With an atom through the ages - part 3
Content
Rutherford's planetary model of the atom was closer to reality than Thomson's "raisin pudding." However, the life of this concept lasted only two years, but before talking about a successor, it's time to unravel the next atomic secrets.
1. Hydrogen isotopes: stable prot and deuterium and radioactive tritium (photo: BruceBlaus/Wikimedia Commons).
nuclear avalanche
The discovery of the phenomenon of radioactivity, which marked the beginning of unraveling the mysteries of the atom, initially threatened the basis of chemistry - the law of periodicity. In a short time, several dozen radioactive substances were identified. Some of them had the same chemical properties, despite the different atomic mass, while others, with the same masses, had different properties. Moreover, in the area of the periodic table where they should have been placed due to their weight, there was not enough free space to accommodate them all. The periodic table was lost due to an avalanche of discoveries.
2. Replica of J.J. Thompson's 1911 mass spectrometer (photo: Jeff Dahl/Wikimedia Commons)
Atomic nucleus
This is 10-100 thousand. times smaller than the entire atom. If the nucleus of a hydrogen atom were to be enlarged to the size of a ball with a diameter of 1 cm and placed in the center of a football field, then an electron (smaller than a pinhead) would be in the vicinity of a goal (over 50 m).
Almost the entire mass of an atom is concentrated in the nucleus, for example, for gold it is almost 99,98%. Imagine a cube of this metal weighing 19,3 tons. Everything nuclei of atoms gold have a total volume of less than 1/1000 mm3 (a ball with a diameter of less than 0,1 mm). Therefore, the atom is terribly empty. Readers must calculate the density of the base material.
The solution to this problem was found in 1910 by Frederick Soddy. He introduced the concept of isotopes, i.e. varieties of the same element that differ in their atomic mass (1). Thus, he called into question another postulate of Dalton - from that moment on, a chemical element should no longer consist of atoms of the same mass. The isotopic hypothesis, after experimental confirmation (mass spectrograph, 1911), also made it possible to explain the fractional values of the atomic masses of some elements - most of them are mixtures of many isotopes, and atomic mass is the weighted average of the masses of all of them (2).
Kernel Components
Another of Rutherford's students, Henry Moseley, studied X-rays emitted by known elements in 1913. Unlike complex optical spectra, the X-ray spectrum is very simple - each element emits only two wavelengths, the wavelengths of which are easily correlated with the charge of its atomic nucleus.
3. One of the X-ray machines used by Moseley (photo: Magnus Manske/Wikimedia Commons)
This made it possible for the first time to present the real number of existing elements, as well as to determine how many of them are still not enough to fill the gaps in the periodic table (3).
A particle carrying a positive charge is called a proton (Greek proton = first). Another problem immediately arose. The mass of a proton is approximately equal to 1 unit. Whereas atomic nucleus sodium with a charge of 11 units has a mass of 23 units? The same, of course, is the case with other elements. This means that there must be other particles present in the nucleus and not having a charge. Initially, physicists assumed that these were strongly bound protons with electrons, but in the end it was proved that a new particle appeared - the neutron (Latin neuter = neutral). The discovery of this elementary particle (the so-called basic "bricks" that make up all matter) was made in 1932 by the English physicist James Chadwick.
Protons and neutrons can turn into each other. Physicists speculate that they are forms of a particle called a nucleon (Latin nucleus = nucleus).
Since the nucleus of the simplest isotope of hydrogen is a proton, it can be seen that William Prout in his "hydrogen" hypothesis atom construction he was not too wrong (see: “With the atom through the ages - part 2”; “Young Technician” No. 8/2015). Initially, there were even fluctuations between the names proton and "proton".
4. Photocells at the finish - the basis of their work is the photoelectric effect (photo: Ies / Wikimedia Commons)
Not everything is allowed
Rutherford's model at the time of its appearance had a "congenital defect". According to Maxwell's laws of electrodynamics (confirmed by radio broadcasting already functioning at that time), an electron moving in a circle should radiate an electromagnetic wave.
Thus, it loses energy, as a result of which it falls on the nucleus. Under normal conditions, atoms do not radiate (spectra are formed when heated to high temperatures) and atomic catastrophes are not observed (the estimated lifetime of an electron is less than one millionth of a second).
Rutherford's model explained the result of the particle scattering experiment, but still did not correspond to reality.
In 1913, people "got used" to the fact that energy in the microcosm is taken and sent not in any quantity, but in portions, called quanta. On this basis, Max Planck explained the nature of the spectra of radiation emitted by heated bodies (1900), and Albert Einstein (1905) explained the secrets of the photoelectric effect, i.e., the emission of electrons by illuminated metals (4).
5. Diffraction image of electrons on a tantalum oxide crystal shows its symmetrical structure (photo: Sven.hovmoeller/Wikimedia Commons)
28-year-old Danish physicist Niels Bohr improved Rutherford's model of the atom. He suggested that electrons move only in orbits that meet certain energy conditions. In addition, electrons do not emit radiation as they move, and energy is only absorbed and emitted when shunted between orbits. The assumptions contradicted classical physics, but the results obtained on their basis (the size of the hydrogen atom and the length of the lines of its spectrum) turned out to be consistent with the experiment. new born model of the atom.
Unfortunately, the results were valid only for the hydrogen atom (but did not explain all the spectral observations). For other elements, the calculation results did not correspond to reality. Thus, physicists did not yet have a theoretical model of the atom.
Mysteries began to clear up after eleven years. The doctoral dissertation of the French physicist Ludwik de Broglie dealt with the wave properties of material particles. It has already been proven that light, in addition to the typical characteristics of a wave (diffraction, refraction), also behaves like a collection of particles - photons (for example, elastic collisions with electrons). But mass objects? The suggestion seemed like a pipe dream for a prince who wanted to become a physicist. However, in 1927 an experiment was carried out that confirmed de Broglie's hypothesis - the electron beam diffracted on a metal crystal (5).
Where did atoms come from?
Like everyone else: Big Bang. Physicists believe that literally in a fraction of a second from the "zero point" protons, neutrons and electrons, that is, the constituent atoms, were formed. A few minutes later (when the universe cooled and the density of matter decreased), the nucleons merged together, forming the nuclei of elements other than hydrogen. The largest amount of helium was formed, as well as traces of the following three elements. Only after 100 XNUMX For many years, conditions allowed electrons to bind to nuclei - the first atoms were formed. I had to wait a long time for the next one. Random fluctuations in density caused the formation of densities, which, as they appeared, attracted more and more matter. Soon, in the darkness of the universe, the first stars flared up.
After about a billion years, some of them began to die. In their course they produced nuclei of atoms down to iron. Now, when they died, they spread them throughout the region, and new stars were born from the ashes. The most massive of them had a spectacular end. During supernova explosions, the nuclei were bombarded with so many particles that even the heaviest elements were formed. They formed new stars, planets, and on some globes - life.
The existence of matter waves has been proven. On the other hand, an electron in an atom was considered as a standing wave, due to which it does not radiate energy. The wave properties of moving electrons were used to create electron microscopes, which made it possible to see atoms for the first time (6). In subsequent years, the work of Werner Heisenberg and Erwin Schrödinger (on the basis of the de Broglie hypothesis) made it possible to develop a new model of the electron shells of the atom, completely based on experience. But these are questions beyond the scope of the article.
The dream of the alchemists came true
Natural radioactive transformations, in which new elements are formed, have been known since the end of the 1919th century. In XNUMX, something that only nature has been capable of until now. Ernest Rutherford during this period was engaged in the interaction of particles with matter. During the tests, he noticed that the protons appeared as a result of irradiation with nitrogen gas.
The only explanation for the phenomenon was the reaction between helium nuclei (a particle and the nucleus of an isotope of this element) and nitrogen (7). As a result, oxygen and hydrogen are formed (a proton is the nucleus of the lightest isotope). The alchemists' dream of transmutation has come true. In the following decades, elements were produced that are not found in nature.
Natural radioactive preparations emitting a-particles were no longer suitable for this purpose (the Coulomb barrier of heavy nuclei is too large for a light particle to approach them). The accelerators, imparting enormous energy to the nuclei of heavy isotopes, turned out to be "alchemical furnaces" in which the ancestors of today's chemists tried to obtain the "king of metals" (8).
Actually, what about gold? Alchemists most often used mercury as a raw material for its production. It must be admitted that in this case they had a real “nose”. It was from mercury treated with neutrons in a nuclear reactor that artificial gold was first obtained. The metal piece was shown in 1955 at the Geneva Atomic Conference.
Fig. 6. Atoms on the surface of gold, visible in the image in a scanning tunneling microscope.
7. Scheme of the first human transmutation of the elements
The news of the achievement of physicists even caused a short stir on the world stock exchanges, but the sensational press reports were refuted by information about the price of the ore mined in this way - it is many times more expensive than natural gold. Reactors will not replace the precious metal mine. But the isotopes and artificial elements produced in them (for the purposes of medicine, energy, scientific research) are much more valuable than gold.
8. Historic cyclotron synthesizing the first few elements after uranium in the periodic table (Lawrence Radiation Laboratory, University of California, Berkeley, August 1939)
For readers who would like to explore the issues raised in the text, I recommend a series of articles by Mr. Tomasz Sowiński. Appeared in "Young Technics" in 2006-2010 (under the heading "How they discovered"). The texts are also available on the author's website at: .
Cycle "With an atom for ages» He began with a reminder that the past century was often called the age of the atom. Of course, one cannot fail to note the fundamental achievements of physicists and chemists of the XNUMXth century in the structure of matter. However, in recent years, knowledge about the microcosm is expanding faster and faster, technologies are being developed that allow manipulating individual atoms and molecules. This gives us the right to say that the real age of the atom has not yet arrived.
