Our Atomic World: the Story of Atomic Energy

Part 1

OUR ATOMIC WORLD

by C. Jackson Craven

THE STORY OF ATOMIC ENERGY

U.S. ATOMIC ENERGY COMMISSION Division of Technical Information Understanding the Atom Series

The Understanding the Atom Series

Nuclear energy is playing a vital role in the life of every man, woman, and child in the United States today. In the years ahead it will affect increasingly all the peoples of the earth. It is essential that all Americans gain an understanding of this vital force if they are to discharge thoughtfully their responsibilities as citizens and if they are to realize fully the myriad benefits that nuclear energy offers them.

The United States Atomic Energy Commission provides this booklet to help you achieve such understanding.

{Edward J. Brunenkant} Edward J. Brunenkant, Director Division of Technical Information

UNITED STATES ATOMIC ENERGY COMMISSION

Dr. Glenn T. Seaborg, Chairman James T. Ramey Wilfrid E. Johnson Dr. Theos J. Thompson Dr. Clarence E. Larson

OUR ATOMIC WORLD

by C. Jackson Craven

CONTENTS

THE GREEKS WERE CURIOUS ABOUT MATTER 1 THE ATOMIC THEORY IS CONFIRMED 2 CATHODE RAYS SHOW ATOMS CONTAIN SMALLER PARTS 3 RADIOACTIVE ATOMS DISCOVERED 5 RUTHERFORD FINDS THE ATOMIC NUCLEUS 6 THE PROTON IS RECOGNIZED 8 ISOTOPES ARE DISCOVERED 9 THE ALCHEMISTS’ DREAM COMES TRUE 10 SOME PARTICLES HAVE NO ELECTRIC CHARGE 13 MATTER IS ENERGY; ENERGY IS MATTER 14 NUCLEI CONTAIN ENERGY 15 CHRONOLOGY 18 FISSION IS EXPLAINED 20 THE FISSION BOMB IS EXPLODED 23 NUCLEAR ENERGY IS NEEDED FOR THE FUTURE 25 FUSION HAS POTENTIAL 26 ISOTOPES HAVE MANY USES 29 RADIOISOTOPES AT WORK 30 THE ATOMIC ENERGY COMMISSION 31 TOWARD AN INTERNATIONAL ATOM 33 SUGGESTED REFERENCES 35

United States Atomic Energy Commission Division of Technical Information Library of Congress Catalog Card Number: 63-64918 1963; 1964 (Rev.)

C. JACKSON CRAVEN is a teacher’s teacher as well as a student’s teacher, and has had an active career aiding understanding of atomic energy as a member of the University of Tennessee faculty and on the staff of the Oak Ridge Institute of Nuclear Studies. He has conducted short courses to instruct groups of high school science teachers in nuclear energy, and has served in a key capacity in training Institute demonstration-lecturers who visit high schools throughout the nation.

Dr. Craven worked during World War II for the Manhattan Project, which built the first atomic bomb. He earned bachelor’s and graduate degrees at the University of North Carolina, and later taught physics and mathematics at Delta State Teachers College and at Furman and Emory Universities.

His research interests include infrared spectroscopy, gaseous diffusion through porous media, and the physical properties of fibers.

OUR ATOMIC WORLD

By C. Jackson Craven

The story of atomic energy evolves from the curiosity of people concerning the nature and structure of matter, the stuff of which all material things are made.

The Greeks Were Curious About Matter

Certain philosophers of ancient Greece—Democritus for one—were fascinated by the question: what is matter? You can imagine one of the philosophers saying to his pupils:

“Gentlemen, let us consider a piece of cheese. With a knife we can cut it in two, thus obtaining smaller pieces. We can then cut one of these smaller pieces in two, obtaining still smaller pieces. We can think about repeating this process over and over to get smaller and smaller pieces of cheese. Now can this process be continued without limit, or will a time come when we arrive at the smallest possible piece of cheese? In other words, is there a piece so small that we must have at least that much or none, with no choice in between?”

It is probable that most people who thought about this question at all during the next two thousand years answered the last question in the negative. The prevailing notion was that matter was continuous, with no theoretical limit as to how small a piece of cheese, or anything else, might be.

This concept was humorously expressed by the British mathematician Augustus De Morgan (1806-1871) in these lines:

Great fleas have little fleas upon their backs to bite ’em, And little fleas have lesser fleas, and so, ad infinitum.

The Atomic Theory Is Confirmed

Part 2

De Morgan evidently did not keep up with the latest developments in science, however, because two years before his birth, John Dalton, an English schoolteacher, had changed the atomic theory of matter from a philosophical speculation into a firmly established principle. The evidence that convinced Dalton and many other contemporary scientists of the reality of atoms came from quantitative chemical analysis.

Dalton knew that many chemical substances could be separated into two or more simpler substances. Chemicals that could be separated further were called compounds; those that could not were called elements. Careful experiments by Dalton and others showed that whenever two or more elements combined chemically to make a compound the relative amounts of the elements had to be carefully adjusted to fit a definite proportion in order to have no elements left over after the reaction was finished. For example, if hydrogen and oxygen were combined to form water, the weight of oxygen had to be eight times the weight of hydrogen; otherwise, either some hydrogen or some oxygen would be left over.

This fundamental truth is now called the Law of Definite Proportions. Another important principle, called the Law of Multiple Proportions, is illustrated by hydrogen peroxide, which is made up of the same two elements that are found in water. The weight of oxygen in hydrogen peroxide, however, is 16 times the weight of hydrogen or exactly twice the relative weight found in water.

These principles of chemical combination convinced Dalton that each chemical element consists of small, indivisible units, all just alike, called atoms, and that each chemical compound also has basic units, called molecules, which cannot be divided without reducing the compound into its elements—that is, destroying it as a compound. He visualized a molecule of a compound as formed by the uniting of individual atoms of two or more elements. It was obvious to him that in any molecule of a compound, the weight of each atom of a component element bore a proportionate relationship to the weight of the entire molecule which was equal to the proportion, by weight, of all that element in the compound. And although Dalton had no idea how heavy any individual atom really was, he could tell how many times heavier or lighter it was than an atom of another element.

Incidentally, Dalton mistakenly thought that one atom of oxygen was eight times as heavy as one atom of hydrogen instead of 16 times as heavy. He assumed a water molecule to be HO instead of H₂O.

Cathode Rays Show Atoms Contain Smaller Parts

Curiosity about the fundamental nature of matter was matched by equally avid curiosity about the fundamental nature of electricity. Before 1850 much had been learned about the behavior of electric charge and electric currents flowing through solids and liquids. Real progress in understanding electric charge, however, had to wait for the development of highly efficient vacuum pumps.

About 1854 Heinrich Geissler, a German glassblower, developed an improved suction pump, and also succeeded in sealing into a glass tube two wires attached to metal electrodes inside the tube. Experimenters were then able to study the flow of electricity through a near-vacuum. A Geissler tube is diagramed in Figure 1.

By the 1890s it had become clear that the flow of electricity through a highly evacuated tube consisted of a negative electric charge moving at a very high speed along straight lines between sealed-in electrodes. Since it originated at the negative electrode, or cathode, the invisible stream of charge was named “cathode rays.”

CURRENT SOURCE CATHODE (-) STREAM OF ELECTRONS VACUUM PUMP ANODE (+)

Although many investigators contributed to knowledge about cathode rays, the experiments of Joseph J. Thomson, a British physicist, are generally considered to have been the most enlightening. Thomson arranged a cathode-ray tube so that the rays could be deflected by magnets and by electrically charged metal plates. By applying certain well-known principles of physics, he was able to confirm an impression already held by physical chemists, namely, that electric charge, like matter, was “atomized”—the stream of charge consisted of a swarm of very small particles, all alike. He succeeded also in determining that the speed of the particles was about one-tenth the speed of light.

Probably Thomson’s most significant result was determining the ratio of the charge of each little particle to its weight. He was able to do this by measuring the magnetic force required to divert a stream of charged particles. (You can do this experiment yourself with relatively simple equipment.) This charge-to-weight ratio proved to be nearly 2000 times greater than the already known charge-to-weight ratio for a positively charged hydrogen atom, or ion, which until then was thought to be the lightest constituent of matter. It remained to be determined whether charge or weight caused the difference. Further experimentation showed that the charges were approximately the same amount in the two cases. It was therefore proven that the weight of the hydrogen atom, lightest of all the atoms, was nearly 2000 times as great as the weight of one of the little negative particles.

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