 8.
This, therefore, ought to have a great tendency to happen.
If it did happen, though, the sodium atom, minus 1 electron, would have
a unit positive charge and would be Na⁺, a positively charged ion.
Fluorine with 1 electron in excess would become F⁻, a negatively charged
ion. The 2 ions, with opposite charges, would cling together, since
opposite charges attract, and thus the molecule of sodium fluoride (NaF)
would be formed.
In 1916 the American chemist Gilbert Newton Lewis (1875-1946) carried
this notion farther. Atoms could cling together not only as a result of
the outright transfer of 1 or more electrons, but through sharing pairs
of electrons. This sharing could only take place if the atoms remained
close neighbors, and it would take energy to pull them apart and break
up the shared pool, just as it would take energy to pull 2 ions apart
against the attraction of opposite charges.
In this way the vague notions of atoms clinging together in molecules
and being forced apart gave way to a much more precise picture of
electrons being transferred or shared. The electron shifts could be
dealt with mathematically by a system that came to be called “quantum
mechanics” and chemistry was thus made a more exact science than it had
ever been before.

The Energy of the Sun
The most serious problem raised by the law of conservation of energy
involved the sun. Until 1847, scientists did not question sunlight. The
sun radiated vast quantities of energy but that apparently was its
nature and was no more to be puzzled over than the fact that the earth
rotated on its axis.
Once Helmholtz had stated that energy could neither be created nor
destroyed, however, he was bound to ask where the sun’s energy came
from. It had, to man’s best knowledge, been radiating heat and light,
with no perceptible change, throughout the history of civilization and,
from what biologists and geologists could deduce, for countless ages
earlier. Where, then, did that energy come from?
The sun gave the appearance of being a huge globe of fire. Could it
actually be that—a large heap of burning fuel, turning chemical energy
into heat and light?
The sun’s mass was known and its rate of energy production was known.
Suppose the sun’s mass were a mixture of hydrogen and oxygen and it were
burning at a rate sufficient to produce the energy at the rate it was
giving it off. If that were so, all the hydrogen and oxygen in its mass
would be consumed in 1500 years. No chemical reaction in the sun could
account for its having given us heat and light since the days of the
pyramids, let alone since the days of the dinosaurs.
Was there some source of energy greater than chemical energy? What about
the energy of motion? Helmholtz suggested that meteors might be falling
into the sun at a steady rate. The energy of their collisions might then
be converted into heat and light and this could keep the sun shining for
as long as the supply of meteors held out—even millions of years.
This, however, would mean that the sun’s mass would be increasing
steadily, and so would the force of its gravitational pull. With the
sun’s gravitational field increasing steadily, the length of earth’s
year would be decreasing at a measurable rate—but it wasn’t.
In 1854 Helmholtz came up with something better. He suggested that the
sun was contracting. Its outermost layers were falling inward, and the
energy of this fall was converted into heat and light. What’s more, this
energy would be obtained without any change in the mass of the sun
whatever.
Helmholtz calculated that the sun’s contraction over the 6000 years of
recorded history would have reduced its diameter only 560 miles—a change
that would not have been noticeable to the unaided eye. Since the
development of the telescope, two and a half centuries earlier, the
decrease in diameter would have been only 23 miles and that was not
measurable by the best techniques of Helmholtz’s day.
Working backward, however, it seemed that 25 million years ago, the sun
must have been so large as to fill the earth’s orbit. Clearly the earth
could not then have existed. In that case, the maximum age of the earth
was only 25 million years.
Geologists and biologists found themselves disturbed by this. The slow
changes in the earth’s crust and in the evolution of life made it seem
very likely that the earth must have been in existence—with the sun
delivering heat and light very much in the present fashion—for many
hundreds of millions of years.
Yet there seemed absolutely no other way of accounting for the sun’s
energy supply. Either the law of conservation of energy was wrong (which
seemed unlikely), or the painfully collected evidence of geologists and
biologists was wrong (which seemed unlikely),—or there was some source
of energy greater than any known in the 19th century, whose existence
had somehow escaped mankind (which also seemed unlikely).
Yet one of those unlikely alternatives would have to be true. And then
in 1896 came the discovery of radioactivity.

The Energy of Radioactivity
It eventually became clear that radioactivity involved the giving off of
energy. Uranium emitted gamma rays that we now know to be a hundred
thousand times as energetic as ordinary light rays. What’s more, alpha
particles were being emitted at velocities of perhaps 30,000 kilometers
per second, while the lighter beta particles were being shot off at
velocities of up to 250,000 kilometers per second (about 0.8 times the
velocity of light).
At first, the total energy given off by radioactive substances seemed so
small that there was no use worrying about it. The amount of energy
liberated by a gram of uranium in 1 second of radioactivity was an
insignificant fraction of the energy released by a burning candle.
In a few years, however, something became apparent. A lump of uranium
might give off very little energy in a second, but it kept on for second
after second, day after day, month after month, and year after year with
no perceptible decrease. The energy released by the uranium over a very
long time grew to be enormous. It eventually turned out that while the
rate at which uranium delivered energy did decline, it did so with such
unbelievable slowness that it took 4.5 billion years (!) for that rate
to decrease to half what it was to begin with.
If _all_ the energy delivered by a gram of uranium in the course of its
radioactivity over many billions of years was totalled, it was
enormously greater than the energy produced by the burning of a candle
with a mass equal to that of uranium.
Let’s put it another way. We might think of a single uranium atom
breaking down and shooting off an alpha particle. We might also think of
a single carbon atom combining with 2 oxygen atoms to form carbon
dioxide. The uranium atom would give off 2,000,000 times as much energy
in breaking down, as the carbon atom would in combining.
The energy of radioactivity is millions of times as intense as the
energy released by chemical reactions. The reason mankind had remained
unaware of radioactivity and very aware of chemical reactions was,
first, that the most common radioactive processes are so slow that their
great energies were stretched over such enormous blocks of time as to be
insignificant on a per second basis.
Secondly, chemical reactions are easily controlled by changing
quantities, concentrations, temperatures, pressures, states of mixtures,
and so on, and this makes them easy to take note of and to study. The
rate of radioactive changes, however, could not apparently be altered.
The early investigators quickly found that the breakdown of uranium-238,
for instance, could not be hastened by heat, pressure, changes in
chemical combination, or, indeed, anything else they could think of. It
remained incredibly slow.
But despite all this, radioactivity had at last been discovered and the
intensity of its energies was recognized and pointed out in 1902 by
Marie Curie and her husband Pierre Curie (1859-1906).
Where, then, did the energy come from? Could it come from the outside?
Could the radioactive atoms somehow collect energy from their
surroundings, concentrate it several million-fold, and then let it out
all at once?
To concentrate energy in this fashion would violate something called
“the second law of thermodynamics”. This was first proposed in 1850 by
the German physicist Rudolf Julius Emmanuel Clausius (1822-1888) and had
proved so useful that physicists did not like to abandon it unless they
absolutely had to.
Another possibility was that radioactive atoms were creating energy out
of nothing. This, of course, violated the law of conservation of energy
(also called “the first law of thermodynamics”) and physicists preferred
not to do that either.
The only thing that seemed to remain was to suppose that somewhere
within the atom was a source of energy that had never made itself
evident to humanity until the discovery of radioactivity. Becquerel was
one of the first to suggest this.
It might have seemed at first that only radioactive elements had this
supply of energy somewhere within the atom, but in 1903 Rutherford
suggested that all atoms had a vast energy supply hidden within
themselves. The supply in uranium and thorium leaked slightly, so to
speak, and that was all that made them different.
[Illustration: _The room in which the Curies discovered radium. Pierre
Curie’s writing is on the blackboard._]
But if a vast supply of energy existed in atoms, it was possible that
the solution to the puzzle of the sun’s energy might rest there. As
early as 1899 the American geologist Thomas Chrowder Chamberlin
(1843-1928) was already speculating about a possible connection between
radioactivity and the sun’s energy.
If it were some variety of this newly discovered source of energy (not
necessarily ordinary radioactivity, of course) that powered the
sun—millions of times as intense as chemical energy—then the sun might
be pouring out energy for hundreds of millions of years without
perceptible physical change—just as uranium would show scarcely any
change even in so mighty a time span. The sun would not have to be
contracting; it would not have had to fill the earth’s orbit 25,000,000
years ago.
This was all exciting, but in 1900 the structure of the atom had not yet
been worked out and this new energy was just a vague supposition. No one
had any idea of what it actually might be or where in the atom it might
be located. It could only be spoken of as existing “within the atom” and
was therefore called “atomic energy”. Through long habit, it is still
called that much of the time. And yet “atomic energy” is not a good
name. In the first couple of decades of the 20th century, it became
apparent that ordinary chemical energy involved electron shifts and
those electrons were certainly components of atoms. This meant that a
wood fire was a kind of atomic energy.
The electrons, however, existed only in the outer regions of the atom.
Once Rutherford worked out the theory of the nuclear atom, it became
apparent that the energy involved in radioactivity and in solar
radiation had to involve components of the atom that were more massive
and more energetic than the light electrons. The energy had to come,
somehow, from the atomic nucleus.
What is involved then in radioactivity and in the sun is “nuclear
energy”. That is the proper name for it and in the next section we will
consider the subsequent history of the nuclear energy that broke upon
the startled consciousness of scientists as the 20th century opened and
which, less than half a century later, was to face mankind with untold
consequences for good and for evil.


FOOTNOTES

[1]“Mass” is the correct term, but “weight”, which is a somewhat
different thing, is so commonly used instead that in this book I
won’t try to make any distinction.


QUOTATION CREDIT

Inside front cover Copyright © by Abelard-Shuman, Ltd., New York.
Reprinted by permission from _Inside the Atom_,
Isaac Asimov, 1966.


PHOTO CREDITS

Cover The Metropolitan Museum of Art
Page facing inside The “Horsehead” Nebula in Orion. Hale
cover Observatories.
Author’s Photo Jay K. Klein
Contents page & Lick Observatory
page 4
Page
7 New York Public Library
9 From _Discovery of the Elements_, Mary E. Weeks,
Chemical Education Publishing Company, 1968.
12 Library of Congress
15 Sir George Thomson
18 Burndy Library
19 New York Public Library
21 Copyright © 1965 by Barbara Lovett Cline,
reprinted from her volume _The Questioners:
Physicists and the Quantum Theory_ by permission
of Thomas Y. Crowell Company, Inc., New York.
22 & 23 Curie Foundation, Institute of Radium
26 Academic Press, Inc.
29 Van Nostrand Reinhold Company
31 Top, Nobel Institute; bottom, from _Discovery of
the Elements_, Mary E. Weeks, Chemical Education
Publishing Company, 1968.
32 From _Discovery of the Elements_, Mary E. Weeks,
Chemical Education Publishing Company, 1968.
34 Top, Nobel Institute; bottom, Niels Bohr
Institute.
36, 42, 44, & 45 Nobel Institute
48 Academic Press, Inc.
49 From _Discovery of the Elements_, Mary E. Weeks,
Chemical Education Publishing Company, 1968.
60 Curie Foundation, Institute of Radium
★ U.S. GOVERNMENT PRINTING OFFICE: 1975—640—285/13

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technologies for converting coal to synthetic gas and liquid fuels,
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