When Leucippus and his disciple Democritus first advanced the notion of atoms, they pictured the atom as the ultimate, indivisible particle. Dalton, over two thousand years later, retained that view. It seemed necessary to suppose the atom to have no internal structure by definition. If the atom would be divided into still smaller entities, then would not those entities be the true atoms?
Throughout the nineteenth century this view of the atom as a featureless, structureless, indivisible particle persisted. When the view broke down, it was through a line of experimentation that was not chemical in nature at all. It came about through studies of the electric current.
If a concentration of positive electric charge exists in one place and a concentration of negative electric charge exists in another, an electric potential is set up between the two. Under the driving force of this electric potential a current of electricity flows from one point of concentration to the other, this current tending to equalize the concentration.
The current flows through some materials more easily than others. For Instance, the various metals are conductors and even a small electric potential suffices to drive a current through them. Substances such as glass, mica, and sulfur are non-conductors or insulators, and it requires enormous electric potentials to drive even small currents through them.
Nevertheless, given enough electric potential, a current could be driven through any material, solid, liquid, or gaseous. Some liquids (a salt solution) conduct electric currents quite easily as the first experimenters learned early in their studies. Then, too, a lightning bolt represents an electric current momentarily being carried through miles of air.
It seemed reasonable to the nineteenth-century experimenters to carry the matter one step further and to make attempts to drive an electric current across a vacuum. To obtain meaningful results one had to have a vacuum sufficient to allow the current to cross (if it were possible at all) without significant interference from matter.
Faraday's attempts to drive electricity through a vacuum failed for lack of a sufficient vacuum. In 1855 a German glassblower, Heinrich Geissler (1814-1879), devised a method for producing vacuums better than any that had previously been obtained. He prepared glass vessels, so evacuated. A friend of his, the German physicist Julius Plucker (1801-1868), used such Geissler tubes for electrical experimentation.
Plucker had two electrodes sealed in such tubes, set up an electric potential between them, and succeeded in driving a current across. The current produced glowing effects within the tube, and those effects changed according to quality of the vacuum chamber. If the vacuum was very good the glow would fade out, but the glass of the tube around the anode gave off a green light.
The English physicist William Crookes (1832-1919) had devised, by 1875, a still better evacuated tube (a Crookes tube), in which the electric current through a vacuum could more easily be studied. It seemed quite clear that the electric current started at the cathode and traveled to the anode, where it struck the neighboring glass and created the glow of light. Crookes demonstrated this by placing a piece of metal in the tube and showing that it cast a shadow on the glass on the side opposite the cathode. (The electrical experimenters of the eighteenth and nineteenth centuries, beginning with Benjamin Franklin, had assumed that the current flowed from the concentration arbitrarily named positive to that named negative. Crookes had now shown that, in actual fact, the assumption was wrong and that the flow was from negative to positive.)
At the time physicists did not know what an electric current might be and they could not easily tell just what it was that was moving from the cathode to the anode. Whatever it was, it traveled in straight lines (as it cast sharp shadows), so without committing oneself to any decision as to its nature, one could refer to it as a "radiation". Indeed, in 1876, the German physicist Eugen Goldstein (1850-1930) named the flow cathode rays.
It seemed natural to suspect that the cathode rays might be a form of light, and be made up of waves. Waves traveled in straight lines, like light, and, like light, seemed unaffected by gravity. On the other hand, it might equally well be inferred that the cathode rays consisted of speeding particles which, because they were so light or moved so quickly (or both), were affected by gravity not at all or only by undetectable amounts. For some decades the matter was one of considerable controversy, with the German physicists strongly for waves and the English physicists strongly for particles.
One way to decide between the alternatives would be to find out whether the cathode rays were deflected to one side by the action of a magnet. Particles might themselves be magnets, or might carry an electric charge, and in either case, they would be far more easily deflected by such a field than waves would.
Plucker himself had actually shown this deflection to exist, and Crookes had done so independently. There still remained a problem. If the cathode rays consisted of charged particles, an electric field should deflect them, too, but at first that effect was not demonstrated.
Then, in 1897, the English physicist Joseph John Thomson (1856-1940), working with very highly evacuated tubes, was finally able to show cathode ray deflection in an electric field That was the final link in the chain of evidence, and it had to be accepted that the cathode rays were streams of particles carrying a negative electric charge. The amount by which the cathode ray particle is deflected in a magnetic field of given strength is determined by its mass and by the size of its electric charge. Thomson was therefore able to measure the ratio of the mass to the charge, though he couldn't measure either separately.
The smallest mass known was that of the hydrogen atom, and of the cathode-ray particle was assumed to have that mass it would have to carry an electric charge hundreds of times greater than the smallest charge known (that on the hydrogen ion). If, on the other hand, the cathode-ray particle was assumed to have the minimum charge observed in ions, then its mass would have to be only a small fraction of that of the hydrogen atom. One of these alternatives was necessary, from Thomson's determinations of the mass-charge ratio.
There were ample reasons for preferring the latter alternative and assuming the cathode-ray particle to be much smaller than any atom. By 1911 this was proven definitely by the American physicist Robert Andrews Milikan (1868-1953), who succeeded in measuring, quite accurately, the minimum electric charge that could be carried by a particle.
If this charge were carried by the cathode-ray particle, it would have to be only 1/1837 as massive as a hydrogen atom. It was thus the first of the sub-atomic particles to be discovered.
Ever since the time of Faraday's laws of electrolysis, it had seemed that electricity might be carried by particles. In 1891, the Irish physicist George Johnstone Stoney (1826-1911) had even suggested a name for the fundamental unit of electricity, whether particle or not. He suggested the name electron.
Now here, at last, in the form of the cathode-ray particle, was the "atom of electricity" about which men had speculated for over half a century. Those particles came to be called electrons, as Stoney had suggested, and J. J. Thomson is therefore considered to have discovered the electron.
The Photoelectric Effect
It remained to be determined whether there was any connection between the electron and the atom. The electron might be the particle of electricity and the atom might be the particle of matter; and both might be structureless, ultimate particles, completely independent of each other.
It seemed quite clear that the independence could not be complete. Arrhenius, in the 1880's, had advanced his theory of ionic dissociation. He had explained the behavior of ions by assuming them to be electrically charged atoms or groups of atoms. At the time this had seemed nonsense to most chemists, but it seemed nonsense no longer.
Suppose an electron attached itself to a chlorine atom. In that case, one would have a chlorine atom carrying a single negative charge, and this would be the chloride ion. If two electrons attached themselves to an atom-group made up of a sulfur atom and four oxygen atoms, the results would be a doubly-charged sulfate ion, and so on. In this way one could easily explain all negatively charged ions.
But how would one explain positively charged ions? The sodium ion, for instance, was a sodium atom carrying one positive charge. No positively charged particle quite analogous to the electron was then known, so one could not take the easy way out of supposing that atoms might attach themselves to such positively charged particles.
An alternative suggestion was that the positive charge might be created by withdrawing an electron or two from the atom, an electron or two that had been present as part of the atom itself.
This revolutionary possibility was made the more plausible because of a phenomenon first observed in 1888 by the German physicist Heinrich Rudolf Hertz (1857-1894) during the course of experiments in which he discovered radio wave.
While sending an electric spark across an air gap from one electrode to another, Hertz found that when ultraviolet light shone on the cathode, the spark was more easily emitted. This, together with other electrical phenomena brought about by the shining of light upon metal, was eventually termed the photoelectric effect.
In 1902, the German physicist Philipp Eduard Anton Lenard (1862-1947) who, in earlier life, had been an assistant in Hertz's laboratory, showed that the photoelectric effect was brought about by the emission of electrons from metal.
A wide variety of metals was subject to photoelectric effects, and all these metals could emit electrons on the impact of light even when there was no electric current or electric charge in the vicinity. It seemed reasonable to suppose that metal atoms (and, presumably, all atoms) contained electrons.
But atoms in their normal state did not carry an electric charge. If they contained negatively charged electrons, they must also contain a balancing positive charge. Lenard thought that atoms might consist of clouds of both negative and positive particles equal in all respects but charge. This possibility seemed quite unlikely, for if it were so, why were not positively charged particles ever emitted by the atom? Why was it always, and only, the electrons?
J. J. Thomson suggested that the atom was a solid sphere of positively charged material with negatively charged electrons stuck to it, like raisins in poundcake. In the ordinary atom the negative charge of the electrons just neutralized the positive charge of the atom itself. Adding additional electrons gave the atom a net negative charge, while prying loose some of the original electrons gave it a net positive charge.
However, the notion of a solid, positively charged atom did not hold up. While positively charged particles exactly comparable to an electron remained unknown in the early decades of the twentieth century, other kinds of positively charged particles were discovered.
In 1886, Goldstein (who had given cathode rays their name) did some experimenting with a perforated cathode in an evacuated tube. When cathode rays were given off in one direction toward the anode, other rays found their way through the holes in the cathode and sped off in the opposite direction.
Since these other rays traveled in the direction opposite to the negatively charged cathode rays, it seemed that they must be composed of positively charged particles. This hypothesis was confirmed when the manner in which they were deflected in a magnetic field was studied. In 1907, J. J. Thomson named them positive rays.
The positive rays differed from electrons in more then charge. All electrons had the same mass, but the positive-rays particles came in different masses, depending on what gases were present (in traces) in the evacuated tube. Furthermore, whereas the electrons were only 1/1837th as massive as even the lightest atom, the positive ray particles wee full as massive as atoms.
The New Zealand-born physicist Ernest Rutherford (1871-1937) finally decided to accept the fact that the unit of positive charge was a particle quite different from the electron, which was the unit of negative charge. He suggested, in 1914, that the smallest positive ray particle, the one as massive as the hydrogen atom, be accepted as the fundamental unit of positive charge. He was confirmed in this view by his later experiments on nuclear reactions when he frequently found himself producing an identical particle as a hydrogen-nucleus. In 1920, Rutherford suggested that this fundamental positive particle be called the proton.
Positively charged particles turned up, also, by way of a completely different line of experimentation.
A German physicist, Wilhelm Konrad Rontgen (1845-1923), was interested in the ability of cathode rays to cause certain chemicals to glow. In order to observe the faint light that was produced, he darkened the room and enclosed his evacuated tube in thin, black cardboard. In 1895 he was working with such a tube when a flash of light that did not come from the tube caught his eye. Quite a distance from the tube was a sheet of chemically coated paper, glowing away. It glowed only when the cathode rays were in action, not otherwise.
Rontgen concluded that when the cathode rays struck the anode some form of radiation was created which could pass through the glass of the tube and the surrounding cardboard and strike materials outside. In fact, if he took the chemically coated paper into the next room, it still glowed whenever the cathode rays were in action, so one had to conclude that the radiation was capable of penetrating walls. Rontgen called this penetrating radiation x-rays, and they have retained that name to the present. (It was eventually determined that x-rays were like light waves in nature, but much more energetic.
The world of physics grew very interested in x-rays at once, and among those who began experiments in connection with it was the French physicist Antoine Henri Becquerel (1852-1908). He was interested in the ability of some chemicals to glow with a characteristic light of their own (fluorescence) upon being exposed to sunlight. He wondered if the fluorescent glow contained x-rays.
In 1896, Becquerel wrapped photographic film in black paper and put it in sunlight, with a crystal of a uranium compound resting on it. The crystal was a fluorescent substance, and if the glow were simply ordinary light, it would not pass through the black paper or affect the photographic film. If x-rays were present, they would pass through the paper and fog the film. Sure enough, Becquerel found his film fogged. He discovered, however, that if the crystal was not exposed to the sun and was not fluorescing, it fogged the photographic film anyway. In short, the crystals were emitting penetrating radiation at all times!
Marie Sklodowska Curie (1867-1934), the first famous woman scientist, gave this phenomenon the name of radioactivity. She determined that it wasn't the whole uranium compound but the uranium atom specifically that was radioactive. Whether the atom was in the metallic elementary form, or combined in any compound whatever, it was radioactive. In 1898 she discovered that the heavy metal, thorium, was radioactive also. Mme. Curie, a Pole by birth, did her research with the help of her French husband, Pierre Curie, a physicist of note.
The radiation given off by uranium and thorium was found soon enough to be complex in nature. When a stream of such radiation was passed through a magnetic field, some was slightly deflected in one direction, some was strongly deflected in the opposite direction, and some was unaffected. Rutherford have these three components of the radiation the names alpha rays, beta rays, and gamma rays, respectively, from the first three letters of the Greek alphabet.
Since the gamma rays were undeflected by the magnetic field, it was decided they were light-like radiation, like x-rays, in fact, but even more energetic. The beta rays were deflected in the direction and by the amount that cathode rays would have been deflected. Becquerel had decided that these rays were composed of speeding electrons. The individual electrons emitted by radioactive substances are therefore beta particles. That left the nature of alpha rays still to be determined.
Experiments with alpha rays in magnetic fields showed deflection opposite to that of the beta rays. The alpha rays had to be positively charged. They were only very slightly deflected, so they must be very massive; in fact, as it turned out, they were four times as massive as the particles Rutherford had named protons.
This ratio of weights seemed to indicate that alpha rays might consist of particles made up of four protons each. But if this were so, one of the particles out to have a positive charge equal to that of four protons; however, as was discovered, its charge was equal to that of only two protons. For that reason, it had to be assumed that, along with the four protons, the alpha particle also contained two electrons. These electrons would neutralize two of the positive charges while adding virtually nothing to the mass.
For about thirty years this proton-electron combination was believed to be the structure of alpha particles. Similar combinations were believed to make up other massive, positively charged particles. However, this inference created problems. There were theoretical reasons for doubting that the alpha particle could possibly be made up of as many as six smaller particles.
Then, in 1932, in experiments suggested by Rutherford, the English physicist Sir James Chadwick (1891-1974) discovered a particle just about as massive as the proton, but carrying no electric charge at all. Because it was electrically neutral, it was named the neutron.
Werner Karl Heisenberg (1901-1976), a German physicist, at once suggested that it was not proton-electron combinations that made up the massive, positively charged particles but proton-neutron combinations. The alpha particle, according to this suggestion, would be made up of two protons and two neutrons for a total positive charge of two and a total mass four times that of a single proton.
Physicists found that an alpha particle made up of four subatomic particles, rather than six, would fit their theories beautifully. The proton-neutron structure has been accepted ever since.