Quantum physics was born of light and atoms. It was through researching the true nature of each that the great “quantizations” of Planck, Einstein, and Bohr were made. The other unifying theme of these works was that the critical link between light and atoms was tied to the electron. Not only was it a fundamental constituent to atoms, and therefore all types of matter, but whenever it moved—or even “jumped”—light was bound to follow. We should therefore not be surprised to learn that the next great advance on the path to quantum physics would hinge prominently on the humble electron.
Many scientists (and nonscientists) had been messing around with electrons for generations before they knew that they were actually using pieces of atoms. They were storing electric charge, drawing huge sparks, and generating electric current without knowing the exact nature of the energy they were manipulating.
J. J. Thomson gets the credit for “discovering” the electron; that is, correctly identifying and measuring the characteristics of this, the first “subatomic” particle so isolated. It turned out that the flow of electrons was responsible for practically all of the electrical and magnetic phenomena that had been observed for several decades.
As the nineteenth century came to a close, Thomson clearly established that the electron was a discrete particle which carried a negative electric charge. About a decade later, Robert Millikan and Harvey Fletcher were able to measure the exact amount of charge on an electron. They confirmed that every electron carried the same amount of charge, and that the charge on an electron was the smallest unit of charge in the observable world.
This is another way to say that electric charge is quantized. All sorts of objects large and small may be electrically charged, and that charge can have negative or positive values. However, the amount of charge is always some whole-number multiple of the electron’s charge, either positive or negative (if it isn’t zero, of course).
Having isolated beams of electrons, Thomson could measure the ratio between the electron’s charge and its mass by watching how much the beams bent when manipulated with electromagnetic fields. When Millikan had directly measured its charge, the mass of the electron could be precisely calculated, and as we’ve seen, it turned out to be a very small fraction (less than a tenth of a percent) of the total mass of the atoms in which they dwell.
When Rutherford and his pals were firing alpha particles at all kinds of atoms, they clearly showed that the negative electrons occupied most of the volume of an atom, probably in some sort of constant motion. All of the positive charge was located in an incredibly dense “nucleus” that was very tiny relative to the whole atom. Although the nucleus was physically small, it contained almost all of the mass of the atom (99.95 percent or so).
ATOM TRAP
We sometimes hear about scientists “splitting the atom,” or labs containing “atom smashers.” While removing electrons could technically be considered “disassembling” an atom, when scientists talk about splitting or smashing atoms they are really referring to taking apart the nucleus of an atom. Taking out electrons is a lot easier than doing anything to change the nucleus; the latter requires (and potentially releases) a lot more energy. This is also why nuclear is the better term for bombs and power plants, rather than atomic.
While your average atom is electrically neutral, it is also possible to have atoms with a little extra negative or positive charge (called ions). Although the negatively charged electrons are attracted to the positive nucleus, this attraction can be overcome and an electron can be stripped away. Remove one or more electrons, and you end up with a positive ion. It is also possible to pile on a few extra electrons to get a negatively charged ion.
This is all to say there were lots of reasons to focus on electrons at this stage of the game in the early 1920s. Electrons were common and relatively easy to remove from atoms, so they could be isolated and studied directly. There was something fundamental about them, since they possessed the basic unit of electrical charge. Because of their charge and low mass, they could be moved around with electromagnetic fields. Electrons clearly played a major role in chemistry, too, due to their manner of packing into an atom’s outermost shell. Perhaps most important, classical physics was unable to explain exactly what they were doing when bound in atoms.
QUANTUM LEAP
All atomic nuclei are made up of protons and neutrons, with one exception. The nucleus of your ordinary hydrogen atom contains only one proton, with no neutrons at all. Thus the hydrogen atom is a very simple system: one electron bound to a single proton. Not only that, but if the electron is stripped off to make a positive hydrogen ion, that ion is an isolated proton ready to be manipulated and studied.
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