Since Dalton discovered the atom, the evolution history of the atomic model has gone through several important theories and changes. The details are as follows:
1. Discovery:
Since the British chemist and physicist J. John Dalton (1766 ~ 1844) (pictured on the right) founded the atomic theory, for a long time people thought that the atom was like an extremely small solid glass Ball, there are no more tricks in it.
Since the German scientist Hitov discovered cathode rays in 1869, a large number of scientists such as Crookes, Hertz, Lerner, and Thomson have studied cathode rays for more than 20 years. Finally, Joseph John Thomson discovered the existence of electrons. Normally, atoms are uncharged. Since negatively charged electrons that are 1,700 times smaller than their mass can escape from the atom, this shows that there is still structure inside the atom, and it also shows that there are still positively charged things in the atom. It should neutralize the negative charge carried by the electrons, making the atoms neutral.
2. History of model evolution:
1. Neutral atom model
In 1902, German physicist Philipp Edward Anton Lenard (1862- 1947) proposed the neutral particle kinetic submodel. Leonard's early observations showed that cathode rays could pass through the aluminum windows in the vacuum tube and reach the outside of the tube. Based on this observation, he used absorption experiments to prove in 1903 that high-speed cathode rays can pass through thousands of atoms. According to the prevailing semi-materialist views at the time, most of the volume of atoms is empty space, and rigid matter is only about 10% (that is, one hundred thousandth) of its total volume. Leonard imagined that "rigid matter" was a composite of several positive and negative charges scattered in the inner space of atoms.
2. Solid charged ball atomic model
The famous British physicist and inventor Lord Kelvin (1824～1907) was originally named W. Thomson (William Thomson). For his contribution to the first Atlantic submarine cable, the British government knighted him in 1866 and was promoted to Lord Kelvin in 1892, and he began to use the name Kelvin. Kelvin's research range is wide-ranging and he has made contributions in thermal, electromagnetic, fluid mechanics, optics, geophysics, mathematics, engineering applications, etc. He published more than 600 papers in his lifetime and obtained 70 invention patents. He enjoyed a high reputation in the scientific community at that time. Kelvin proposed the solid charged ball atomic model in 1902, which regarded atoms as uniformly positively charged spheres with negatively charged electrons buried in them. Under normal conditions, they are in electrostatic equilibrium. This model was later developed by J.J. Thomson and was later known as the Thomson atomic model.
3. Raisin cake model
Joseph John Thomson (1856-1940) continued to conduct more systematic research and tried to depict the atomic structure. Thomson believed that atoms contain a uniform positive sphere with a number of negative electrons running within this sphere. Following Alfred Mayer's research on the equilibrium of floating magnets, he proved that if the number of electrons does not exceed a certain limit, a ring formed by these running electrons will be stable. If the number of electrons exceeds this limit, it will form two rings, and so on to multiple rings. In this way, the increase in electrons results in periodic structural similarities, and the repeated recurrence of physical and chemical properties in Mendeleev's periodic table may also be explained.
In this model proposed by Thomson, the distribution of electrons in a sphere is a bit like raisins dotted in a piece of cake. Many people call Thomson's atomic model the "raisin cake model." It can not only explain why atoms are electrically neutral and how electrons are distributed in atoms, but also explain the phenomenon of cathode rays and the phenomenon that metals can emit electrons under ultraviolet irradiation.
Moreover, based on this model, the size of the atom can also be estimated to be about 10-8 centimeters, which is a great thing. Because the Thomson model can explain many experimental facts at that time, it is easily accepted by many physicists.
4. Saturn model
Japanese physicist Nagaoka Hantaro (1865-1950) gave an oral presentation at the Tokyo Mathematical Physics Society on December 5, 1903, and in 1904 The paper "Electron Movement in Atoms Explaining Linear and Band Spectroscopy and Radioactive Phenomena" was published in Japanese, British and German magazines respectively. He criticized Thomson's model, believing that positive and negative charges could not penetrate each other, and proposed a structure he called the "Saturn model"-that is, an atomic model with a ring of electrons rotating around a positively charged core. A massive positively charged ball has a circle of equally spaced electrons on the periphery that move in a circle at the same angular velocity. The radial vibration of electrons emits a line spectrum, and the vibration perpendicular to the ring surface emits a band spectrum. The electrons flying out of the ring are beta rays, and the positively charged particles flying out of the central sphere are alpha rays. This Saturn-like model was very influential in his later establishment of the nuclear model of the atom. In 1905, he analyzed experimental results such as the measurement of the charge-to-mass ratio of alpha particles and concluded that alpha particles were helium ions. In 1908, Swiss scientist Leeds proposed the magnetic atom model.
Their models can explain some experimental facts at that time to a certain extent, but they cannot explain many new experimental results that will appear in the future, so they have not been further developed. A few years later, Thomson's "raisin cake model" was overturned by his own student Rutherford.
5. Solar system model
The British physicist Ernest Rutherford (1871-1937) came to the Cavendish Laboratory in the UK in 1895 and followed Thomson studied and became Thomson's first graduate student from overseas. Rutherford was studious and diligent. Under Thomson's guidance, Rutherford discovered alpha rays while doing his first experiment, the radioactive absorption experiment.
In an ingenious experiment designed by Rutherford, he placed uranium, radium and other radioactive elements in a lead container, leaving only a small hole in the lead container. Because lead can block radiation, only a small portion of the radiation emerges from the hole into a very narrow beam of radiation. Rutherford placed a strong magnet near the radiation beam and found that there was a type of ray that was not affected by the magnet and kept traveling in a straight line. The second type of ray is affected by the magnet and deflects to one side, but not too much. The third type of ray is highly deflected.
Rutherford placed materials of different thicknesses in the direction of the radiation and observed how the radiation was absorbed. The first type of ray is not affected by the magnetic field, which means that it is uncharged and has strong penetrating power. Ordinary materials such as paper and wood chips cannot block the progress of the ray. Only relatively thick lead The plate can completely block it, which is called gamma rays. The second type of ray will be affected by the magnetic field and deflected to one side. It can be judged from the direction of the magnetic field that this ray is positively charged. The penetrating power of this ray is very weak, and it can be completely blocked with a piece of paper. This is the alpha ray discovered by Rutherford. The third type of ray is judged to be negatively charged based on the direction of deflection, and has the same properties as fast-moving electrons, and is called beta rays. Rutherford was particularly interested in alpha rays, which he discovered himself. After in-depth and detailed research, he pointed out that alpha rays are streams of positively charged particles, and these particles are ions of helium atoms, that is, helium atoms with two electrons missing.
The "counting tube" was invented by Hans Geiger (1882-1945), a student from Germany, and can be used to measure electrically charged particles that are invisible to the naked eye. When the charged particles pass through the counting tube, the counting tube emits an electrical signal. When this electrical signal is connected to the alarm, the instrument will make a "click" sound and the indicator light will light up. Invisible and intangible rays can be recorded and measured using very simple instruments. This instrument is called a Geiger counter. With the help of Geiger counter tubes, the Manchester laboratory led by Rutherford rapidly developed its research on the properties of alpha particles.
In 1910, E. Marsden (1889-1970) came to the University of Manchester. Rutherford asked him to use alpha particles to bombard gold foil, do practice experiments, and use a fluorescent screen to record those particles that passed through the gold foil. Alpha particles. According to Thomson's raisin cake model, electrons with tiny masses are distributed in a uniformly positively charged substance, and the alpha particle is a helium atom that has lost two electrons, and its mass is thousands of times greater than the electrons. When such a heavy shell hits an atom, the tiny electrons are no match for it. The positive matter in gold atoms is evenly distributed throughout the entire atomic volume, and it is impossible to withstand the bombardment of alpha particles. In other words, alpha particles will easily pass through the gold foil. Even if they are blocked a little, it will only change the direction of the alpha particles slightly after passing through the gold foil. Rutherford and Geiger had done this type of experiment many times, and their observations were in good agreement with Thomson's raisin cake model. The alpha particle changes direction slightly due to the influence of the gold atoms, and its scattering angle is extremely small.
Marsden and Geiger repeated this experiment that had been done many times, and a miracle occurred! They not only observed scattered alpha particles, but also observed alpha particles reflected back by the gold foil. Rutherford described the scene in a speech in his later years. He said: "I remember that two or three days later, Geiger came to me very excited and said: 'We got some alpha particles reflected back... ....', this is the most incredible event in my life. It is as incredible as shooting a 15-inch cannonball at a cigarette paper and being hit by the reflected cannonball. After thinking about it, I realized. This kind of backscattering can only be the result of a single collision. After calculation, I saw that it is impossible to obtain this order of magnitude without considering that most of the atomic mass is concentrated in a very small nucleus." /p>
What Rutherford said "after thinking" was not thinking for one or two days, but thinking for one or two years. After doing a lot of experiments and theoretical calculations and careful consideration, he boldly proposed the nucleated atom model, overturning his teacher Thomson's solid charged ball atom model.
After Rutherford verified that the reflected alpha particles in his student's experiment were indeed alpha particles, he carefully measured the total number of reflected alpha particles. Measurements showed that under their experimental conditions, one alpha particle was reflected back for every eight thousand incident alpha particles. Thomson's solid charged sphere atomic model and the scattering theory of charged particles can only explain the small-angle scattering of α particles, but cannot explain the large-angle scattering. Multiple scattering can produce large-angle scattering, but calculation results show that the probability of multiple scattering is extremely small, which is far from the above-mentioned observation that one out of eight thousand alpha particles is reflected back.
The Thomson atomic model cannot explain alpha particle scattering. After careful calculation and comparison, Rutherford found that only assuming that the positive charges are concentrated in a small area, when alpha particles pass through a single atom, Only then can large-angle scattering occur. That is, the positive charge of the atom must be concentrated in a very small nucleus in the center of the atom. On the basis of this assumption, Rutherford further calculated some laws of alpha scattering and made some inferences. These inferences were soon confirmed by a series of beautiful experiments by Geiger and Marsden.
The atomic model proposed by Rutherford is like a solar system. The positively charged atomic nucleus is like the sun, and the negatively charged electrons are like planets orbiting the sun. In this "solar system", the force that governs them is the electromagnetic interaction force. He explained that the positively charged matter in the atom is concentrated in a very small core, and most of the atomic mass is also concentrated in this very small core. When an alpha particle is shot directly at the core of an atom, it is likely to bounce back. This satisfactorily explains the large-angle scattering of alpha particles. Rutherford published a famous paper "Scattering of alpha and beta particles by matter and its principle structure".
Rutherford's theory opened up new ways to study atomic structure and made immortal contributions to the development of atomic science. However, for a long time at that time, Rutherford's theory was ignored by physicists.
The fatal weakness of Rutherford's atomic model is that the electric field force between positive and negative charges cannot meet the stability requirements, that is, it cannot explain how electrons stay stably outside the nucleus. The Saturn model proposed by Nagaoka Hantaro in 1904 failed because it could not overcome the difficulty of stability. Therefore, when Rutherford proposed the nuclear atom model, many scientists regarded it as a conjecture or just one of various models, while ignoring the solid foundation on which Rutherford proposed the model. Experimental basis.
Rutherford had extraordinary insight and was often able to grasp the essence and make scientific predictions. At the same time, he has a very rigorous scientific attitude and makes the conclusions that should be made based on experimental facts. Rutherford believes that the model he proposed is still imperfect and needs further research and development. He stated at the beginning of the paper: "At this stage, it is not necessary to consider the stability of the atoms mentioned, because it will obviously depend on the fine structure of the atoms and the movement of the charged components." He also wrote to a friend that year Said: "I hope that within one or two years we can have some clearer insights into the structure of the atom."
6. Bohr model
Rutherford's theory attracted a A young man from Denmark, his name is Niels Bohr (1885-1962). Based on the Rutherford model, he proposed the quantized orbit of electrons outside the nucleus and solved the problem of the stability of the atomic structure. sexual problem, and sketched out a complete and convincing theory of atomic structure.
Bohr was born in a family of professors in Copenhagen and received a doctorate from the University of Copenhagen in 1911. He studied in Rutherford's laboratory from March to July 1912, during which his atomic theory was conceived. Bohr first extended Planck's quantum hypothesis to the energy inside the atom to solve the stability difficulties of the Rutherford atomic model. It was assumed that an atom can only change its energy through discrete energy quantons, that is, an atom can only It is in discrete stationary states, and the lowest stationary state is the normal state of the atom. Then, inspired by his friend Hansen, he reached the concept of stationary transition from the combination law of spectral lines. He published three parts of a long paper "On Atomic Structure and Molecular Structure" in July, September and November 1913.
Bohr's atomic theory gives such an atomic image: electrons move in a circle around the nucleus in some specific possible orbits, and the farther away from the nucleus, the higher the energy; the possible orbits are determined by the angular momentum of the electron. It is determined by an integer multiple of h/2π; when the electron moves in these possible orbits, the atom does not emit or absorb energy. Only when the electron jumps from one orbit to another orbit does the atom emit or absorb energy, and emit or absorb energy. The radiation is single frequency, and the relationship between the frequency and energy of the radiation is given by E=hν. Bohr's theory successfully explained the stability of atoms and the regularity of the spectral lines of hydrogen atoms.
Bohr's theory greatly expanded the influence of quantum theory and accelerated the development of quantum theory. In 1915, German physicist Arnold Sommerfeld (1868-1951) extended Bohr's atomic theory to include elliptical orbits, and considered the effect of special relativity in which the mass of electrons changes with their speed, and derived the fine details of the spectrum. The structure is consistent with the experiment.
In 1916, Albert Einstein (1879-1955) started from Bohr’s atomic theory and used statistical methods to analyze the process of material absorbing and emitting radiation, and derived Planck’s radiation law. . This work of Einstein synthesized the achievements of the first stage of quantum theory and combined the work of Planck, Einstein and Bohr into a whole.
7. Nuclear model
There are more than a dozen Nobel Prize winners among Rutherford’s students, the famous ones are Bohr, Chadwick, and Cockrov Te, Kapitsa, Hahn, etc. After the discovery of the atomic nucleus, Rutherford used alpha rays to bombard the nitrogen nucleus in 1919, achieving "alchemy" and the first nuclear reaction in human history. From now on, the elements are no longer eternal things.
Through a series of nuclear reactions, Rutherford discovered that protons, that is, hydrogen ions, are the components of all atomic nuclei, and predicted the neutron. The neutron was later discovered by his student Chadwick, and finally established the basis of protons and neutrons. Basic nuclear structure model. After the Pauli exclusion principle was established, the periodic law of elements was also explained. Rutherford later became known as the father of nuclear physics. Of course, while Britain was in a state of turmoil, don't forget the Curies in France, because the atomic bombs required for Rutherford's series of discoveries were alpha particles emitted by radioactive elements (especially radium). At this time, the Curie Laboratory was established in France. Curie died in a car accident. Marie won another Nobel Prize in Chemistry for her achievements in radioactivity. The famous book "General Theory of Radioactivity" was handed down. After the Curie Laboratory, the younger Curies and his wife: Hosted by Joliot Curie and Hélène Curie, the event is also full of talents and is not inferior to the three major holy places. The Curies were a bit unlucky. Chadwick discovered the neutron first, Anderson discovered the positron first, and Hahn discovered nuclear fission first. The opportunity was fleeting. But in the end he won the Nobel Prize for the discovery of artificial radioactivity. Today, there are thousands of radioactive isotopes, most of which are artificially produced, thanks to the younger and younger Curies.
The nuclear model was experimentally successful, but it was in serious conflict with the basic theory at the time. According to classical electrodynamics, due to the circular motion of electrons, they will definitely radiate electromagnetic waves. Due to the loss of energy, they will fall into the nucleus within 1 ns and emit a continuous spectrum at the same time. In other words, theoretically there is no such thing as an atom. But atoms do exist and are stable, emitting linear spectra, supported by a large number of experimental facts and chemistry as a whole. In 1911, a 26-year-old Danish young man came to Cambridge and later transferred to the Rutherford Laboratory in Manchester, where he learned about the amazing discovery of the atomic nucleus. Eventually, he found a fundamental modification to the nucleated model that could both account for the stability of the atom and calculate its radius. He is Niels Bohr, who is as famous as Einstein.
In 1885, Balmer, a Swiss mathematics teacher, discovered an empirical formula for the visible spectrum of hydrogen atoms, which was later promoted as the Rydberg formula by the Swedish physicist Ridberg. In 1900, German physicist Max Planck proposed the concept of energy quantization and explained the blackbody radiation spectrum. In 1905, Einstein proposed the concept of light quanta. These conclusions inspired Bohr greatly. Under these inspirations, Bohr applied the concept of quantization to the atomic model in 1913 and proposed Bohr's hydrogen atom model. The key to this model are three assumptions introduced by Bohr. Stationary assumption: Electrons can only move in some discrete orbits and do not radiate electromagnetic waves. Frequency condition assumption: The energy level difference is the same as the photon energy absorbed (or emitted) by the atom. Angular momentum quantization hypothesis: The angular momentum of an electron is an integer multiple of approximately Planck's constant. Through a series of derivation, the mystery of the hydrogen spectrum gradually emerged and achieved great success. Bohr won the 1922 Nobel Prize. Although Bohr's model seems to be relatively rough now, its significance does not lie in the model itself, but in the concepts introduced when establishing the model: stationary state, energy levels, transitions, etc. Bohr introduced the correspondence principle to reconcile the conflict between the hydrogen atom model and classical mechanics. After Bohr's success, he rejected the invitation of his mentor Rutherford, returned to his motherland, and established an institute in Copenhagen (later renamed the Bohr Institute). The Bohr Institute attracted a large number of outstanding young physicists from all over the world. Scientists, including Heisenberg, Pauli and Dirac, the founders of quantum theory, formed a strong academic atmosphere. At this time, Copenhagen began to explore basic physical laws.