In the last video we talked about how people are confirming the existence of atoms step by step, and although we can't see the atoms themselves, physicists at the end of the 19th century saw a series of changes in the laboratory that atoms produce. These include the radioactivity of elements, the transmutation of elements, and the discovery of electrons.
Especially in 1905, Einstein also confirmed the existence of atoms from a theoretical point of view, which is what we know as Brownian motion.
Now that we know the truth of atoms, the next step is to build an atomic model.
The first person to propose the atomic model was the discoverer of the electron, John Thomson.
Thomson was born in Manchester, his father's publishing house often printed textbooks for the university, the family conditions were quite good, and he had frequent access to university professors and various books.
Thomson was admitted to the University of Manchester at the age of 14, and in 1876, at the age of 21, Thomson was sent to Cambridge University, where he received his doctorate in 1880 and stayed on to teach.
In 1884, Lord Ruili, director of the Cavendish Laboratory at cambridge university, directly abandoned it after the 5-year term, the name Ruili is very familiar to us, when explaining the distribution of blackbody radiation, we have talked about the Ruili formula, and when we explain why the sky is blue, we often mention Ruili scattering.
Lord Ruili was the second director of the Cavendish Laboratory, the first was Maxwell, after Ruili resigned, Cambridge University began to look for a new head of physics, successively looking for Lord Kelvin, and Germany's Helmholtz, these two people are almost one hundred and thirty years old, and they have achieved fame, so they directly declined the invitation.
But for young people, this position is not dreamed of. However, Thomson at Cambridge University saw the opportunity to flip his life and dared to apply to the school, since no one did it, let me try it first, with the support of Ruili, 28-year-old Thomson became the third head of Cavendish Laboratory.
Thomson proved to be a good leader, producing nine Nobel laureates under his command, including Onast Rutherford.
This one is better than his teacher, and under his hands has produced 11 Nobel Laureates, which can be called the Nobel Prize Kindergarten. Later we will mention Rutherford, who is also our protagonist today.
In 1897, Thomson discovered that cathode rays are actually electrons, which made him win the Nobel Prize in 1906, after the discovery of electrons, Thomson believed that electrons are parts of atoms, and atoms are electrically neutral, then there must be positively charged parts of atoms.
In the absence of experimental evidence, in 1903 Thomson came up with his own atomic model purely by imagination, known as the "raisin pudding" model.
He sketched the picture of atoms in the shape of a spherical shape, electrons embedded in a cloud of positive charge in concentric circles, and the whole structure looking like a pudding embedded with raisins.
And Thomson can't say how many electrons there are in the atom of an element, but gives a rough range of quantities.
For example, a hydrogen atom with an atomic weight of 1 may have only one electron, and a helium atom with an atomic weight of 4 may have two electrons, or it may be 3 or 4.
The main reason is that at this time, people do not know the order of the elements in the periodic table, in fact, according to the number of positive charges in the nucleus, of course, do not know what the number of positive charges in the nucleus is, so Thomson is not clear how many electrons should be in an atom.
At that time, people relied on the index of the order of the elements as the weight of the atom, that is, the atomic weight.
For example, the atomic weight of the hydrogen atom is 1, it ranks first, the next element with the smallest atomic weight is helium, its atomic weight is 4, ranking second, and the third atomic weight is 6, that is, lithium, ranking third.
It can be seen that people's understanding of the periodic table at that time had great limitations, and did not clarify the most basic arrangement law, especially in 1907, after the discovery of isotopes, the periodic table of the periodic table arranged in atomic weight became chaotic.
It wasn't until 1910 that Ball gave the right answer for the first time. We will elaborate when we talk about Pol, and we will skip it here for the time being.
Obviously, the atomic model proposed by Thomson is very different from the actual situation, but no one knew the structure of the atom at that time, and since Thomson proposed the model, we will do experiments to verify it.
The person who did the experiment was none other than Thomson's student, Rutherford.
Born in 1871 in a new Zealand village, Rutherford was 8 years older than Einstein and 14 years older than Pol, his father was a worker in a linen textile factory, his mother was a primary school teacher, and the family had 12 children.
Rutherford's academic career went on a scholarship, and in 1895 Rutherford came to Cambridge University and laid the foundation for Thomson at The Cavendish Laboratory.
It just so happened that this year Röntgen discovered X-rays, which caused a huge sensation, and Thomson, who liked to study atomic phenomena, became extremely interested in this ghostly X-ray.
His research project for Rutherford was to measure the effects of X-rays passing through gases, confirmed that X-rays can ionize gases, and over the next two years, Rutherford published 4 papers on X-rays, earned a doctorate, and made Rutherford stand out in the scientific community.
As we also mentioned in the previous video, Rutherford divided Becquerel rays into α rays and β rays by studying the penetration of various rays, naming the radioactive radiation discovered by Gerald Schmidt as γ rays.
Later, it was also confirmed that X-rays and gamma rays are electromagnetic waves, β rays are electrons, and α rays are helium nuclei.
With these achievements, Rutherford had become a pioneer in radioactivity research at that time, and it was a bit too condescending to continue to work as an assistant to Thomson, and Rutherford had the ability to carry out scientific research independently.
In April 1898, when McGill University in Montreal, Canada, was recruiting full professors, Thomson wrote a passionate letter of recommendation for Rutherford, and at the end of September, the 27-year-old Rutherford arrived in Montreal and stayed there for nine years.
In 1901, Rutherford and the 25-year-old chemist Sody discovered the transmutation of elements, an achievement that earned him the 1908 Nobel Prize in Chemistry.
In 1905, when he was in Montreal, Rutherford bombarded mica flakes with α particles and found that the trajectory of α particles had shifted, although there was no in-depth study, Rutherford kept this phenomenon in mind.
In May 1907, Rutherford came to the University of Manchester as a full professor, and his main research topic was the scattering of α particles.
Rutherford devised a simple experiment in bombarding the gold leaf with α particles to observe the trajectory of α particles after passing through the gold leaf, which needed to be carried out in an all-black environment to see the faint flash left on the screen after the α particles passed through the gold leaf.
And the need to bombard the gold leaf with α particles for hours, Rutherford gave the arduous task to his new assistant Hans Geiger;
Later, a graduate student, Marsden, who also participated in the study, found that most of the α particles can pass through the gold leaf in a straight line, some α particles will be deflected at small angles, a small part of the α particles will be deflected at large angles, and a very few α particles will be directly bounced back.
In June 1909, Geiger and Marsden published the procedure and results of the experiment, and Rutherford did not comment on the results in the paper.
Because of this result, Rutherford was puzzled, in Rutherford's view, the α particles were directly bounced back, just like using a 38 cm shell to bombard a piece of paper, but the shell returned to hit him, which was simply incredible.
According to the atomic model proposed by his teacher Thomson, as long as there is a α particle that can pass through the gold leaf smoothly, then all α particles should be able to pass through, and there is no case of being directly bounced back.
Rutherford boldly abandoned Thomson's atomic model and in December 1910 proposed his own atomic model, historically known as the "planetary atomic model".
He conceived of a picture in which Rutherford believed that in the center of the atom, there was a nucleus that carried all the mass of the atom and was positively charged, but the size of the nucleus was only 1/100,000 of the atom, generally speaking, the size of the atom was 10^-8 cm, and the size of the nucleus was 10^-13 cm.
For example, if the size of an atomic nucleus is the tip of a needle, then the electron is 100 meters away, orbiting the nucleus like a planet orbiting the sun. Such an atomic model fits the scattering experiment of α particles, and the tiny α particles that bounce back are hit by a head-on nucleus.
Rutherford himself is a very typical experimental physicist, he does not like unfounded speculation, of course, he does not like the research style of theoretical physicists, the beginning depends on assumptions, the process is all guessing.
So Rutherford did not rush to publish his own atomic model, but proposed a mathematical formula that predicts how many α particles will be deflected at a particular deflection angle.
Through experiments, Geiger found that the distribution of α particles after being scattered was exactly the same as Rutherford's prediction.
It was then that Rutherford boldly announced his atomic model in 1911. I believe that you can see that Rutherford's atomic model has made great progress compared with Thomson's atomic model, closer to the real situation, but the same problem is that Rutherford can not say the number of electrons in the atom, nor can he say how electrons are distributed outside the nucleus.
And Rutherford's atomic model also faces a very serious problem, electrons like planets outside the nucleus around the nucleus, Newtonian mechanics tells us that circular motion is accelerated motion, Maxwell electromagnetic theory tells us that accelerated electrons will release electromagnetic radiation, constantly depleting energy.
In this way, the electron will fall into the nucleus in a very short time, the whole atom will completely collapse, and the existence of the objective world strongly shows that there is a problem of instability in Rutherford's atomic model.
This problem was solved by Pol in 1913, when Pohr was still a student of Rutherford. In the next few videos, I'll talk in detail about how Bohr built a quantized atomic model.
After Rutherford published his own atomic model, he received a letter from Japan written by Nagaoka Hantaro.
The letter said that 5 years ago, I had proposed an atomic model similar to your atomic model, which I call saturn. Rutherford admits that his atomic model is similar to Nagaoka's, but there are essential differences.
In Hantaro Nagaoka's model of Saturn, the nucleus is as large as Saturn, taking up most of the atom's space, and the electrons orbit Saturn like saturn's moons.
Obviously, Nagaoka Hantaro's model was probably guessed, and if he had done the scattering experiments of α particles like Rutherford, he would not have proposed that the nucleus of an atom was as huge as Saturn.
So in general, we rarely mention the Nagaoka model.
Well, today's video is here, the protagonist of the next video will be Pol's turn to appear, he and Planck, Einstein are known as the three giants of the old quantum theory, in Pol's hands the old quantum theory will also reach its peak, the next will be the world of young men.