How is it that electrons move but protons do not

 

We will talk about atoms and atomic nuclei; The first thing to do is to get used to a few orders of magnitude and numbers. The examples I give are taken from the Feynman lectures by Richard Feynman, the Nobel Prize winner in 1965.

Imagine a drop of water about 6mm in diameter.

The first step is to enlarge it two thousand times. This is the extreme limit of the magnification that can be achieved with light microscopes.

Our drop has a diameter of 12 m; it is as big as a small hall. The surface still looks quite smooth; we may occasionally notice an object the size of a football floating around in the water; it is a paramecium.

We increase again by a factor of 2000. Now our drop has a diameter of 15 miles. And now we notice an irregular movement of the water particles; it creates an impression like the random movement of a crowd of spectators in a crowded stadium. It is the so-called Brownian molecular movement. That is the random, incessant movement of the molecules. Wherever masses move, there is kinetic energy, kinetic energy. Of course, the Brownian molecular movement is not an ordered, targeted movement of the whole body as it is when the drop moves as a whole. The kinetic energy of the irregular movement of the molecules is well known to us, it is the heat energy.

We are not yet able to see the individual molecules exactly. Now we enlarge again by a factor of 25. And only now, since our drop covers all of Germany, do we see the molecules, consisting of an oxygen atom about half a centimeter in diameter and two hydrogen atoms about half as large.

Or another comparison. Take an apple. Imagine the apple has grown to the size of the globe. Then one molecule of the apple is as big as the apple before it was enlarged. A molecule is to an apple like the apple is to the globe.

How is an atom structured? It consists of a core and a shell. In the shell are the electrons, which are negatively charged elementary particles of incredibly small mass (about 10-30 kg), which to this day nobody knows whether they have any extent at all. They are electrically negatively charged; they carry the negative elementary charge; i.e. all charges occurring in the world are integer multiples of this smallest charge. The core is a hundred thousand times (!!) smaller than the shell, but it contains almost the entire mass of the atom. Think of the atomic nucleus as a marble one centimeter in diameter. Then it is half a kilometer to the edge of the atom, and the closest atomic nucleus is one kilometer away, if packed as closely as possible. Now one understands Lenard's statement: "The atom is as empty as the universe".

The atomic shell is responsible for the chemical behavior. All chemistry takes place in the atomic shell; the core remains unaffected by any chemical processes. Whether a substance is burned, digested, melted down or dissolved somewhere, none of this affects the nucleus. These are all processes of the shell.

The atomic nucleus is made up of protons and neutrons. Both have roughly the same mass, but the proton carries the positive elementary charge. The neutron has no electrical charge.

There are as many protons in the core as there are electrons in the shell. This number, also known as the ordinal number, determines which element is involved. For example, the carbon atom has the atomic number 6. Its nucleus consists of 6 protons and usually 6 neutrons; that's the C-12 atom. There is also a type of carbon that contains 8 neutrons in addition to the 6 protons, which is the C-14 atom. Chemically, it behaves in exactly the same way as the C-12 atom, but it is not stable, it decays. But there we are already in the middle of the radioactivity; We'll put that back for a moment. There are 6 electrons in the shell of the carbon atom. When coal burns, one carbon atom combines with two oxygen atoms to form a carbon dioxide molecule. The three reaction partners assemble their previously three atomic shells into a single molecular shell. During this reaction, energy is released in the form of heat.

Let us turn to another behavior of the atomic shell, which is of interest for our topic today. We will briefly discuss two consequences of Heisenberg's uncertainty principle. One is the existence of the so-called quantum mechanical kinetic energy, which is a direct result of the so-called positional uncertainty.

Suppose you wanted to try to hold an elementary particle, say an electron, in one place, to pin it down, so to speak. It would react very strangely. The tighter we pound it in, the more violent and irregular it would be. It would gain kinetic energy. This is the so-called quantum mechanical kinetic energy. It is definitely reminiscent of Brownian molecular motion, the underlying motion is also chaotic, but the quantum mechanical kinetic energy only becomes noticeable when the particles are compressed to atomic size, so it is out of the question for molecules.

If one wanted to lock an electron in the area of ​​an atomic nucleus, it would gain a kinetic energy that would be so great that it could no longer be held there. It would fly out of the core with an energy that would be many millions of times greater than the greatest energy it has at its usual location, in the nuclear envelope. Could, would? This scene happens all the time! It is the? Decay that occurs when a neutron is converted into a proton in the nucleus. This creates an electron which, for the reasons mentioned, cannot withstand it in the nucleus. It flies out with tremendous energy, and now we've already discussed one of three naturally occurring radioactive decays.

Perhaps it is now puzzling to you that an atomic nucleus can hold together at all. There are in a very confined space - diameter 10-15m - protons and neutrons crammed together. They have to move around each other like mad, because their quantum mechanical kinetic energy must be considerable. In addition, the electrical force - charges of the same name repel each other - drives the protons apart. Why can an atomic nucleus be stable at all?

There are two reasons. First, the quantum mechanical kinetic energy is inversely proportional to the mass, so it is much smaller for neutrons and protons than for electrons. But at least it is so big that the neutrons and protons actually move like mad in the nucleus. In addition, the electrical repulsion drives the protons apart. What holds the core together? It is the so-called strong interaction, a tremendously strong attractive force between elementary particles, the so-called Yukawa force, which only works when the particles come together very closely, i.e. at a distance of around 10-15 m. And now it is clear that once the core wobbles, tears, and the fragments come so far apart that the Yukawa force is no longer effective, the fragments are left off the chain and blow apart. That is exactly the energy that we talk about in nuclear decay and fission. It is a good ten million times greater than the energy that can be released in a typical chemical elementary process, by which we understand, for example, the above-described connection of a carbon atom with two oxygen atoms to form a carbon dioxide molecule.

But let's go back to the shell. And to the second episode of Heisenberg's uncertainty principle. We talked about the blurring of the location. It makes it impossible to pin an electron. But maybe you can get it to move in an orderly fashion, say on a circular or elliptical path, as the so-called planetary model of the atom leads us to believe? Unfortunately it doesn't work either. If you wanted to try that, you would no longer be able to specify the location of the electron. It would smear, so to speak, spread over the whole shell. How can you imagine that? Not at all, nobody really understands it. If the electron were a lion that you wanted to let go around in a nice circle, it would gradually disappear, but without dissolving into nothing, it would roar all over the room and no one could say exactly where the roar came from.

The electrons stay somewhere and somehow in the shell, they move here and there with a certain probability, but at one point they have an absolutely iron order. That is in their energy. It is very precisely determined. Even more. The energies that they can have in the shell are precisely graduated. You can only absorb certain portions (quanta) of energy and release precisely these portions again. This happens when you change energy levels. And these portions are typical for the atomic shell and thus for the element like a fingerprint. You all know these portions. Because when an atom gives off such a portion, it happens in the form of electromagnetic radiation, mostly visible light, and the size of the portion of energy that the shell is giving off determines the color of the light. And this is how the spectra are created that are characteristic of the individual atom. And the atom can also take up the portions that it can release, so the absorption spectra and emission spectra are the same.

And now we want to let our imagination run wild a little. Suppose a fabric is exposed to daylight, which contains all possible colors. Its atomic shells absorb certain colors, certain light quanta. And suppose that the atoms do not give up the quanta again immediately, but with a delay of perhaps a few hours. In the meantime it has got dark. The effect? Yes, the afterglow, the luminescence as you know it from the luminous digits on your wristwatch.

Or you can stimulate the atomic shells in such a way that certain chemical processes are allowed to take place and, in turn, the absorbed energy is released somewhat delayed in the form of light. The effect? Well, the glow of the fireflies, the chemiluminescence.

Some of the sun's rays are swallowed up by the ground. Where is the energy? A part of it somehow spills, maybe warms up the ground, in any case the atomic and molecular shells of the earth's surface are not stimulated as strongly as the quanta of visible light actually make it possible. But a little less. And these slightly reduced portions are then broadcast again. Instead of visible light, the earth radiates back infrared light with less energy, heat radiation. And that cannot pass through the atmosphere as well as visible light: the greenhouse effect.

Or you want to get the whitest white of your life in your shirt. Well, even the best shirt can't reflect more light than it can. But daylight also contains ultraviolet light, the quanta of which are more energetic than those of the light that is visible to us. Let's make visible light out of it. How? By the effect just described. You put a chemical in the detergent that swallows ultraviolet light, spills some of it in some form and emits the remaining part as visible light. Hence the "disco lighting".

Semiconducting substances can be composed in such a way that the material contains electron shells in which there is still space for an electron. If you send a current through it, which consists of electrons, the electrons can fall from their free state into the shell; energy is released in the form of light. This is how a light emitting diode works. Because of the exact energy level, it is understandable that light-emitting diodes usually only emit a certain color and that white light has to be constructed using color addition.

Enough of the atomic shell. Long before the facts just described were known, at the end of the nineteenth century, there was a lot of experimentation with luminous colors. They had only just been invented and were attractions at world fairs. And so Becquerel also experimented with luminous colors in 1896. He left these phosphorescent substances - so the technical term - in the dark for days to see how long they afterglow. When he tried that with the uranium-containing material pitchblende, he was surprised. The afterglow did not stop at all, rather the photo plates that Becquerel used for detection were blackened with the same intensity even after months. And he was completely surprised when he happened to leave a piece of pitchblende lying on well-packed photo plates. The piece of pitchblende was clearly visible on the photo plates, as if it had been photographed through the completely opaque cover. There was radiation that couldn't be seen, but was very penetrating, very similar to the X-rays discovered a short time later. Indeed, a second type of radiation called? - Radiation, very closely related to X-rays. Just like this, it is electromagnetic radiation, nothing more than a particularly short-wave light with very strong energy portions (quanta). Only that, in contrast to X-rays, which still result from the transitions of energy states of the electrons, it comes from the nucleus.

The Curie couple succeeded in isolating a substance from the mineral pitchblende that radiated a million times more intensely than the mineral itself; she had discovered the radium. Let's say it right away, radium is a? Radiator, it consists of helium nuclei (2 protons, 2 neutrons), which are emitted during certain nuclear transformations. With which we have also named the third type of naturally occurring radioactive radiation.

Using the example of radiation from radium, an initial quantitative estimate of the enormous energies contained in the newly discovered radiation was made. It is possible to use calorimetric measurements to determine that 1 gram of radium has an output of 0.015 watts. If a battery were to deliver this power per gram of its own weight, it would be empty in a few hours; But radium has a half-life of 1500 years. After 1500 years, halfway through, so to speak, this gram has given off more than ten thousand times the energy that is released in a typical chemical process, the combustion of one gram of carbon. From the turn of the century, there was intensive research into radioactivity, and now Otto Hahn and Lise Meitner take the stage.

Otto Hahn was born on March 8, 1879 in Frankfurt am Main, where he attended high school. After studying chemistry, doing his doctorate and working as an assistant in Marburg in 1904/05 with Ramsay in London, there first discoveries about radioactivity (Th 228). 1906 in Montreal near Rutherford, discovery of further radioactive elements. Return to Germany, assistant to Emil Fischer at the University of Berlin, habilitation there in 1907. This year begins a 30-year collaboration with the Austrian physicist Lise Meitner. Discovery of further radioactive elements, especially discovery of the recoil of the ejected? - Particle and use of this effect for an analytical method. From 1910 the first essential discoveries about the spectrum of radioactive radiation (magnetic line spectra of beta rays) and thus the first indications of the shell structure of atomic nuclei. This path culminated in the discovery of core isomerism in 1920 (Pa 234).

In 1911 the Kaiser Wilhelm Society for the Advancement of Science was founded; Hahn became head of the radioactivity department at the Kaiser Wilhelm Institute for Chemistry in Berlin-Dahlem, and from 1928 he was its director.

Hahn was nominated for the Nobel Prize in Chemistry for the first time in 1914, then five more times up to 1935 (for a total of four different discoveries), and finally in 1946 he received the Nobel Prize in Chemistry for 1944.

In addition to this fundamental work, Hahn also dealt with the applications of radioactivity. You all know the applications of the tracer method in medicine, perhaps also one or the other method of determining the age of rocks.

A less publicly known application, which was introduced by Otto Hahn himself, is the determination of the surfaces of amorphous bodies, such as glass powders. If these contain known quantities of radioactive substances, one can deduce the surface of the substance from the quantity of escaping gaseous decay products.

After all, the most famous age determination on earth, which was carried out in the sixties, has its origins in Otto Hahn's work from the twenties.

A substantial part of the work of the whole institute concentrated on the chemical and physical detection of decay products. Usually a tiny, incalculable amount of some kind of indicator substance was added to the radiant preparation in order to detect a product by precipitation. This precipitation could also result in mixed crystals from the indicator substance and the desired product. Among other things, it emerged that lead isotopes are built into the crystal lattice of table salt and rock salt.Since uranium and thorium are present in seawater, their stable end products, certain lead isotopes, must also be found in them, and these in turn would have to be detectable in the rock and salt deposits of northern Germany. This conclusion has been tested and proven experimentally. A previously incomprehensible finding, the presence of helium in these camps, could also be understood in this way. At that time it was not technically possible to determine the age from the mass ratios of uranium and lead and of thorium and lead in seawater; this happened in the 1960s and the earth was found to be around 4.5 billion years old.

But we come to Otto Hahn's most famous discovery, the fission of the U-235 by slow neutrons (1938).

The discovery of nuclear fission was only a matter of time after the Englishman Chadwick had detected the neutron in 1932 and at the same time found a neutron source sufficient for laboratory scales. Do you bring beryllium-9 with that? - Polonium emitters together, this radiation causes an element conversion to carbon in the beryllium, with a neutron being released. This is very fast, but can be slowed down simply by letting it collide with hydrogen nuclei - that is, protons. Hydrogen is abundant in water or in paraffin - remember that for radioactive processes, chemistry is indifferent. And so the tube with beryllium powder, which is mixed with polonium and is located in the middle of a paraffin block, became an instrument that opened the gate to nuclear fission.

Because slow neutrons can firstly penetrate the atomic nucleus unhindered by electrical repulsive forces, secondly they are so slow that they interact with it and do not simply cross or pass through it. However, no one could or would get used to the idea that a neutron, which is deposited on the largest atomic nucleus available at the time, uranium, which is more than two hundred times as heavy as it is itself, causes it to, with great force, roughly two to burst fragments of the same size. It was of the opinion that in this way an unstable, superheavy core would be obtained for a short time, which would pass into neighboring cores by the known pathways of radioactive decay. But it was different; Otto Hahn and Fritz Strassmann used chemical methods to prove that the U-235 core had disintegrated into barium and krypton.

Before I go into the path of Otto Hahn in the post-war period, I should of course report on Lise Meitner. Lise Meitner was born in Vienna in 1878, prepared privately with a small group of girls with a later professor of physics for the matriculation examination at a Viennese school for boys, the Academic Gymnasium, and then studied mathematics and mathematics at the University of Vienna from 1901 to 1905 Physics, quite unusual for a young woman at this time. Graduated in 1905 with a doctorate in physics, then went on to study in Berlin, the then Mecca of physics, where Max Planck, as she gratefully noted in her memories, welcomed her in a friendly manner and introduced her to the circle of young physicists and chemists, both professionally and privately had developed in the institute and in the Planck house.

By chance she met Otto Hahn, who already had a name in radioactivity, and received permission from the reluctant Emil Fischer to work with Otto Hahn in a small laboratory to familiarize herself with radioactivity. Little by little, a group of physicists and chemists formed in Berlin who met for the so-called Wednesday Colloquium, in which Lise Meitner was welcome and in which there were several Nobel Prize winners of the twenties (including M. v. Laue and J. Franck). Incidentally, Lise Meitner was employed for years as Max Planck's assistant, so much to the prejudice that Max Planck did not like to see women work in science.

The First World War did not completely interrupt the joint work, although Otto Hahn had been drafted into Fritz Haber's unit and Lise Meitner, as an Austrian citizen, was serving in a hospital at the front.

After the First World War, a time began for Lise Meitner that she describes as particularly happy in her memoir. She was not only fully accepted in Berlin, but was also an active participant in all important international congresses, especially the Copenhagen meetings organized by Nils Bohr. Every month brought a new discovery in the field of physics, and Berlin, Göttingen, and Copenhagen were centers of this lively development. Lively not least because, as Lise Meitner emphasizes, because she was constantly surrounded by the best students, who constantly raised new questions and, in their own way, advanced science. In 1934 Lise Meitner and Otto Hahn took up the research direction suggested by Enrico Fermi, to bombard the heavy atomic nuclei with the newly discovered neutrons in order to find out what would then happen.

Lise Meitner was of Jewish descent, her Austrian citizenship initially protected her from gross attacks by the Nazis after 1933. But the first gloomy clouds came: Lise Meitner was expelled from the University of Berlin in autumn 1933 and lost her teaching license. In January 1934 Otto Hahn declared his departure from the University of Berlin in solidarity with Lise Meitner. Otto Hahn refused several requests to join the NSDAP; In 1935 he gave the commemorative speech at the memorial service for Fritz Haber, which had been banned by the Prussian minister of education.

In 1938, after Austria's annexation, Lise Meitner's previous protection was lost. She fled Germany to Stockholm on July 13, 1938. On December 17, 1939 Otto Hahn and Fritz Strassmann discovered the splitting of the uranium nucleus. Both published their results in January 1939, Lise Meitner could only participate remotely and together with her nephew Otto Robert Frisch published a calculation of the energy balance of the fission process.

With the exodus of many important scientists from Berlin and Germany, the heyday of physics and chemistry in this country came to an end; Otto Hahn, now over sixty years old, limited himself to some additional work on the decay of uranium, Lise Meitner was only able to work under difficult conditions in exile. Fritz Strassmann, professor of chemistry in Mainz after the war, put his life in danger when he and his wife hid the persecuted Jew Andrea Wolffenstein in their apartment for two months. Otto Hahn was inaugurated. It was probably this and a whole series of less risky aids that led to Fritz Strassmann being posthumously accepted into the series of "Righteous Nations" by Yad Vashem in Israel in 1985. Otto Hahn was also always welcome in Israel; The Weizman Institute has its own "Otto Hahn Wing".

In 1946 Otto Hahn became President of the still existing Kaiser Wilhelm Society, and in 1948 President of its successor, the Max Planck Society. He is well known for his conflict with Defense Minister Strauss, when he repeatedly expressed the public opinion that a nuclear deterrent policy would be ineffective in the long term. This controversy reached its climax in 1957, when Otto Hahn and 17 colleagues in their "Göttingen Declaration of 18 Atomic Researchers" opposed nuclear armament in the Federal Republic.

What is less well known is that Otto Hahn rejected the offers of Bayer and Hoechst in 1953 to become a member of their supervisory boards, that of all things the French trade union CGT suggested him for the Nobel Peace Prize, that he turned down an invitation from President Eisenhower to the White House, that he refused honorary membership in the Soviet Academy of Sciences.

In the last decade of his life he received many honors; President Johnson, together with Lise Meitner and Fritz Strassmann, presented him with the Enrico Fermi Prize of the American Atomic Energy Commission.

The friendship with Lise Meitner, who spent the last years of her life in England with her nephew, lasted a lifetime. Lise Meitner was the godmother of her son Hanno, born in 1922, and of her grandson Dietrich Hahn, born in 1946. In 1968 Otto Hahn died, a few months later Lise Meitner.

The data on the curriculum vitae and the achievements of Otto Hahn and Lise Meitner come from the book: Lise Meitner: Memories of Otto Hahn, Ed. Dietrich Hahn, Hirzel-Verlag.

I would like to thank Mr. Dietrich Hahn for detailed additional information and explanations that I received from him in long and extremely pleasant conversations.

Saarbrücken, May 8, 2008
Dr. Gerd Brosowski