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The electron. The first subatomic particle to be discovered, and much of it is thanks due to the developments in electrical engineering during the Industrial Revolution. But proving something that can't be seen is hard, an extraordinary amount of evidence was required.
And how extraordinary it was! |
"As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter." - J. J. Thomson, Philosophical Magazine, (1897)
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In a previous article on the periodic table, we mentioned how one of the ways chemists and physicists attempted to sort out elements are through the number of valence electrons. And later in the article on organic chemistry, we mentioned how important these outermost electrons are in chemistry in general, and carbon chemistry in particular.
But what exactly are they? We know they supposedly orbit around the nucleus of an atom, but why? And why aren’t protons orbiting an electron-based nucleus instead? To answer these questions, we must go back and ride the train of thought of the past great chemists and physicists who parried with this secretive subatomic speck. Although several people have suggested that an atom is made of fundamental units during the 19th century, this hypothesis was yet to be proven even considering the developments in electrodynamics of the time. Ironically, the mode of power used in the turn of the century was never truly understood before it was used, except in engineering terms. Daring, yes, and is certainly proof of human audacity. It wasn’t until 1897 that an English physicist proposed to prove this hypothesis by suggesting that the motive force behind electricity was actually a subatomic particle. Joseph Thomson postulated that the rays emanating from passing electricity through vacuum tubes (a novel instrument invented by Michael Faraday now known as cathode ray tubes) are particles smaller than the atoms of hydrogen. The claim was unprecedented, and as in all extraordinary claims it requires extraordinary proof. |
Joseph John Thomson (1856 - 1940)
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Thomson’s Experiments
Before Thomson, cathode rays were thought to be similar to light, though tainted by whatever material it emanates from (to explain the unearthly green glow). Thomson disproves this by experimenting with multiple elements to discharge the rays, which yielded the same results. Thus the rays are not tainted by atomic particles, but rather something else altogether. Similarly he discovered that the discharged beam can be deflected magnetically, a property that does not correspond with light. It provides a clue to the electromagnetic nature of the beam, similar to the accidental discovery of Hans Christian Ørsted when his compass was deflected at the proximity of live wires. But how do we confirm that the beams are electrical in nature?
Luckily for Thomson an instrument was already invented for this: the electrometer. All he needed to do is to direct the flow of the cathode beam towards it to see whether it confirms the hypothesis that the beam itself is electrically charged, rather than just inert light. So he did, and it was. So now it is confirmed that the beam is electrical, and can be manipulated magnetically. But will it work the other way? Will an electric field affect it all the same?
Unfortunately for his last experiment, it had been done before. Others have tried to deflect the beam with an electric field and failed. This does not sit well with the hypothesis that the beams are electrically charged. Thomson then postulates that if these are indeed subatomic particles, the only possible reason for previous failures are due to the inadequate vacuum in them. It took him quite an effort to produce a cathode ray tube with as near to a perfect vacuum as possible, and attempted the experiment again.
An interesting discovery was made when he did. When an electric field was passed between the beam, it deflected, but that is expected as it was earlier proven that the beam is electromagnetic and carries an electrical charge. What was surprising was how it had deflected. The electric field seems to repel the beam from the negative pole of the field. Eureka!
Thus from these experiments, Thomson had proven that cathode rays are (1) electromagnetic, (2) carries an electric charge, and (3) it’s a negative charge.
Before Thomson, cathode rays were thought to be similar to light, though tainted by whatever material it emanates from (to explain the unearthly green glow). Thomson disproves this by experimenting with multiple elements to discharge the rays, which yielded the same results. Thus the rays are not tainted by atomic particles, but rather something else altogether. Similarly he discovered that the discharged beam can be deflected magnetically, a property that does not correspond with light. It provides a clue to the electromagnetic nature of the beam, similar to the accidental discovery of Hans Christian Ørsted when his compass was deflected at the proximity of live wires. But how do we confirm that the beams are electrical in nature?
Luckily for Thomson an instrument was already invented for this: the electrometer. All he needed to do is to direct the flow of the cathode beam towards it to see whether it confirms the hypothesis that the beam itself is electrically charged, rather than just inert light. So he did, and it was. So now it is confirmed that the beam is electrical, and can be manipulated magnetically. But will it work the other way? Will an electric field affect it all the same?
Unfortunately for his last experiment, it had been done before. Others have tried to deflect the beam with an electric field and failed. This does not sit well with the hypothesis that the beams are electrically charged. Thomson then postulates that if these are indeed subatomic particles, the only possible reason for previous failures are due to the inadequate vacuum in them. It took him quite an effort to produce a cathode ray tube with as near to a perfect vacuum as possible, and attempted the experiment again.
An interesting discovery was made when he did. When an electric field was passed between the beam, it deflected, but that is expected as it was earlier proven that the beam is electromagnetic and carries an electrical charge. What was surprising was how it had deflected. The electric field seems to repel the beam from the negative pole of the field. Eureka!
Thus from these experiments, Thomson had proven that cathode rays are (1) electromagnetic, (2) carries an electric charge, and (3) it’s a negative charge.
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Thomson's three experiments proving the existence of the electron
C. T. R. Wilson (1869 - 1959)
Observing the Unobservable
Thomson’s breakthrough had answered much of the questions of what the electron is, but only partially. For it to be confirmed as a particle, it must have one thing above all: observation. In the spirit of empiricism of that age, an observation is required to prove something, and unlike contemporary particle physicists who require only data collected from instruments, the physicist of the age demand naked-eye observation – something have to be seen to be believed.
But how do you observe a single atom, let alone it’s constituencies like the electron? You don’t , you look for what it affects instead.
In the few last years of the 19th century, over the cloud cloaked highlands of Scotland, a meteorologist woke up in the wee hours of sunrise and caught a spectacle in the sky. It’s called a Broken spectre, he saw his shadow projected onto a cloud and haloed by a rainbow of colours. Greatly affected by the vision, he chose to artificially reproduce the effect by literally making clouds in a laboratory.
In 1895, Charles Thomson Rees (“CTR”) Wilson built an elaborate instrument, intended to create a contained supersaturate of air and water vapor, very much replicating the conditions in the Earth’s troposphere. What he found instead was that his artificial atmosphere tend to fog up when it’s irradiated with the recently-discovered x-ray. Wilson soon realised why the x-rays caused clouds to form in what we now call his ‘cloud chamber’. The radiation was stripping electrons from the air molecules in the chamber, and water droplets would then grow around the remaining electron-less atoms (called ions).
In 1910 Wilson built his masterpiece and realised his dream: a cloud chamber that could visualise particle tracks – resembling something like the vapour trails left in the wake of an airplane. The next year he took his first photographs of tracks, exclaiming excitedly, “they are as fine as little hairs”. For the first time, physicists could see the activity of the subatomic world.
Probable Orbits
But why do electrons surround the nucleus? For one thing it’s because of their smaller mass. Just as the moon orbits the Earth, and the Earth orbits the Sun, the entity with the smaller mass will always orbit the larger one. Though to be fair, they do affect each other, as the Sun also wobbles due to the Earth’s gravitational pull.
But the celestial and subatomic similarity stops there. As particle physics marches on in the 20th century, the ‘orbit’ model became obsolete with the advent of quantum mechanics. The prevailing theory now is that the nucleus is surrounded by a probability cloud of its electrons. The number of course does not change, a hydrogen atom still have only one electron, but it has a probability to be everywhere around the nucleus at any given time. Certain areas have a higher probability, but that doesn’t constitute a fixed ‘orbit’.
Before your brain melts, maybe we should leave the realm of quantum mechanics for a different article to cover. Now, I believe you were looking for some painkillers for that headache you’re having ?
Thomson’s breakthrough had answered much of the questions of what the electron is, but only partially. For it to be confirmed as a particle, it must have one thing above all: observation. In the spirit of empiricism of that age, an observation is required to prove something, and unlike contemporary particle physicists who require only data collected from instruments, the physicist of the age demand naked-eye observation – something have to be seen to be believed.
But how do you observe a single atom, let alone it’s constituencies like the electron? You don’t , you look for what it affects instead.
In the few last years of the 19th century, over the cloud cloaked highlands of Scotland, a meteorologist woke up in the wee hours of sunrise and caught a spectacle in the sky. It’s called a Broken spectre, he saw his shadow projected onto a cloud and haloed by a rainbow of colours. Greatly affected by the vision, he chose to artificially reproduce the effect by literally making clouds in a laboratory.
In 1895, Charles Thomson Rees (“CTR”) Wilson built an elaborate instrument, intended to create a contained supersaturate of air and water vapor, very much replicating the conditions in the Earth’s troposphere. What he found instead was that his artificial atmosphere tend to fog up when it’s irradiated with the recently-discovered x-ray. Wilson soon realised why the x-rays caused clouds to form in what we now call his ‘cloud chamber’. The radiation was stripping electrons from the air molecules in the chamber, and water droplets would then grow around the remaining electron-less atoms (called ions).
In 1910 Wilson built his masterpiece and realised his dream: a cloud chamber that could visualise particle tracks – resembling something like the vapour trails left in the wake of an airplane. The next year he took his first photographs of tracks, exclaiming excitedly, “they are as fine as little hairs”. For the first time, physicists could see the activity of the subatomic world.
Probable Orbits
But why do electrons surround the nucleus? For one thing it’s because of their smaller mass. Just as the moon orbits the Earth, and the Earth orbits the Sun, the entity with the smaller mass will always orbit the larger one. Though to be fair, they do affect each other, as the Sun also wobbles due to the Earth’s gravitational pull.
But the celestial and subatomic similarity stops there. As particle physics marches on in the 20th century, the ‘orbit’ model became obsolete with the advent of quantum mechanics. The prevailing theory now is that the nucleus is surrounded by a probability cloud of its electrons. The number of course does not change, a hydrogen atom still have only one electron, but it has a probability to be everywhere around the nucleus at any given time. Certain areas have a higher probability, but that doesn’t constitute a fixed ‘orbit’.
Before your brain melts, maybe we should leave the realm of quantum mechanics for a different article to cover. Now, I believe you were looking for some painkillers for that headache you’re having ?
Ponder this
Considering how much Thomson's hypothesis deviates from the mainstream idea of his time - that atoms are the basic, indivisible constituents of all matter, how do you think had he plan his experiments to turn that prevailing theory? Why did he chose those three experiments?
Why were the water vapour in Wilson's cloud chamber attracted to the ionized atoms? Would a less-saturated atmosphere result in the same? Considering that our atmosphere is constantly bombarded by cosmic radiation, why don't we have foggy skies all the time?
Discuss
New knowledge and evidences allows science to be revised. Germ theory of diseases replaced the theory of miasma, Einsteinian mechanics partly replaces Newtonian mechanics, and evolution through natural selection replaces Lamarckism and creationism.
How do we go about disproving an established hypothesis? List and elaborate on the steps taken in the scientific method to solve this. Try applying it to currently prevailing theories such as gravity, evolution, germ theory of disease.
Further readings
Joseph John Thomson, the discoverer of the electron, at the Chemical Heritage Foundation.
J. J. Thomson and Charles Thomas Rees Wilson, at the Nobel Prize Foundation. Do read their lectures from the ceremony, very enlightening indeed.
Thomson's cathode ray experiments, at Stanford Encyclopedia of Philosophy.
The role of experiments in physics, at Stanford Encyclopedia of Philosophy. Covers the scientific method and philosophy behind experimentation.
