This post is the next in a series dating back several years. As I have previously indicated, a more correct title would be something along the lines of "59 Years of Science". But I like the original title so, even though it becomes less accurate every year, I am sticking with it. By now we are up to the eleventh post in the series. And you can go to sigma5.blogspot.com/2017/04/50-years-of-science-links.html to find an index with links to all the previous posts in the series. I will update that post to include a link to this post right after this post is published.
I take Isaac Asimov's book "The Intelligent Man's Guide to the Physical Sciences" as my baseline for the state of science as it was 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 chapter Asimov titled "Radioactive Elements". I will then move on to the chapter he titled "Electrons".
It turns out that X-Rays were critical to understanding the structure of atoms. And the structure of atoms is critical to the understanding of radioactivity. One of the many reasons scientists do experiments is because they sometimes turn out quite differently than expected. In 1896 Becquerel did an experiment that involved a Uranium containing compound and X-Rays. The idea was that sunlight would react somehow with the Uranium containing compound and make X-Rays. And that seemed to be what was happening. But it turned out that sunlight was not necessary. The compound would fog photographic film that had been covered in black paper even if no sunlight was involved at all. Uranium produced X-Rays all by itself.
The person most responsible for moving the story along from there was Marie Sklodowska. She is better known under her married name of Curie. Mrs. Curie decided that Uranium threw off "radiation" through a process she named "radioactivity". (She and her husband Pierre were also responsible for naming and studying the "piezo-electric effect".) She also identified Thorium as being radioactive. Further experimentation determined that Uranium and Thorium emitted not X-Rays but an even more powerful kind of radiation called "Gamma Rays". Gamma Rays can also fog photographic plates. Hence the initial confusion.
The Curies laid the foundation of what is now called nuclear chemistry. Just as chemical reactions change the properties of compounds, nuclear reactions change the properties of atoms. In particular, radioactivity and "transmutation", the changing of an atom from one element into an entirely different element, go hand in hand. Asimov gets into this in more detail later in his book so I am going to leave it at that for now.
Uranium was not as valuable then as it is now but it was valuable. And the way you got Uranium was by extracting if from something called Pitchblende. Pitchblende turned out to be more radioactive than it would have been had it been pure Uranium. That caused Marie and her husband to go searching for more radioactive elements in the Pitchblende. This led to the discovery of a hitherto unknown element, which they named Polonium after the country of Marie's birth. Further detective work led to the discovery of a second new element, which they named Radium.
A key to what they were able to do is the simple fact that not all radioactive elements are equally radioactive. We tend to think of Uranium as being very radioactive. It isn't. Polonium is much more radioactive than Uranium. And Radium is much more radioactive than Polonium. What's going on can be simply explained.
Radioactive atoms are "unstable". Left to themselves their nucleuses will literally explode into pieces. This process is called "nuclear fission". And that's why nuclear chemistry comes into play. What kind of an element an atom is is determined by how many Protons it has in its nucleus. Uranium famously has 92. When a Uranium atom explodes multiple nucleuses are created, one for each piece that the Uranium atom explodes into. The explosion may throw off what is called an "Alpha particle". An Alpha particle has two Protons. That means it literally is a Helium atom. If the rest of the Uranium atom stays in one piece it will have 90 Protons and will be an atom of Thorium.
More commonly, one of the pieces will have 82 Protons and will, therefore, be an atom of Lead. There are many different ways a Uranium atom can explode. But nuclear physicists have been studying this for a long time. So they can assign probabilities for each combination. But that level of understanding came well after the work of the Curies.
As a result of work by the Curies and others the list of radioactive elements increased quickly. And further careful study of pitchblende yielded three other hitherto unknown elements: Actinium, Radon, and Protactinium. If you have read the earlier installments you will remember the periodic table and the search to fill holes found in it. The exploration of radioactivity was responsible for filling many of these holes in. The Curie work still left holes to be filled but that became harder and harder. Why?
Well, remember the business about some elements being more radioactive than others. Early on it seemed likely that transmutation was going on. Uranium would disappear over time and other elements would appear. And it wasn't just Uranium. Radioactive elements had a habit of disappearing. But when they did other elements would appear. And these new elements might be radioactive and they might not. It took a long time to figure out what was going on, that atoms were fissioning into other atoms. But this "Y" appears when radioactive element "X" disappears was noted early on.
For instance, as Uranium disappears, Thorium and Radium appear. But then if you wait a while longer some of the Thorium would disappear and other elements would appear. These patterns acquired the name "decay chain". "X" decays into "Y" which, in turn, decays into "Z".
Eventually a concept called "half life" emerged. After a fixed amount of time, which was specific to the element, exactly half would disappear to replaced by elements in its decay chain. And the more radioactive an element was, the shorter the half life. So the half life of Uranium was a long time. The Half life of Radium was a short time. And the half life of Polonium was somewhere in the middle.
The reason holes in the periodic table became harder and harder to fill turned out to be related to half life. Let's say the half life of element "X" is a million years. That sounds like a long time. But it is very much shorter than the amount of time the Earth has been around. So say at the time of Earth's formation 1% of it is element "X". If we wait a million years element "X" will only constitute 1/2% of the composition of the Earth. That's still a lot.
After ten million years we would have to multiply 1/2 by itself ten times. It turns out the result is about a thousandth. And after another ten million years we will have a thousandth of a thousandth or only a millionth of the original amount of element "X" remaining. But at this point we are still more than 4.5 billion years away from the appearance of humans on Earth.
Unless a radioactive element has an extremely long half life, one measured in billions of years, little or none of what was originally there will remain. And that was the secret. All of the holes in the periodic table that lasted to the start of the twentieth century belonged to elements that had relatively short half lives. So whatever amount of the element that was around when the Earth formed would be long gone by now.
And let me confess to a wee bit of a simplification. Elements don't have half lives. Isotopes do. Elements come in multiple types called isotopes. All isotopes of a particular element have the same chemical properties. What makes two isotopes of the same element different is the number of Neutrons in its nucleus. Neutrons don't affect chemical properties but they do affect radioactivity properties. Some isotopes are stable. Other isotopes are radioactive. Each isotope of each element has it's own distinct half life. If the isotope is stable we can think of its half life as being infinitely long.
Carbon-12 has 6 Protons and 6 Neutrons. It is stable. Carbon 14 has the same 6 Protons but it has 8 Neutrons. It is extremely radioactive. We know this because it's half life is less than 6,000 years. Uranium-235 is not very radioactive. It's half life is 700 million years. Uranium-238 is even less radioactive with a half life of 4.5 billion years. The difference in half life between Uranium-235 and Uranium-238 is one of the reasons that almost all the Uranium now on earth is Uranium-238.
Now, it sounds like all this means that by now there should be no Thorium left. There is no Thorium left from the time when the Earth was created. But there is a way to make more. You just let Uranium-238 decay. That's why we have Carbon-14 around today in spite of its extremely short half life. All the original Carbon-14 is long gone by now but new Carbon-14 is continuously being created by various processes. So it is found pretty much everywhere but only in trace amounts.
To cut to the chase, all of the holes from one (Hydrogen) to 92 (Uranium) were filled in before the start of World War II. This constitutes the list of "naturally occurring elements", elements we expect to find in nature. But as far back as 1934 people were searching for evidence of the existence of "trans-uranic elements", elements with a number higher than 92.
If it is possible to break an atom apart (fission it) is it also possible to build it up instead, to "fusion it", by fusing two atoms together to create a new element with an atomic number (number of Protons) equal to the sum of the atomic numbers of the constituent parts? It took a while to confirm that the answer was "yes". And at the same time things got more complicated.
Atomic number is literally the number of Protons in the nucleus of an atom. In the early part of the twentieth century a strange particle called a Neutron was discovered. Besides contributing to an element's atomic number a Proton has an electrical charge And it is this electrical charge that turns out to confer the specific chemical properties of each element. So Hydrogen is Hydrogen specifically because it has a nucleus with one Proton in it and that Proton confers exactly one unit of positive electrical charge on the nucleus of the Hydrogen atom.
Uranium is Uranium because its nucleus has 92 Protons in it resulting in a total positive electrical charge of 92 units. If you know the number of Protons in the nucleus you know the chemical properties of the atom you are talking about. But the Neutron has zero electrical charge (and the zero electrical charge is why it has no effect on chemical properties). It is electrically neutral, hence the name. But other than that, the Neutron seems very similar to the Proton. Both particles weigh roughly the same, for instance.
An obvious question is "can you turn a Neutron into a Proton or vice versa"? In 1934 Fermi was trying to answer questions like that. And it turned out that is some cases you could. If you slammed a Neutron into an atom you could sometimes get the Neutron to stick. Beyond that, you could sometimes get it to magically change into a Proton. And that meant the atomic number went up by one and an atom of "X" turned into an atom of "X+1". Very interesting.
Fermi didn't actually do what he was trying to do, which was turning Uranium into a new element with an atomic number of 93. But he did succeed in doing a lot of other important things, things that have justifiably made him famous. Element 93 (Neptunium) and element 94 (Plutonium) were both discovered in 1940. They constituted the first of many trans-uranics. The atomic number list now goes all the way to 118. (It ended at 105 when Asimov wrote the book.) And traces of several of the "not found in nature" trans-uranics were eventually found in tiny quantities in nature, once scientists knew exactly what to look for.
Asimov delves no deeper into the subject of radioactivity at this point so I won't either. On to Electrons.
Protons are heavy subatomic particles that have one unit of positive electric charge. Electrons are light subatomic particles that have one unit of negative electric charge. Like Protons, Electrons are critical to understanding why Chemistry works the way it does. But that doesn't mean they were easy to detect and study. The first to take a shot at it was Faraday. He got nowhere. It turns out he didn't have access to a good enough vacuum pump to make his experiments work. But in 1854 Geissler perfected a pump that was good enough.
With a "Geissler tube" you could make part of its wall glow green under the right conditions. In 1876 Goldstein hypothesized that the effect was caused by some kind of radiation being emitted from a negative electrode placed in the vacuum inside the tube. This radiation was then hitting the glass wall of the Geissler tube. Since the negative electrode was then called a "cathode" the radiation was called "cathode rays". (Older readers should recognize this as the origin story of the formerly ubiquitous but now rare CRT or Cathode Ray Tube.)
Crookes improved on Geissler by improving the quality of the vacuum and by adding small magnets inside the tube. The magnets could deflect the path of the rays. And Crookes decided they were particles, not rays, anyhow. Thompson in 1897 went a step farther and showed that cathode rays could also be deflected by electric fields. Further experimentation showed that these particles (the consensus position by now) were unbelievably light. Two thousand of them weighed about as much as a single Hydrogen atom, the then lightest thing known. That meant they pretty much had to be their own thing. In 1891 Stoney suggested they be called "electrons" and the name stuck.
Given how light they were it seemed possible that they were a part of an atom. But the whole idea behind atoms was that they were the smallest thing possible and indivisible. They were supposed to have no internal structure. But a number of experiments seemed to indicate that if you kicked an atom in one of several ways an electron would come flying out. If you started with a neutral atom (one with no electrical charge) then what got kicked out had exactly one unit of negative charge and what was left behind had exactly one unit of positive charge. The books stayed balanced. That was interesting.
And it turned out that there were lots of ways to get an atom to kick out an electron. That suggested that the electron was located at or near the outside edge of the atom. Still more interesting was what happened in the reverse situation. Suppose an atom absorbed an electron. Then it would emit some light. And there was a complicated relationship between the color of the light and the conditions under which the electron was absorbed. This electron emission/absorption business changed the electrical charge of the atom but did not change which element it was. All this took a lot of sorting out.
And by this time it was well understood that "visible light" was just one kind of electromagnetic radiation. Various members of the family might seem like they were different but in reality the differences were superficial. The electromagnetic spectrum stretched from radio waves to gamma rays, with visible light roughly in the middle. But the same rules applied to all of them.
Radio was ultra-low frequency and ultra-low energy. Gamma rays were super-ultra-high frequency and super-ultra-high energy. And frequency is just wavelength turned inside out. So low frequency is just long wavelength and high frequency is just short wavelength. If you know the energy or the frequency or the wavelength you can directly calculate the other two.
Why is this discussion relevant to the issue at hand? Because it was determined that if you did a certain specific thing to an electron often light (well, technically electromagnetic radiation) of a specific frequency was emitted. Now, this "light" might actually be X-Rays (more energy than regular light) or microwaves (less energy than regular light). But if you calculated the energy of whatever kind of electromagnetic radiation it was, patterns quickly emerged.
Mosely found that you could associate a "characteristic" X-Ray wavelength with a specific metal. In other words, you could identify the type of metal from the X-Rays it emitted. And there was a specific mathematical progression in the characteristic wavelengths as you walked up and down the periodic table a step at a time. He made the leap to suggesting that there was a certain amount o energy associated with each electron in an atom so you could use X-Ray wavelengths to calculate electron counts.
For a neutral atom Proton counts give you electron counts and vice versa. That led to a simple idea of how an atom is put together. You have a "nucleus" with a bunch of Protons (and later Neutrons too) in it and an outer "shell" with a bunch of electrons in it. And it is possible to give an atom an negative charge by somehow stuffing some extra electrons into the shell. You could also give an atom a positive charge by somehow stealing some electrons so the shell was a couple of electrons short.
Chemists were already familiar with "Ions". These were versions of molecules that had either extra positive charge or extra negative charge. This simple model of the atom brought it all together. Negative ions featured one or more atoms with extra electrons in their shells. Positive ions featured one or more atoms that been short changed one or more electrons from their standard compliment. Of course, this model turned out to be vastly too simple but it was a good starting point.
And one of the problems with the above model was there there was a dominant wavelength that could be associated with a specific atom, particularly when it came to the metals. But atoms could emit or absorb lots of different wave lengths, not just the dominant one. That meant there was more going on.
But again, patterns emerged. If you kicked a Hydrogen atom just right it would emit or absorb one of several wavelengths. But if you were doing a certain thing you always saw the same set of wavelengths. Some of these sets showed up so frequently that they were assigned names like "K series", "L series", etc. And there was a simple mathematical relationship between the various permitted wavelengths. The pattern was obvious but scientists initially did not know what to make of it.
An early idea was that of "shells". Instead of a single shell electrons could occupy one of a number of shells. The wavelength of the light gave you the amount of energy. And that was presumably exactly the amount of energy it took to kick an electron from a certain specific shell to a certain other specific shell. The idea worked fine but soon there were lots of shells. And the shells ended up having very odd attributes. For instance, they were definitely not simple rings or spheres.
Generations of what started out as simple models of the atom were by necessity replaced by more and more complicated models. And each model seemed to fail in ways that suggested that the failed model had just not been weird enough. Cutting to the chase, eventually Quantum Mechanics emerged, very roughly in 1930, and finally produced models that fully worked. Quantum Mechanics has since been replaced by the "standard model". But the standard model pulls many of its key concepts directly from Quantum Mechanics.
Both Quantum Mechanics and the standard model are based on mathematics that are beyond me. But frankly we got to them because all the simpler and more "natural" models failed. It turns out that the atom is quite complex. Atoms have structure. They are composed of particles like the Proton and the Neutron. But it turns out both of these particles are actually composed of smaller, more fundamental, sub-sub-particles like Quarks.
And it is important to remember that at a certain point (the arrival of Quantum Mechanics) things stopped getting more complicated. We have had pretty much the same complicated model involving shells for a long time now. Nothing new has shown up for a long time now that would necessitate a major change to the model.
Electrons can and do jump up from one shell to another, higher energy shell. To do this the electron must absorb a photon of light containing a sufficient amount of energy. Or an electron can jump down to another, lower energy shell. It does this by emitting a photon of light. The wavelength of that photon is precisely determined by the energy difference involved. Remember that a specific amount of energy translates into a specific wavelength.
I have left out a lot of details like hyperfine splits but all of it is just that, detail. And nothing behaves anywhere near what sounds sensible. But if say electrons behaved at all sensibly, none of it would work. For instance, we are all used to every system containing at least some friction. With that in mind, consider an electron orbiting a nucleus. Throw some friction in. It doesn't matter if you throw a little in or a lot. What happens is that the electron spirals into the nucleus and atoms don't work.
How much friction you throw in just determines how long it takes the electron to crash into the nucleus. Any friction at all will cause the crash to happen sooner or later. The "Quantum" rules are batshit crazy but absolutely necessary. Since the orbit of an electron can't decay a little bit at a time (Quantum rules forbit it) a small amount of friction becomes impossible and the electron can merrily keep orbiting in the same grove forever (or until something gives it a Quantum sized kick).
Asimov does a nice job of explaining how this business of shells explains chemistry. Most of the chemical attributes of a specific element are determined by how many elements are in its outer shell. This is the shell that is easiest for other atoms to see. And the Quantum rules determine how many electrons it takes to fill that shell. There is an urge to get to full shells. This results in chemical bonds.
Let's say two atoms have an outer shell that holds 8 electrons. And let's say that in their natural states one atom has 6 electrons in its outer shell and the other atom has two. If they can somehow share electrons then each atom can behave sort of like it has an outer shell filled with 8 electrons. And in this case, 8 is the magic number. What I have just described is called an "ionic bond". There are other kinds of sharing arrangements resulting in other kinds of bonds.
The specifics of the sharing arrangements depend on Quantum Mechanics so I am going to skip them. But they have been worked out and they tell Chemists what combinations of atoms want to form into molecules and how "stable" (resistant to disruption) the resulting molecule will be. Let me end with a quick observation about why "Noble Gasses" are the way they are.
Noble gasses don't like to combine with anything to make molecules. Why? Well, Helium has 2 electrons in its outer shell and the magic number for that shell is 2. Neon has 8 electrons in its outer shell and the magic number for that shell is 8. The same is true for Argon and the other Noble gasses. All of them feature the exact number of electrons in their outer shell to fill it completely. This means that their outer electrons don't want to go anywhere. And that means that these elements really don't like to be part of a molecule.
Asimov goes into all this and much more in considerable detail. (This chapter is the longest in the whole book.) He is a great explainer so if you want to know more about shells and chemical bonds and the like, find his book and read this chapter. But my summary gives you the general idea so I am going operate on the "enough is plenty" principle and spare you any more of it.
And just so you know, we are at the half way point in the book. I don't know whether that sounds like good news to you or not.
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