August 1959 Popular Electronics
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Saunder Harris wrote in this 1959 of edition Popular Electronics
that the concept of atoms has been around for more than 2,500 years
since Greek philosopher Democritus suggested that a particle existed
which was basic to all matter. It has only been in the last century
and a half that we have learned that even the atom itself is made
up of even more basic particles - the electron, proton, and neutron
(J.J. Thompson found the electron in 1897, which was postulated
by G. Johnstone Stoney in 1947). It wasn't until the 1930s that
even those three entities were thought to be constructed of yet
more fundamental particles - quarks, bosons, and leptons. Modern
science believes it has fully defined the set of subatomic particles,
particularly with the Higgs boson having been finally seen in the
Large Hadron Collider (well, maybe). Does anyone really believe
this is the final word on fundamental particles after having been
proved wrong so many times?
The World Within the Atom
... an atomic detective story
cloud chamber "footprints" have shed new light on the
inner world of the atom. By Saunder Harris
Believe it or not, our Atomic Age is over 2500 years
old. It all started back with the ancient Greek philosophers. One
in particular, named Democritus, suggested that a particle existed
which was basic to all matter. This particle, he said, was invisible
and could not be divided. The Greeks had a name for it ... they
called it the atom. Early Atomics.
idea of a basic particle or substance was more hunch than scientific
theory, and it took thousands of years before it could be put to
test. Our present concept of the atom began with the work of John
Dalton, an English chemist, who first described the laws of chemical
compounds and elements in 1802. He separated matter down to its
basic building blocks, the elements.
To visualize Dalton's
discoveries, imagine that we have a basket of mixed citrus fruit.
The complete basket with all the various fruits would be comparable
to a chemical compound. If we took out the fruits and separated
them into groups of lemons, oranges, grapefruit and so on, we would
be breaking the compound down into its elements. Then, if we set
apart one orange, for example, we would be isolating a single atom.
The next step would be to peel the orange and take a bite of the
fruit within. In the case of the atom, this first bite was taken
by the English physicist, Sir Joseph J. Thomson, in 1897.
Discovering the Electron.
Thomson was able to calculate the ratio
of the electron's charge to its mass by bending the electron
beam with known electrostatic and magnetic fields.
Cloud chambers such as the one above at the Brookhaven
National Laboratory provide clues for atomic detectives. The
device bombards atomic nuclei with billion-volt nuclear particles.
Atomic fragments leave a path in the moist, gas-filled air which
can be photographed and interpreted.
shows basic construction of cloud chamber.
left by high-speed protons on a sheet of photographic
film are shown at left. The dotted horizontal lines were made
by protons. The "star" was made when an atom disintegrated
in the photographic emulsion. (Brookhaven Lab photo)
at the Brookhaven National Laboratory accelerates
particles to energies of two or three billion volts. Inside
diameter of the Cosmotron is over 60 feet.
Bohr's model of the hydrogen atom. Solid
line indicates normal orbit of the planetary electron. When
electron moves to inner orbit, energy in the form of light is
given off. When energy in the form of heat is applied, the electron
wll move to the outer orbits.
During the middle years
of the 19th century, scientists had discovered that if an electric
current were passed between two electrodes placed at the ends of
a partially evacuated glass tube, a visible beam of unknown nature
would travel from negative to positive electrode. Experiments indicated
that this beam was negative in its electrical charge.
Sir J. J. Thomson, using the apparatus shown in Fig. 1, was able
to compute the ratio of the charge of a single particle in the beam
to its mass. In so doing he proved that the beam was composed of
individual, negatively charged particles. This was the discovery
of the electron.
In other experiments, Thomson tried using
various gases in the tube, but in each case his results were the
same. The particle was independent of the material from which it
came. Thomson therefore concluded that the electron was a basic
constituent of all atoms.
You can see that Thomson's
apparatus was similar to our cathode-ray tube. In fact, the picture
tube in your TV set is a direct descendant of the one Thomson used.
If you take a strong magnet and place it against the face of the
tube while the set is on, you will see a distortion caused by the
magnetic field bending the tube's negative electron beam. This
is essentially the same effect that led Thomson to identify the
electron. The Proton.
The discovery of
the electron was only the first step in the exploration of the inner
world of the atom. Since the atom was known to be electrically neutral,
the physicist now began to search for the positive particles which
would balance out the negative charge of the electron.
In 1914, another English physicist, Sir Ernest Rutherford, found
such a positive particle and called it the proton. The electron's
charge was assigned a value of -1 and the proton's charge a
value of +1. Besides the difference in charge, it was also discovered
that the proton was much greater in mass than the electron. It was,
in fact, 1836 times the mass of its smaller opposite.
Obviously, a proton is too small to be seen directly, and you may
wonder how it was detected. Consider a trail left in the sky by
a jet plane traveling at high speed. By looking at such a trail
you can follow the flight of the jet without actually seeing the
plane. On a smaller scale, this is how atomic particles are observed.
Figure 2 shows a cloud chamber, a major tool in the detection of
As high-speed particles pass through
the cloud chamber, they produce ions in the chamber's gas-filled
atmosphere. When the piston in the bottom of the cloud chamber is
suddenly lowered, this gas, which is saturated with water vapor,
expands and drops in temperature. Water vapor condenses on the ions
and outlines the path of atomic particles through the gas. Photographs
can be made of the ion tracks, and by studying photos of the trails,
physicists are able to identify the mass and charge of the various
particles. Bohr's Atomic Model.
In physics, when
a theory is proposed, the known facts are often organized by fitting
them into a model. Based on this model, observations are explained
and predictions are made. Our present understanding of the atom
has come about in such a manner.
The first proposed
model of the atom suggested that it was spherical, like a golf ball,
and that its mass consisted of protons with rings of electrons between
them. This model, however, did not explain certain phenomena such
as atoms giving off light when excited electrically or by heat.
It remained for a Danish physicist, Niels Bohr, to offer a model
which would explain these phenomena.
of the simplest atom, the hydrogen atom, consisted of a positively
charged nucleus with a "planetary" electron in orbit around
it. To move around the nucleus, the electron had to be influenced
by some force. This force, Bohr said, was the electrostatic attraction
of the positive proton nucleus for the outer electron. Figure 3
shows a model of the Bohr atom. Bohr was able to explain with mathematics
many of the experimental results which were obtained through the
use of his model. Isotopes.
In 1932 a new particle
was unexpectedly discovered. While experimenting with radioactive
polonium, German scientists detected a strong, penetrating radiation.
In France, the Curies noticed that the placing of a substance containing
hydrogen in the path of this radiation caused the release of high
energy protons. These results were analyzed in the laboratory of
James Chadwick, an English physicist, and it was determined that
the radiation was a new type of particle which had no charge. This
third particle was called the neutron.
The fact that
various atoms of the same element had been found to have different
weights could now be explained by the difference in the number of
neutrons in their nuclei. For example, there are three types of
has a nucleus which contains one proton.
has a proton and a neutron in the nucleus. The heaviest,
, so rare that only three pounds of it are thought
to exist on earth, has one proton and two neutrons in the nucleus.
Atoms with an "excess" of neutrons are called isotopes.
Isotopes can appear in all elements. The important thing
to remember is that planetary electrons in the outer orbits balance
the number of protons in the nucleus. Energy and Radioactivity.
With our three particles, the electron, the proton, and the neutron,
we could set up a mechanical model of the atom such as Bohr's.
This model would account for most of the things physicists have
observed. What it would not do is explain how mass could be converted
into energy (and energy into mass) without loss. In other words,
the fly in the atomic ointment would be Einstein's famous equation
E = MC2
(energy = mass times speed of light squared).
According to Einstein, it is possible to change these
two, mass and energy, into each other without a loss. The three-particle
model of the atom could not mathematically explain how this is possible.
Physicists were thus forced to the realization that there was more
to the atom than the electron, the proton, and the neutron.
The first inkling that there was energy in the atom came about through
the studies of Curie and Becquerel in the field of radio-activity.
Three different types of radiation were found to be given off by
naturally radioactive radium and uranium: alpha, beta and gamma
rays. The alpha and beta rays were high-speed particles while the
gamma rays were found to be powerful streams of energy with 100
times the penetrating power of beta particles.
these radiations in the light of Einstein's equation, physicists
found that the energy given off did not balance with the loss of
mass. In order for everything to balance, the Italian physicist,
Enrico Fermi, suggested still another particle. This he called the
neutrino or "little neutral one." Fermi theorized that
the neutrino would have to be almost pure energy to make the scales
balance. It would also be very difficult to detect because of its
high speed and lack of charge and mass. It was finally found in
1956 through delicate atomic detective work, a major scientific
triumph. Atom Smashers.
When the big "atom
smashers" were built. scientists were given the necessary tools
for probing the inner atom. There are many types of atom smashers,
or particle accelerators, their scientific name. Among them are
the cyclotron, the betatron, and the cosmotron.
going into detail on their operation, it is enough to understand
that the atom smashers whirl ions of gas around in circular paths
by electrical and magnetic means. These ions increase in velocity
until they approach the speed of light. They are then deflected
magnetically into an opening where they bombard the nuclei of substances
under study. If you have ever whirled a stone on a string and had
the string break, you will understand the principle of an atom smasher.
With the aid of these giant scientific instruments,
some of them filling huge buildings, more new particles were discovered.
Many of these had been mathematically predicted - and now they were
revealed. A particle was found which was the same as the electron,
but opposite in charge: it was called a positron or positive electron.
Then in the 1950's an important announcement ... the discovery
The French physicist, Dirac, mathematically concluded that each
of the basic particles should have an opposite, or anti-particle.
Four such anti-particles were found.
very difficult to detect because of its short life; the anti-matter
particles combine with their opposites and annihilate each other
almost instantly. Gamma rays equal to their former mass are given
off. It is thought that anti-matter differs from its opposite only
in that its magnetic poles are reversed, with each particle being
considered a small magnet.
Next, two other particles were found: the meson,
whose existence was predicted by the Japanese physicist, Yukawa,
and the hyperon, the most massive of all atomic particles. These
are each actually families of particles, rather than single units.
Mesons and hyperons are believed to" act as a "glue"
which binds the particles of the nucleus together.
Where do we go from here? Our 2500- year search for an understanding
of matter is far from being finished. There is still no final model
of the atom. Man has yet to find the. complete answer to nature's