However, in 19 10, Rutherford and his students conducted an experiment in his laboratory that went down in history. They bombarded an extremely thin gold foil with alpha particles (positively charged helium nuclei), trying to confirm the size and properties of "raisin pudding" by scattering. However, an incredible situation has emerged: the scattering angle of a few α particles is over 90 degrees. Rutherford himself described this situation very vividly: "It's like bombarding a piece of paper with a 15 inch shell, but the shell bounced back and hit itself."
Rutherford carried forward the fine character of Aristotle's predecessor, "I love my teacher, but I love truth more", and decided to modify Thomson's raisin pudding model. He realized that the alpha particles must have bounced back because they collided with an extremely hard and dense core in the gold foil atom. This core should be positively charged and concentrate most of the mass of the atom. However, from the point of view that only a few alpha particles have large angle scattering, the space occupied by the core is very small, less than one tenth of the atomic radius.
So Rutherford published his new model the following year (19 1 1). In the atomic diagram he described, there is a "core" at the center of the atom that occupies most of the mass. And around this nucleus, negatively charged electrons move around it along a specific orbit. This is very similar to a planetary system (such as the solar system), so this model is naturally called the "planetary system" model. Here, the nucleus is like our sun, and the electrons are the planets orbiting the sun.
However, this seemingly perfect model has its own serious difficulties that cannot be overcome. Because physicists quickly pointed out that negatively charged electrons revolve around positively charged nuclei, this system is unstable. There will be strong electromagnetic radiation between them, which will make the electrons lose energy bit by bit. At the cost, it had to gradually reduce its radius of operation until it finally "hit" the nucleus, and the whole process only took a blink of an eye. In other words, even if the world is described by Rutherford, it will be destroyed by the collapse of the atom itself in an instant. Nuclei and electrons are bound to radiate, neutralize each other, and then turn Rutherford and his laboratory, as well as the whole of England, the whole earth and the whole universe into chaos.
However, of course, despite the pessimistic predictions made by theorists, the sun still rises on time every day, and everyone lives well. Electrons still revolve around atoms happily without any warning of losing energy. Niels bohr, a young man from Denmark, arrived in Manchester safely and began to write a colorful page in the history of physics.
Bohr did not give up this theory because of the difficulty of Rutherford model. After all, it is strongly supported by alpha particle scattering experiments. On the contrary, Bohr has considerable doubts about whether electromagnetic theory can act on atoms, which is a level that people have never set foot in. For Bohr, life in Manchester is obviously much more comfortable than that in Cambridge, although he and Rutherford have such different personalities. The latter is a quick-tempered and energetic person, while Bohr is like a shy big boy, saying a word is like lisping. But they are obviously a great team. Bohr's genius was fully stimulated under the leadership of the boss Rutherford, and soon stirred up waves in history.
19 12 In July, Bohr finished his first paper on atomic structure, which historians later often called "Manchester Memorandum". Bohr has begun to try to combine the concept of quantum with Rutherford model to solve the problem that classical electromagnetic mechanics can't explain. However, everything is just the beginning. In that virgin land that no one has ever set foot in, Bohr can only grope his way forward step by step. No one told him where the direction should be, and his motivation was only the belief in Rutherford model and the great enthusiasm unique to young people. Bohr knew nothing about atomic spectrum at that time, and of course he couldn't see its decisive significance for later atomic research. However, the direction of the revolution has been determined, and nothing can change the fact that quantum theory is about to appear.
A glimmer of light appeared in the cloudy sky. Although it turned out to be just a meteor, this beam of light undoubtedly injected new vitality into the already rigid and aging material world, a kind of vitality with fresh breath and hope. This light lit the torch in people's hands and guided them to find the real A Ring of Endless Light.
Finally, on July 24th, Bohr finished his study in England and went to Denmark. There, his lovely fiancee Margaret is anxiously waiting for him, and the future of physics is about to open to him. Before leaving, Bohr showed his paper to Rutherford and was eagerly encouraged. However, did Rutherford ever wonder to what extent this young man would change people's ultimate view of the world?
Yes, yes, it's time. The great trilogy is about to come out, and the era that truly belongs to quantum has finally arrived.
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After-dinner gossip: the kindergarten of Nobel Prize winner
There is no doubt that Rutherford himself was a great physicist. But at the same time, he is also a great physics teacher. He discovered people's genius with keen eyes, cared for them with great personality and tapped their potential. Most of Rutherford's assistants and students proved to be excellent later, including a large number of science masters.
We are familiar with niels bohr, one of the greatest physicists in the 20th century, the winner of the 1922 Nobel Prize in Physics, and the founder and symbol of quantum theory. Follow Rutherford in Manchester.
Paul dirac, one of the founders of quantum theory, is also a great scientist and the winner of the 1933 Nobel Prize in Physics. His major achievement was made in Cavendish Laboratory in Cambridge (Rutherford succeeded J·J· Thomson as the director of the laboratory at that time). Dirac was only 3 1 year old when he won the prize. He told Rutherford that he didn't want to win the prize because he hated his reputation in the public. Rutherford advised that if you don't accept the prize, the reputation will be even louder.
James chadwick, the discoverer of neutrons, spent two years in Rutherford Laboratory in Manchester. 1935 won the nobel prize in physics.
Blackett resigned as a naval captain after World War I and went to Cambridge to study physics with Rutherford. Later, he improved the Wilson Cloud Room and made great contributions to cosmic rays and nuclear physics, for which he won the Nobel Prize in Physics at 1948.
1932, E.T. Swarton and Caulklaugh Croft built a powerful accelerator at Cavendish Laboratory in Rutherford to study the internal structure of atomic nuclei. Two disciples of Rutherford shared the Nobel Prize in Physics at 195 1.
There are countless examples like this: Englishman Soddy won the Nobel Prize in chemistry with 192 1. Swede Hevesy won the Nobel Prize in Chemistry with 1943. Otto hahn, German, 1944 Nobel Prize in Chemistry. Cecil frank powell, England, 1950 Nobel Prize in Physics. American hans bethe, 1967 Nobel Prize in Physics. Soviet Union P.L. Kapisa, 1978 Nobel Prize in Chemistry.
Except for some slightly alienated cases, Rutherford trained at least 10 Nobel Prize winners (not counting himself) in his life. Of course, there are some outstanding names among his students who didn't win the Nobel Prize, such as Hans Geiger (who was later famous for inventing Geiger counter), Henry Mosley (a young man with infinite genius, but unfortunately died in the battlefield of World War I) and Ernest marsden (who made α with Geiger).
Rutherford's laboratory was called "the kindergarten for Nobel Prize winners" by later generations. His head appeared on the largest face value of New Zealand currency-100 yuan, as the highest respect and commemoration for the country.
five
On August 19 12, Bohr and Margaret got married in a small town not far from Copenhagen, and then they went to England for their honeymoon. Of course, there is one person who must not forget to visit, and that is Professor Rutherford, one of the best friends of the Bohr family.
Although it was in the honeymoon period, the pictures of atoms and quantum still did not disappear from Bohr's mind. He and Rutherford exchanged views seriously again, which deepened their beliefs. After returning to Denmark, he devoted himself to this work with 200% enthusiasm. The dream of uncovering the secrets of the atom is too tempting for Bohr to resist.
In order to keep up with our story, let's describe what Bohr was facing at that time. Rutherford's experiment showed a new look of the atom: the center of the atom has a dense core, and electrons move around this center, just like planets move around the sun. However, this model faces serious theoretical difficulties, because the classical electromagnetic theory predicts that such a system will inevitably release radiation energy, which will eventually lead to the collapse of the system. In other words, Rutherford's atoms cannot exist stably for more than 1 sec.
Bohr was faced with the choice of abandoning Rutherford model or Maxwell and his great theory. Bohr bravely chose to give up the latter. He foresaw with profound insight that at such a small level of atoms, the classical theory would no longer be established, and new revolutionary ideas, namely Planck's quantum and his H constant, must be introduced.
It should be said that this is a rather arduous task. How to overthrow Maxwell's theory is second, and the key is that the new theory can perfectly explain all the behaviors of atoms. In the year when Bohr worked hard in Copenhagen, Mendeleev's periodic law of elements had been discovered for a long time, and the theory of chemical bonds had been firmly established. There are indications that there is a potential law governing their behavior and forming a certain pattern inside the atom. The atomic world is like a pyramid with infinite treasure, but how to find the passage into it is a puzzling problem.
However, like Belzoni in those days, Bohr also possessed the most precious qualities of an explorer: insight and intuition, which enabled him to grasp obscure but fleeting clues, thus opening the door to a new world. 19 13 At the beginning of this year, Danish youth Hans Marius Hansen asked Bohr how to explain the atomic spectral lines in his quantized atomic model. Bohr didn't think much about this problem before. Atomic spectrum is both strange and complicated to him. Thousands of spectral lines and strange effects are so confusing that he seems unable to extract any useful information from them. However, Hansen told Bohr that there are rules, such as Balmer's formula. He urged Bohr to care about balmer's work.
Suddenly, just as Hyon (Ion) discovered the linen with the gorgon painted on it, everything became clear. There is no way to go because of the winding water flow in the mountains, and a mountain village suddenly appears in the willow-green flower bay. No one expected that quantum had made a decisive breakthrough. 1954, Bohr recalled: When I saw balmer's formula, everything became clear.
Looking back at the development of spectroscopy from the beginning, we have to start with the great Bunsen and Kirchhoff, which is bound to be another long text. In view of the space, we only need to have a brief understanding of the background knowledge, because this historical story was not intended to describe all aspects in detail. To sum up, people at that time already knew that any element would emit light with a specific wavelength when heated. For example, we know from the flame experiment in middle school that sodium salt emits bright yellow light, potassium salt is purple, lithium is red, copper is green, and so on. These rays are projected onto the screen through a spectroscope and spectral lines are obtained. Various elements can be seen in the spectrum: sodium always appears as a pair of yellow lines, lithium produces a bright red line and a dark orange line, and potassium is a purple line. In short, any element will produce a specific and unique spectral line.
However, it is a big problem what laws these spectral lines present and why they have these laws. Take the spectral line of hydrogen atom as an example. This is the simplest atomic spectrum. It is represented by a set of line segments, and each line represents a specific wavelength. For example, in the visible light range, the spectral lines of hydrogen atoms are: 656, 484, 434, 4 10, 397, 388, 383, 380 ... in order of nanometers. There is no doubt that these data are not confusing. 1885, Johann Balmer, a Swiss math teacher, discovered this law and summed up a formula to express the relationship between these wavelengths. This is the famous Balmer formula. Change its original form slightly, and express it by the reciprocal of the wavelength, which is more simple and clear:
ν=R( 1/2^2 - 1/n^2)
Where r is a constant, called Rydberg constant, and n is a positive integer greater than 2 (3, 4, 5, etc. ).
This is a very useful empirical formula for a long time. But no one can explain what the meaning behind this formula is and how it is derived from the basic theory. But in Bohr's eyes, this is undoubtedly a bolt from the blue. It is like a spark, which instantly ignited Bohr's inspiration. All doubts become natural at that moment. Bohr knew the secret hidden in the atom and finally smiled at him.
Let's look at Balmer's formula, which uses a variable n, where n is any positive integer greater than 2. N can be equal to 3,4, but not to 3.5, which is undoubtedly a quantized expression. Bohr took a deep breath. His brain is working fast. Atoms can only emit radiation whose wavelength conforms to some quantum law. What does this mean? Let's recall the classical quantum formula derived by Planck: E = hν. Frequency (wavelength) is a measure of energy. An atom only emits radiation with a certain wavelength, that is, it can only absorb or emit a certain amount of energy inside the atom. And how atoms absorb or release energy? At that time, I had a certain understanding. For example, J.Stark suggested that the spectral lines of the spectrum are emitted by electrons moving between different potentials, and J.W.Nicholson, an Englishman, had a similar idea. Bohr was undoubtedly aware of these tasks.
A bold idea emerged in Bohr's mind: only a certain amount of energy can be released inside the atom, indicating that electrons can only be converted between specific "potential energy positions". That is to say, electrons can only follow certain "definite" orbits, and these orbits must meet certain potential energy conditions, so that when electrons jump between these orbits, they can only release energy that conforms to Balmer's formula.
We can make an analogy like this. If you have attended physics class in middle school, you should know the transformation of potential energy. A person with a weight of 100 kg will get 1 000 joules when he/she jumps off the steps with a height of1meter. Of course, this energy will be converted into kinetic energy when it falls. But if this is the case, we know by some means that a person weighing 1000 kg releases a total of1000 joules of energy after jumping down several steps of the same height. What can we say about the height of each step?
The obvious direct calculation is that this person dropped 1 meter, which strictly limits the height of our steps. In normal times, we will admit that a step can have any height, depending on the interest of the builder. But if this condition is added, the height of each step is no longer arbitrary. We can assume that there is only one step in total, so its height is 1 meter. Or this person jumped two steps altogether, so the height of each step is 0.5 meters. If you jump three times, then each level is 1/3 meters. If you are a fan of spy movies, then you will probably guess that every step is 1/39 meters high. But in any case, we can't draw the conclusion that every step is 0.6 meters high. The reason is obvious: the 0.6-meter-high step is not in line with our observation (a total of 1000 joules of energy is released). If only this step is taken, the energy it brings is not enough. If there are two steps, the total height will reach1.2m, resulting in the released energy exceeding the observed value. If we want to conform to our observation, we must assume that there are one and two-thirds steps in total, which is undoubtedly absurd, because children know that steps can only have integer levels.
The number of steps "must" here is an integer, which is our quantization condition. This condition limits that the height of each step can only be 1 m, or 1/2 m, but not any number in the middle.
The story of atoms and electrons is basically similar to this one in reason. We still remember that in the Rutherford model, electrons revolve around the nucleus like planets. When the electron is closest to the nucleus, its energy is the lowest, which can be regarded as a state on the "flat ground". However, once electrons gain a certain amount of energy, they gain the power to "climb" up one or several steps and reach a new orbit. Of course, if there is no energy supplement, it will fall from the orbit at that height until it returns to a "flat" state, and at the same time release the original energy in the form of radiation again.
The key point is that we now know that in this process, electrons can only release or absorb specific energy (given by Balmer formula of spectrum), rather than continuously. Bohr made a reasonable inference: this shows that the "steps" of electronic climbing must meet certain height conditions and cannot be continuous and arbitrary as assumed by classical theory. Continuity is destroyed, and the quantization condition must become the master of atomic theory.
We have to use the quantum formula E = hν again, please forgive me. Stephen hawking said in his best-selling book A Brief History of Time that inserting any mathematical formula would halve the sales of his works, so he thought twice and only used one formula, E = mc2. Our historical story is a play, we don't consider so much, but even if we list the formulas, we don't force the spectators to understand their mathematical significance. Only this E = hν, I think it is necessary to understand its meaning, which is also conducive to the understanding of the whole historical story. Scientifically speaking, it is no less than Einstein's E = mc2. So I want to repeat the description of this equation: e stands for energy, h is Planck constant and ν is frequency.
Back to the topic, Bohr now knows that the spectral lines of hydrogen atoms represent the energy released by electrons jumping from one specific step to another. Because the observed spectral lines are quantized, the "steps" (or orbits) of electrons must also be quantized, and they cannot take any values continuously, but must be divided into "ground floor", "first floor" and "second floor". Between the two floors, it is a forbidden area for electrons, and it can't appear there. Just like a person cannot float between two steps. If the electron is in the "third layer" and its energy is represented by W3, then when the electron has a whim and decides to jump to the "first layer" (energy W 1), it will release the energy of W3-W 1. The formula we asked everyone to remember came into play again, W3-W 1 = hν. So the direct result of this shift is that a spectral line with the frequency ν appears on the spectrum of the atom.
Bohr's ideas were all transformed into theoretical deduction and mathematical expression, and finally published in the form of three papers. The title of these three papers (or three parts of a big paper) is On the Composition of Atoms and Molecules. The system with only one nucleus and the system with several nuclei were sent to Rutherford in Manchester from March to September in 19 13, and were recommended by the latter to be published in Philosophy magazine. This is an epoch-making document in the history of quantum physics, which is also a great trilogy.
This is indeed the arrival of a new era. If the history of quantum mechanics is divided into three parts, Planck announced the birth of quantum in 1900, then Bohr announced that it entered youth in 19 13. A complete quantum theory system was established for the first time. Although we will see that this system still bears strong traces of the old world, its significance cannot be underestimated in any case. Quantum shocked the whole world for the first time with its power. Although half of its consciousness is still asleep and in the old physics building, its roar has undoubtedly disintegrated the whole old world and shaken the foundation of classical physics that has lasted for hundreds of years. The mythical giant has begun to awaken. Those nobles hiding in ancient castles, tremble!