This post is the next in a series dating back several years. In fact, it has been going on long enough that, as of this year, it would be more accurate to call it "60 Years of Science". But I am going to continue to stick with the old title. Chalk it up to nostalgia. And, as the title indicates, this is the thirteenth post in the series. Your can go to: http://sigma5.blogspot.com/2017/04/50-years-of-science-links.html for a post that contains links to all the entries in the series. I will update that post to include a link to this entry right after I post this entry.
I take Isaac Asimov's book "The Intelligent Man's Guide to the Physical Sciences" as my baseline for the state of the science when he wrote the book (1959 - 60). In these posts I am reviewing what he reported and what's changed since. For this post I am starting with the section he titled "The Nuclear Atom". I will then move on to the section he titled "Isotopes". Both are from the chapter he titled "The Particles".
The book was written at an interesting time in the evolution of our understanding of things subatomic. As Asimov notes "it was known by 1900 that the atom was not a simple, indivisible particle". By the time Asimov wrote the book the situation had reached maximum complexity. Roughly a hundred subatomic particles had been identified. This drove nuclear physicists nuts as there are only about a hundred different elements. The subatomic world was supposed to be simpler (i. e. composed of fewer parts and pieces) than the atomic world, not more complicated.
The impasse was broken a few years later by the introduction of "Quark theory". Quark theory made sense out of this large zoo of subatomic particles. One component of this idea was to organize them into families. Auto makers have developed "lines" of cars. Ford, for instance, used to have (it has now been discontinued) the Ford "Crown Victoria", the Mercury "Grand Marquis", and the Lincoln "Town Car".
To a great extend they were the same car. The Crown Victoria was the least expensive "base line" version for the economy end of the market. The Town Car was the most expensive "luxury" version for the carriage trade. And the Grand Marquis was midway between the two, both is terms of price and in terms of "trim level" and other features. It was fancier (and more expensive) than the Crown Victoria but not as fancy (or expensive) as the Town Car. But all three shared a lot of common design elements, parts, etc.
Nuclear physicists determined that there were similar familial relationships between subatomic particles. Particle families were grouped into "generations". In the case of one family of particles, the first generation was the Electron. It's second generation was the "Muon", originally called the "Mu Mason". Both particles shared a lot of attributes. The principal difference between the two was their mass. The Muon was much heavier and, therefore, held a lot more energy. The third generation was represented by the "Tau", originally called the "Tau Lepton". Again, the principal difference between it and the other two generations was a mass and, therefore, energy that was much larger than the other particles in the same family.
And with the introduction of the generations concept it became possible to line up various generations of one family of particles with the appropriate generational member of other families. So the cousin of the Electron that was a member of the Neutrino family ended up being named the "Electron Neutrino". Similarly, the second generation particle was eventually named the "Muon Neutrino". Unsurprisingly, the third generation ended up being named the "Tau Neutrino".
The second component of the new theory was the Quarks themselves. In the same way that atoms were composed of subatomic particles, some (but not all) of what had been thought to be indivisible subatomic particles like the Proton, turned out to be composites of new and heretofore unsuspected truly fundamental particles. And these newly discovered truly fundamental particles were called Quarks. And, cutting to the chase, Quarks could also be put into the same kind of "three generations" structure I have talked about above. But that's getting ahead of the story. Back to Asimov.
The Electron was identified by J. J. Thompson in about 1900. He was also the first to propose a model of the atom. It was like a cookie, specifically like an oatmeal raisin cookie. An atom consisted of some unspecified material playing the role of the oatmeal batter. Into it was stuck the Electrons, which played the role of the raisins. This model didn't last long but you have to start somewhere. Things quickly got complicated due to the study of radioactivity.
Becquerel did a lot of the early work. He quickly determined that in a lot of cases radioactivity looked like a particle shooting out of the atom. And some of there particles seemed to be Electrons. So far, so good. But another kind of emission was what he called an "Alpha" particle. It had a positive charge so was presumably a chunk of the oatmeal part of the atomic cookie. There were definitely other kinds of emissions. Following the convention he set up, he named a certain class "Beta" particles and another class "Gamma" particles. It didn't take long to determine that a Beta particle and an Electron were the same thing but the name "Beta" stuck and is still in use. And it also turned out that "Gamma" emissions looked like high energy X-Rays but the name "Gamma ray" also stuck and is still in use.
Good experimental work determined that Alpha particles were at least twice as heavy as Hydrogen atoms. More good experimental work soon determined that they were a form of Helium that weighed four times what a single atom of Hydrogen weighed. Other scientists followed up other clues and identified the Proton at about the same time the Alpha work was being done. Protons and electrons have equal but opposite charges. But a proton is roughly two thousand times as heavy as an electron. This large difference in masses was a puzzle that had no solution at the time of Asimov's book.
But the identification of the Proton led to the next iteration of the model for the atom. Now it consisted of Electrons orbiting a "nucleus" consisting of Protons. This was analogous to the solar system where the Sun is in the center (nucleus) and planets (Electrons) orbit it. This model led to a lot of questions. But it also led to some answers. The identity of an element was tied to the number of Protons in the nucleus. Hydrogen is Hydrogen because it has a nucleus containing one Proton. Helium is Helium because it's nucleus contains two Protons. Lithium is Lithium because its nucleus contains three Protons. And so on.
Also, chemistry is all about Electrons. They occupy the outer regions of the atom so when two atoms come close to each other, what they mostly see is the other's Electrons. Remove the Electron from a Hydrogen atom and it is still a Hydrogen atom. It just has a net positive electrical charge that attracts the electrons in the outer regions of other atoms. And that is the basis of how chemical bonds work. Similarly, a Beta particle is a Helium atom from which both outer electrons have been removed. It has a positive electrical charge that is twice as strong as that of a Hydrogen atom whose single electron has been stripped away. This was real progress.
One question that was quickly identified was the "mass" question. The Helium atom should weigh twice as much as a Hydrogen atom but it actually seemed to weigh roughly four times as much. Other, similar discrepancies popped up all over the place. One quick fix to this problem was to assume that a nucleus also contained Electrons. If a Helium nucleus contained four Protons and two Electrons then the mass would come out about right because the electrons weighed so little. And the charge would come out right because the two Electrons would cancel out two of the four Protons.
And there was another, more subtle version of this problem. According to this theory a Helium nucleus contained four Protons and two Electrons. But the weight of the Electrons could be neglected so the mass of the Helium nucleus should be exactly four times that of Hydrogen. But it was off by a bit. All masses for all atoms were off by a bit, a little bit in some cases, and a lot in others. What was going on? The next chapter is "what's going on". So let's move on to "Isotopes".
The obvious base for calculating the relative weights of the various elements is Hydrogen. But, as we have seen with Helium, that doesn't work very well. Helium does not end up having a weight that is exactly four times that of Hydrogen. Various things were tried and eventually it was decided to use Oxygen as the base. It seemed to be the least worst choice. (The reason for this will be explained below.) It was given a standard weight of 16. The weight of other elements then often fell close to an integer number. But not always. Chlorine, for instance, came in at 35.457 instead of a nice round 35. It took a while to figure out what was going on.
Becquerel found that if you purified Uranium, then left it lying around undisturbed for a while, it actually got more radioactive. He speculated that somehow a small portion of the not very radioactive Uranium was mysteriously transforming itself into highly radioactive "Uranium X". And if you carefully separated out the Uranium X then the remaining "regular" Uranium would, over time, just make some more Uranium X.
Rutherford found out that the same thing happened with Thorium. And it had already been determined that Radium, if left alone, would somehow create Radon gas. As this general phenomenon was further investigated it slowly dawned that elements were miraculously transforming themselves into other elements. And in every case radioactivity was involved.
Soddy in 1913 finally cottoned on to what was happening. If a radioactive transformation involved the emission of an Alpha particle then the source element was transformed into a different element that was down two places in the periodic table. What was happening was fission. An element broke into two pieces, One of them was an Alpha particle that carried off two Protons. The element that remained behind retained all of the other Protons but was now a different element due to the smaller number of Protons its nucleus now contained.
There were obviously multiple versions of elements. They all had the same number of Protons so they had to differ in some other way. He called these different versions "Isotopes" without worrying about what the difference was. An obvious "fix" was to assume that "the nucleus consists of a certain number of Protons and a certain number of Electrons". If we added extra Protons but also added the same number of Electrons to the nucleus at the same time then the atomic number stays the same. This trick allows us to account for all then known nuclear transformations.
We still ignore the masses of the nuclear Electrons as they are so light that their effect on the mass of the nucleus is what accountants call "not material". But we now have a new number, the "mass number". The mass number, according to our new theory is the total or "gross" number of Protons. The atomic number is the net Proton count, all nuclear Protons minus however many nuclear Electrons are present. Two isotopes of the same element have the same atomic number (net number of Protons) but different mass numbers (gross number of Protons).
And this isotope business helped to explain why the weight of a particular element did not end up to be a round number. If a typical sample of, say, Helium, contained some Helium -3 (atomic number 2, mass number 3) and Helium-4 (atomic number 2, mass number 4) then the atomic mass of the sample could come out anywhere between three and four depending on the ratio of the two isotopes. Things became clearer when the "mass spectrometer" was invented.
You turn your sample into a gas, then you "ionize" it (strip one or more Electrons off of each atom so it has an electric charge). Then you make it fly through a magnetic field at a constant speed. The magnetic field will make the trajectory of each atom bend. How much will it bend? Well, that depends on the mass, the speed, and the electric charge. If we can keep the speed and electric charge constant then if the mass is higher the sample's trajectory will be bent by a smaller amount. If we can pull this trick off (which is very hard to do in lots of circumstances) then we can weigh each individual particle.
The mass spectrometer allowed many individual isotopes of many elements to be weighed. And by measuring how much of each isotope a representative sample contained the "isotopic composition" of various elements could be determined. And, as an interesting side effect of this work. it was determined that some isotopes of some elements were "stable", they never engaged in radioactive decay. And, of course, some isotopes were determined to be mildly radioactive (they "decayed" slowly into other elements and isotopes) and others were highly radioactive (they quickly decayed into other elements and isotopes).
And it turned out that even Oxygen, the standard against which other elements were weighed when Asimov wrote his book, was a combination of isotopes. It's just that it was 99.9% Oxygen-16 and only a tiny amount of other isotopes of Oxygen. Since Asimov wrote the book, the reference standard against which the relative atomic mass of each isotope of each element is compared, has been changed from "Oxygen" to "Carbon-12".
Carbon is carefully separated out and pure Carbon-12 is isolated. Then it is weighed and given an arbitrary "atomic mass" of 12. The relative atomic mass of other isotopes relative to that of Carbon-12 is determined and that ratio is used to determine that isotope's atomic mass. This resulted in a small change in the atomic masses assigned to other elements.
This change was made because it improved the situation. It brought a lot of atomic masses closer to being integral numbers once isotope ratios were accounted for. And each isotope was now handled separately for the purposes of determining its atomic mass. Most discrepancies are now small, but with the exception of Carbon-12, none of them is an exact integer. The reason for this had been solved by the time Asimov wrote his book. But that's something he gets into later.
So physicists were pretty happy at this point. The "nucleus is a mix of protons and electrons" theory worked very well. But then Rutherford came up with an experimental setup that allowed him to probe the nucleus in new ways. He figured out how to fire Alpha particles at a target. The target was made from Zinc Sulfide which would "scintillate" (throw off a spark of light that could be seen with the naked eye) when hit by an Alpha particle.
He then put a metal disk in the path to see what would happen. At first the scintillations stopped. But then he added Hydrogen to the mix and things changed. He concluded that single Protons, presumably from the Hydrogen, were now striking the target because they had enough energy to penetrate his metal disk. Very interesting.
He tried some different things before switching to what is now called a "Wilson cloud chamber". If you have air with a lot of water vapor in it then lots of things will cause the water vapor to condense into small droplets that are visible using just your naked eye. By carefully tweaking the apparatus you can see the path of ionized particles. If you then add a magnetic field the paths of the ionized particles will bend just like they do in a mass spectrometer. Because you can see the paths of ionized particles you can take the same kinds of measurements. This is a classic example of a better apparatus leading directly to better science.
A careful analysis of an Alpha particle striking the nucleus of a Nitrogen atom led to a determination that the Nitrogen nucleus could sometimes absorb the Alpha particle. It immediately threw off a Proton and transmuted into Oxygen. The Proton's path could be easily seen because at this point it was ionized.
This is the first example of a man-made process that could transmute one element into another. Alchemists had hoped to transmute "base metal", by which they meant lead, into gold. This can now be done. But the process is fantastically expensive. You are far better off just buying gold in the first place.
The method of viewing a cloud chamber as it made the paths of charged particles visible using a "mark one eyeball" was quickly replaced by taking photographs. Photographs could capture more detail and resulted in a permanent record that could be reviewed by others.
Asimov notes that the scientist who nailed down the Nitrogen to Oxygen transmutation had to take and examine 20,000 photographs to find 8 in which the event he was interested in occurred. By the time Asimov's book was published scientists were employing rafts of graduate students to examine hundreds of thousands of photographs looking for interesting events.
But the rate at which scientific instruments could churn out photographs, all of which had to be examined for events of potential interest, kept increasing. It soon reached a practical limit. Fortunately, at about the time the practical limit was reached solid state devices came along that were capable of replacing the cloud chamber.
Detectors capable of collecting the same kind of data (particle path, speed, mass, etc.) that had been extracted from cloud chamber photographs now exist. And they work pretty well for charged particles. But there is only a very limited capability to observe and measure the attributes of uncharged particles. In some cases it is possible to detect the presence of an uncharged particle. It is also sometimes possible to measure the energy it carries. But that's about it. Still, that's better than nothing. There is no doubt that the business of detecting and tracking uncharged particles doesn't work nearly as well as scientists would like.
But it is now possible to hook detectors up to computers and have them look for and measure events. That gets grad students out of the business of going blind by looking at zillions of photographs. It might sound like that puts them out of work, but don't worry. They still have lots to do.
Even after computers do a lot of preliminary work it is still necessary for a trained person to look at the result. The detectors at CERN, the home of the LHC, the largest particle detector in the world, can generate the equivalent of those 20,000 photographs in a small fraction of a second. Even with all the computer filtering the LHC can turn out hundreds of potentially interesting events per day. That's why the staff of each detector runs into the thousands.
We have now reached the point where we have an atom with a nucleus of protons and, as far as we know at this point, some electrons. The nucleus is surrounded by electrons in some mysterious configuration. This is just the beginning of the story. But there is where I must leave it in this installment. To be continued . . .
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