The concept of the element in chemistry

Under the guidance of Aristotle, ancient Greek philosophers believed that the substances that exist in various forms and make up the world around us can be summarized by four basic substances, namely earth, fire, air, and water. This simple notion was revered as fact until the 17th century when chemistry derived from alchemy led to the discovery of other elements. "Earth" is not a single substance, and "qi" is not only composed of one gas. During the 18th century, the elemental lineage experienced dramatic growth. At that time, many new metals were discovered, such as cobalt, nickel, manganese, tungsten, chromium, magnesium, uranium, and new gases, such as hydrogen, nitrogen, oxygen, and chlorine, were isolated for the first time.

The concept of "element" in chemistry was first proposed by Robert Boyle: a substance that cannot be further decomposed by physical processes is an element. Afterward, the French chemist Antoine-Laurent de Lavoisier]formally named the elements in 1789. Lavoisier selected 33 substances, defined elemental states for them, and divided them into four groups: metals, nonmetals, earth, and gases. Later, it was discovered that some of these elements were actually compounds, and others, such as heat and light, were not even chemicals. The following is Lavoisier's table of elements, only the elements marked in red are still considered chemical elements today.

• Air: heat, light, hydrogen, nitrogen, oxygen.

• Soil: alumina, barite, lime, magnesia, silica.

• Metals: antimony, arsenic, bismuth, cobalt, copper, gold, iron, lead, manganese, mercury, molybdenum, nickel, platinum, silver, tin, tungsten, zinc.

• Nonmetals: sulfur, phosphorus, carbon, chlorides, fluorides, and borates.

The "soil" groups here are actually oxides. For example, lime is calcium oxide, and silica is silicon dioxide. However, under the conditions at the time, Lavoisier could not extract oxygen atoms from compounds and independently recognize related elements. . Other pseudoelements are all in the "non-metal" group. Similarly, Lavoisier could not separate these elements into individual elements such as chlorine, fluorine, and boron with the technology of the time. Lavoisier became an enemy of Jean-Paul Marat during the French Revolution and was eventually guillotined during the Reign of Terror in 1794 for his involvement in a state tax scandal. "The Republic does not need geniuses," the judge declared. But just 18 months later, the revolutionary government changed its tune and said that Lavoisier had in fact been wronged.

Then a science teacher from Manchester, England, took this research one step further. In 1805, John Dalton submitted a paper to the Manchester Literary and Philosophical Society explaining the various ways in which the elements could be combined with each other and how the basic components came to be of different weights. At the time, most chemists thought atoms were too small to study. But Dalton was more adventurous, proposing a table of 20 elements and their weights, with symbols that showed how they combined. The matter is represented by pictures, showing patterns of its basic constituent elements. From this table of elements, more compounds can be derived: compound 21 is water, described as HO; compound 22 is ammonia, described as NH. These manifestations were the embryonic form of the chemical equations we know today.

However, Dalton's notation is too complex to use. The chemical symbols we use today come from the Swiss chemist Jöns Berzelius, who was also an admirer of Dalton. Berzelius labeled elements simply by the first letter of their name (sometimes Latin, sometimes even Arabic, such as potassium), or in cases prone to ambiguity, by two letters, such as C for carbon (carbon), while Co stands for cobalt (cobalt). Bundle these symbols together and you can represent chemical compounds, such as H2O. After 1835, this notation was adopted on a large scale and was eventually used in equations representing chemical reactions, such as:

CuSO 4 + 2HCl → H 2 SO 4 + CuCl 2

Dalton was stunned by the new complexity generated by the language of chemistry. After seeing this new scheme, he said

I am afraid that a young chemistry student will have to learn Hebrew.

Tin originally proposed the earth group elements decomposed into real elements. In 1863 there were already more than 60 elements. Is there a limit to the "big bang" of elements?

John Dalton's 20 elements and their weights in 1805

At the time, this was indeed a fascinating question. If there is a limit, what is the number of elements? What factors can really determine this limit?

In the 19th century, many valiant attempts were made to classify elements in terms of weight, properties, etc. All the best chemists of the time would have built a similar system. But, without exception, they were defeated by a Russian chemistry professor from Siberia.

Dmitri Ivanovich Mendeleev was born in 1822 in Topolsk, Siberia, to the headmaster of a local elementary school. He had 13 brothers and sisters. Mendeleev's mother was convinced that her son had special talents and deserved all possible quality education, so she sent her son to school in St. Petersburg. She is right. During the university, Mendeleev's academic performance has always been among the best. He then went to work in France and then to Heidelberg, Germany, as an assistant to the then-rising German chemist Robert Bunsen. Finally, in 1867, Mendeleev returned to St. Petersburg, where he became a professor of chemistry at the university.

One spring day in 1867, Mendeleev was stuck at home due to bad weather and had to take the opportunity to continue working on a new textbook called Principles of Chemistry. He doesn't know how to display and arrange the proliferating number of elements and their properties. So, he wrote the name of each element on a card, along with some properties of the corresponding element, as well as oxides and hydrides. Then he started arranging the cards in various ways, trying to find a pattern: elements with the same valence in the horizontal row, and elements in descending order of atomic weight in the vertical row. Suddenly, he discovered a very distinctive arrangement. He wrote down the results on the back of an old envelope, which can still be seen today in St. Petersburg.

The original periodic table sketched by Mendeleev in 1867

Next, Mendeleev invented a more succinct version. He arranged the first seven elements from lithium to fluorine horizontally in the order of increasing atomic weight and then arranged the seven elements from sodium to chlorine in the same way. Thus, periodicity appears in a column, two elements with similar chemical properties are next to each other. In the 7-column entry, the main valence of the first column element is 1, and the main valence of the next column element is 2, and then respectively It's 3, 4, 3, 1. Next, Mendeleev quickly discovered that the table would be clearer if it was turned over, swapping rows and columns. We can also recognize this result now, although many new elements have been filled in today's table.

The element table has a total of 8 columns or 8 periods. In a larger refinement in 1870, Mendeleev assigned the 63 known elements into 12 rows, starting with hydrogen and ending with uranium, with each element placed in a chemically similar column, and sorted in ascending order of atomic weight.

Mendeleev's tables show results that make an intuitively significant contribution to predicting the existence of new elements. He didn't put all the known elements in a complete periodic table like everyone else did. If it were Aristotle, he would definitely do it. Mendeleev believed that if the periodic table had a logical structure, it meant that there might be gaps in the table. He speculated that new elements would fill in these gaps and that the periodicity of the table could be used to predict atomic weights and atomic densities. Under boron, aluminum, and silicon, he deduced three "undiscovered" elements and named them "boron-like", "aluminum-like" and "silicon-like". These three elements were discovered one after another, and their atomic weight and density were consistent with Mendeleev's prediction: "aluminum-like" was discovered in Paris, France in 1875, called gallium (Gallium, France in Latin); The "boron-like" was discovered in Uppsala, Sweden in 1879, called scandium (scandium in Latin); the "silicon-like" was discovered in Freiberg, Germany in 1886, Known as germanium (germanium, Germany in Latin).

Mendeleev also predicted a new member of the fourth group (titanium) with an atomic weight of around 180. The element was finally discovered in 1923 at the University of Copenhagen, Denmark, with an atomic weight of 178.5 and named hafnium (Copenhagen in Latin).

In 1893 Mendeleev became director of the Russian Metrology Bureau and made admirable contributions. He formally defined the composition of vodka: one molecule of alcohol plus two molecules of water. The molecular weight reveals that the composition of vodka is 38% alcohol and 62% water. In 1894, the legal standards issued by the Russian Metrology Bureau slightly adjusted this figure to 40% alcohol and 60% water. This is 80% alcohol proof (1 proof is equal to 2 times the volume of alcohol).

Gerald Holton once had this wonderful comparison of Mendeleev's achievement and its great influence on his contemporaries: "It's like a librarian putting all the books Put them in a pile, weigh them one by one, and arrange the books on different shelves in ascending order of weight. Then, he suddenly found that the first book on each shelf was about art, the second book was about philosophy, the third is about science, the fourth was about economics...etc. Our librarian may not understand the inner workings of these rules, but once we find that the order of books on one of the shelves is Art-Science-Economics, he would leave a gap between the books of art and science, and start looking for the missing philosophy book of the right weight.

We can see the periodicity of the element table from a property of the element, such as dividing the atomic volume by the atomic weight. This was first discovered by Julius Meyer in 1870. Alkali metals appear at the top of the chart.

Mendeleev never claimed to understand the structure and periodicity of the table. It was a great leap of intuition. He believed that these elements possessed an inherently symmetrical structure, but never imagined that his tables were also a convenient retrieval tool, which eventually led him to make dramatic discoveries and predictions. Although Mendeleev failed to discover the pattern of the elements before him, he knew that this table would help others to do so.

The modern version of this periodic table is divided into 7 rows (periods), and each row places 2, 8, 8, 18, 18, 32, 32 elements respectively. This pattern became understandable after the quantum theory of the atom was discovered. The quantum wave-oriented nature of electrons means that only integer multiples of the wavelength can allow electrons to be "loaded" in surrounding orbits. The increase in the number of elements in each row of the periodic table reflects the increasing number of electrons in orbitals around the nucleus of each atom. Quantum mechanics allows the innermost orbital (called the shell) to contain two electrons, then six, then 10, and then 14.

In the resulting periodic table, the number of elements in each row is the number of electrons if the orbitals were filled, so 8 = 2 + 6, 18 = 2 + 6 + 10, 32 = 2 + 6 +10+14. The elements in each row are arranged in ascending order according to atomic number, and in each column, the elements are arranged according to the number of electrons in the same outermost shell, thus resulting in the modern form of the periodic table. In each row, we regularly add electrons to the orbital until it is full, and what we end up with is the noble (that is, inert) gas on the far right of the periodic table. Then, we open the next row and fill the next level of the track. Remarkably, Mendeleev found this pattern long before the electron and proton were discovered. He studied atomic weight (determined by the number of protons in the nucleus of an element) and valence (determined by the integrity of electrons in orbitals), and used this simple method to find the essence of these two chemical properties.

Today, Mendeleev's periodic table appears on the wall of every chemical laboratory in the world. It seems that his mother's decision back then was right.