A short history of the discoveries of the atom

Compiled by Jim Walker

atom (˘at´am) n. A unit of matter, the smallest unit of an element, consisting of a dense, central, positively charged nucleus surrounded by a system of electrons, equal in number to the number of nuclear protons, the entire structure having an approximate diameter of 10 - 8 centimetres and characteristically remaining undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.

Actually, electricity was thought about before atoms. In about 600 BC Thales of Miletus discovered that a piece of amber, after it is rubbed with fur, attracts bits of hair and feathers and other light objects. He suggested that this mysterious force came from the amber. Thales, however, did not connect this force with any atomic particle.

It wasn’t until about 460 BC, that a Greek philosopher, Democritus, developed the idea of atoms. The question he asked was this: If a piece of matter is broken in half, and then broken in half again, how many breaks will you have to make before you can break it no further? Democritus thought that there was an end point, a smallest possible bit of matter. He called these basic particles, atoms.

Unfortunately, the atomic ideas of Democritus had no lasting effects on other Greek philosophers, including Aristotle. In fact, Aristotle dismissed the atomic idea as worthless. Aristotle’s opinions were considered very important and if Aristotle said the atomic idea is wrong then everybody else thought it was wrong also.

For more than 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter. It wasn’t until the early 1800’s that people began to question the structure of matter.

In the 1800’s an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, was made out of elementary lumpy particles (atoms). Although he did not know about their structure, he knew that they were, somehow, fundamental.

In 1897, the English physicist J.J. Thomson discovered the electron and proposed a model for the structure of the atom. Thomson knew that electrons were negatively charged and thought that matter must be positively charged. His model looked like raisins stuck on the surface of a lump of pudding.

In 1900 Max Planck, a professor of theoretical physics in Berlin showed that when you vibrate atoms strong enough, such as when you heat an object until it glows, it emits radiation in separate bursts of energy. He called these energy bursts quanta.

It was thought at the time that light consisted of waves. But the quanta behaved like particles and, in turn, these particles of light were named photons.

It was found that atoms not only emit photons, they can absorb them also. Albert Einstein won the Nobel Prize for physics in 1908 for discovering that photon absorption can release electrons from atoms. This is called the photoelectric effect.

There was strong controversy for many years on deciding whether light consists of waves or particles. The evidence was strong for both cases and it was later shown that light is both particles and waves.

Other particles were discovered around this time called alpha rays. They were positively charged and it was thought that they were the positive parts of the Thompson atom (it’s now known to be the nucleus of atoms).

In 1911 Ernest Rutherford thought it would be interesting to bombard atoms with these alpha rays, figuring that it could be used to investigate the inside of the atom (sort of like a probe). He used Radium as the source of the alpha particles and shinned them onto the atoms in gold foil. Behind the foil was a fluorescent screen for which he could observe the alpha particles impact.

The results of the experiments were unexpected. Most of the alpha particles went smoothly through the foil. Only an occasional alpha veered sharply from its original path, sometimes bouncing straight back from the foil! Rutherford reasoned that they must be getting scattered by tiny bits of positively charged matter. Most of the space around these positive centres must be empty. He thought that the electrons must be somewhere within this empty space. Rutherford thought that the negative electrons orbited a positive center in a manner like the solar system where the planets orbit the sun.

Rutherford knew that atoms consist of a compact positively charged nucleus, around which circulate negative electrons at a relatively large distance. The nucleus occupies less than one thousand million millionth (10-15) of the atomic volume, but contains almost all of the atom’s mass. If an atom were the size of the earth, the nucleus would be the size of a football stadium.

It wasn’t until 1919 that Rutherford finally identified the particles of the nucleus as discrete positive charges of matter. Using alpha particles as bullets, Rutherford knocked hydrogen nuclei out of atoms of six elements: boron, fluorine, sodium, aluminium, phosphorus, an nitrogen. He named them protons, from the Greek for ‘first’, for they were the first identified building blocks of the nuclei of all elements. He found the protons mass to be 1,836 times as great as the mass of the electron.

But there was something wrong with Rutherford’s model of the atom. The theory of electricity and magnetism predicted that opposite charges attract each other and the electrons should gradually lose energy and spiral inward. In fact, it was reasoned that the atoms should give off a rainbow of colours as they do so. But no one was able to observe this rainbow.

In 1912 a Danish physicist, Niels Bohr came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen. (This was a new approach to science because for the first time, rules were made to fit the observation even though they conflicted with present theories of the time.) Bohr said, ‘Here’s some rules that seem impossible, but they describe the way atoms operate, so let’s pretend they’re correct and use them.’ Two of Bohr’s rules are as follows:

RULE 1: Electrons can orbit only at certain allowed distances from the nucleus.

RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron is boosted from a low-energy orbit to a high energy orbit.

By the 1920s, Bohr’s model of the atom was shown to have troubles. Bohr’s atom seemed to be too simple to describe the heavier elements. In fact it only worked roughly in these cases. The spectral lines did not appear correct when a strong magnetic field was operating on the atoms.

Bohr and a German physicist, Arnold Sommerfeld changed the original Bohr model to explain these variations. According to the Bohr-Sommerfeld model, not only do electrons travel in certain orbits but the orbits have different shapes and the orbits could tilt in the presence of a magnetic field. Some orbits are circular, others are elliptical and they can even swing back and forth through the nucleus in a straight line.

The orbit shapes and various angles to the magnetic field could only have certain shapes, similar to an electron being in a certain orbit. As an example, the fourth orbit in a hydrogen atom can have only three possible shapes and seven possible traits. These added states allowed more possibilities for different spectral lines to appear. This brought the model of the atom into closer agreement with experimental data. The conditions of the state of the orbit are described by quantum numbers. The three states discussed so far are: orbit number (n), orbit shape (l) and orbit tilt (m).

In 1924 an Austrian physicist, Wolfgang Pauli predicted that an electron should spin (kind of like a top) while it is orbiting around the nucleus. The electron can spin in either of two direction. This spin was assigned a fourth quantum number: electron spin (s).

In 1924 a Frenchman named Louis de Broglie was thinking about particles of matter. He thought that if light can exist as both particles and waves, why couldn’t atom particles also behave like waves? In a few equations derived from Einstein’s famous equation, (E=mc ) he showed what matter waves would be like if they existed at all. (It was later proved that he was correct.)

In 1926 the Austrian physicist, Erwin Schrodinger had an interesting idea: Why not go all the way with particle waves and try to form a model of the atom on that basis? His theory worked kind of like harmonic theory for a violin string except that the vibrations travelled in circles.

The world of the atom was, indeed, very strange. It was difficult to form an accurate picture of an atom because there is nothing in our world to really compare it with.

Schrodinger’s wave mechanics did not question what the waves were made of, however, he had to call it something so he gave it a symbol, ψ, from the Greek letter (psi).

In 1926, a German physicist, Max Born had an idea of what ψ was. Born said that they were waves of chance. That is, the ripples that move along in waves of chance are made up of places where there may be particles and places where there are no particles. The waves of chance ripple around in circles when the particle is an electron in an atomic orbit and they ripple back and forth when the electron orbit goes straight through the nucleus and they ripple along in straight lines when a free particle is moving through interatomic space. It may be thought of as waves when travelling through space and as particles whenever they are located in space. However, they can not be both a waves and particles at the same time.

Just before Schrodinger proposed his theory, a German physicist Werner Heisenberg, in 1925, had a theory of his own called matrix mechanics which also explained the behaviour of atoms. The two theories seemed to be based on entirely different sets of assumptions yet they both worked. Heisenberg’s theory was based on mathematical quantities called matrices and fit with the conception of an electron as a particle whereas Schrodinger’s theory was based on waves. Both theories were really mathematically the same.

In 1927 Heisenberg formulated an idea which said that the position and momentum of a quantum particle can never be simultaneously determined. This idea is called the ‘Heisenberg uncertainty principle’. This implies that as the certainty of the position of a particle is known, the uncertainty in the momentum becomes correspondingly larger. Or, if momentum is determined with great accuracy, the knowledge about the particle’s position is correspondingly less.

A visual concept of the atom can now be thought of as an electron cloud which surrounds a nucleus. The cloud is a probability distribution map which determines the most probable location of an electron. For example, if one could take a snap-shot of the location of the electron at different times and then superimpose all of the shots into one photo, then it might look something like the view at the top.

Although the mathematical concept of the atom was getting better, the visual concept of the atom was getting to be vague. For visualisation purposes, chemists usually describe the atom as a simple solar system model similar to Bohr’s model but without the different orbit shapes. The important emphasis for chemistry is to show the groupings of electrons in orbital shells. (The example above shows the first eleven elements.)

Chemical behaviour of the elements form together to create molecules. Molecules may share electrons as the water molecule above illustrates. (Atoms which share electrons are called ions.) It is the outer electron shell of an atom which does the actual sharing and bonding of the atoms. This in turn allows chemists to describe all the interactions of chemistry. Even though the orbit model of the atom is not an accurate model, it is sufficient for all of chemistry.

A mystery of the nature of the nucleus was not completely solved. The nucleus contains most of the atom’s mass as well as the positive charge so the protons were thought to account for this mass. However, a nucleus with twice the charge of another should have twice the number of protons and twice the mass. But this is not so. Rutherford speculated in 1920 that there are electrically neutral particles with the protons that make up the missing mass but no one accepted his idea at the time.

It wasn’t until 1932 that the English physicist James Chadwick finally did discover the neutron and found it to be slightly heavier than the proton with a mass of 1840 electrons and is neutral. The proton-neutron together are called the nucleon.

Although scientists knew that atoms of a particular element have the same number of protons, they discovered that some of these atoms have slightly different masses. They concluded that the variations in mass are due, more or less, to the number of neutrons in the nucleus of the atom. Atoms of an element having the same atomic number but different atomic masses are called isotopes of that element.

When scientists found out about the atomic nucleus, they questioned why the protons, which have positive charge should remain so close without repelling. The scientists realised that there were new forces at work and the secrets must lie within the nucleus. They knew that the force which holds the protons together must be much stronger than the electromagnetic force and that the force must act over very small distances (otherwise they would have seen this force in interactions between the nucleus and the outer electrons).

In 1932, Werner Heisenberg concluded that charged particles bounce photons of light back and forth between them. This exchange of photons is the way that electromagnetic forces act between the particles. The theory is that a proton shoots a photon at the electron and the electron shoots a photon back at the proton. These photon exchanges go on all the time, very rapidly. However, because they are never seen, Heisenberg called these exchange particles, virtual photons. (Virtual meaning, not exactly ‘real’.)

In 1935 a Japanese physicist Hideki Yukawa suggested that exchange forces might also be used to describe the strong force between nucleons. However, virtual photons were not strong enough for this force, so a new virtual particle was needed. Yukawa used Heisenberg’s uncertainty principle to explain that a virtual particle could exist for an extremely small fraction of a second. Since its time of existence is so exact, there would be a great uncertainty in the energy of the virtual particle. This uncertainty allowed the particle to be very strong only at certain times and the particles could slip in and out of existence. He also calculated that these particles should be about 250 times as heavy as an electron. Later, in 1947, the physicist Cecil F. Powell detected this particle and was called the pion.

Although the pions have been described as the transmitters of the strong force, they are not classed with the other force-transmitting particles, such as the photon or the W and Z particles. Pions are now known not to be elementary particles but composites made up of quarks. The strong force is transmitted by the pions only at the larger nuclear level.

It is now thought that all of the forces in the universe are carried by some kind of particle. This theory started in 1928 with Paul Dirac stating that photons are responsible for transmitting electromagnetic forces. This theory is called quantum electrodynamics, or QED, and was worked on by Richard Feynman, Julian Schwinger, and Sin-Itiro in the late 1940’s. The four known forces and their particles are listed below:

ParticleNature and Role
PhotonElectromagnetic force
(magnetism, light, heat,
EMR, electricity)
W+, W-, ZWeak force
Gluon (8 types)Carriers of strong force
(holds the quarks)
GravitonGravitational force
(has not been detected yet)

From 1947 until the end of the 1950’s, physicists discovered many new particles. (There were dozens of them.) The various types of particles needed a new theory to explain their strange properties.

In 1960, Murray Gell-Mann and Yuval Ne’man independently proposed a method for classifying all the particles then known. The method became known as the Eightfold Way. What the periodic table did for the elements, the Eightfold Way did for the particles. In 1964 Gell-Mann went further and proposed the existence of a new level of elementary particles called the quarks. He thought that there were three types of quarks and called them up, down, and strange. From 1974 thru 1984 the theory predicted three more quarks called charm, bottom (or beauty), and top (or truth). And each quark has their corresponding anti-quark. The theory of the quark explains the existence of several particles including the nucleus of the atom. In fact the proton and neutron are each made up of three kinds of quarks and the force which holds the quarks together are made up particles called gluons.


In 1928, Paul Dirac produced equations which predicted an unthinkable thing at the time — a positive charged electron. He did not believe his own theory. In 1932 in experiments with cosmic rays, Carl Anderson discovered the anti-electron, which showed that Dirac was right. Physicists call it the positron.

For each variety of matter there should exist a corresponding ‘opposite’ or antimatter. It is now known that antimatter exists. However, because matter and antimatter annihilates whenever they come in contact, it does not stay around for very long. (By the way, it is a great mystery as to why the universe is filled with mostly regular matter and not an equal amount of antimatter.)

There is not only an anti-electron but in 1955, the anti-proton was found, and later the anti-neutron. This allows the existence for anti atoms, a true form of antimatter.

From the ancient Greeks until today, the concept of the atom has become more and more obscure. The mathematical framework allows us to predict information about the atom with greater and greater accuracy, but, the world of the atom, the quanta of particles are so strange that it is no longer possible to be able to visualise what we are talking about. The particles seem to have a complete random existence, non-existence quality about them. And yet the theory of quantum electrodynamics (QED) is so precise and powerful, it encompasses all of our classical physical laws. The predictions of QED have been verified many times and to a precision of better than one part in a billion. It has such predictive powers that psychics, soothsayers, and prophets can only dream about coming as close. When you delve into the strange world of atoms, you start going crazy and you say things like…

Toto, I don’t think we’re in Kansas anymore.

The original Atoms HyperCard stack was compiled by Jim W. Walker of Flight Engineering, September 1988 from the sources listed below:


Particles , by Michael Chester, 1978, Macmillan Publishing

The Particle Explosion by Frank Close, Michael Marten, Christine Sutton, 1987, Oxford University Press

The Nature of Reality by Richard Morris, 1987, The Noonday Press

Foundations of College Chemistry by Morris Hein, 1967, Dickenson Publishing Co., Inc.

The American Heritage Dictionary , Second College Edition

Other writings on the General Electric Information Services (GEnie):

Death and Time Travel (GEnie file #3861)

Travel to the Past (GEnie file #10920)

The Source of Gravity?

Mechanism of Gravity (a HyperCard stack)

P.O. Box 661133
Miami Springs, FL 33266
(305) 884-1475

Converted to HTML by Ray White, August, 2003.