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?
August 1959 Popular Electronics
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics
was published from October 1954 through April 1985. All copyrights are hereby
acknowledged. See all articles from
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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, WINXL
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. This 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.
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. 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.
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.
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.
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
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.
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 atomic particles.
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
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.
Bohr's conception 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 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 hydrogen. H1 has a nucleus which contains
one proton. H2 has a proton and a neutron in the nucleus.
The heaviest, H3, 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).
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.
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.
Without 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 of anti-matter.
Anti-Matter. 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
and Hyperons. 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
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 atomic puzzle.