While he was casting his eyes left and right, observing the list of chemical elements and their atomic weights, he noticed something interesting: some similar characteristics appeared to be repeated in the elements at regular numerical intervals, so a certain pattern appeared, but it quickly disappeared. It seemed that there was something behind this chemical puzzle, but what was it?
The Russian chemist Dmitri Mendeleev was certain that he was on the verge of a great achievement. There must be an unusual pattern linking these elements that make up all the matter around him. It had been a long day until he felt exhausted from thinking too much, so he finally gave up and rested his shaggy head on his arms and fell asleep, and only then did he see a dream that turned the scales around.
It was the dream that changed the course of the science of chemistry and turned it upside down. It opened new horizons for seeing the world around us, and paved the way for the discovery of more chemical elements and even their prediction. Mendeleev himself wrote , commenting on the dream he saw in his memoirs, saying: “I saw in a dream a table All the elements fall into their correct places as they should, and as soon as I woke up I immediately wrote down on a piece of paper what I had seen.”
Mendeleev was guided to see the correct pattern that connects the elements and places them in a precise table to draw the first features of what the periodic table is today. This table has become the basic foundation for students of chemistry, which summarizes an entire science in approximately 100 squares containing symbols and numbers.
The beginning of March of each year coincides with the anniversary of the founding of the periodic table, which changed the features of chemistry forever, and opened new horizons that contributed to the establishment of the periodic law, which stipulates the existence of close family ties (groups) between known chemical elements, so similar characteristics appear in each family ( group) at regular intervals when arranged according to their atomic weights.
The periodic table also paved the way for predicting more chemical elements that were difficult for scientists to access through the traditional, direct method. Mendeleev notes that before the discovery of the periodic law, chemical elements were merely incidental facts present in nature.
Introduction before Mendeleev's dream
About 155 years after his famous dream, legend says that Mendeleev was able to create the periodic table in one day, on March 1, 1869 to be precise. But in fact, this table was the result of many years' work, and the contributions of other chemists in the past decades cannot be overlooked in any way.
What records state is that the German chemist Johann Dobreiner was one of the first to recognize these common properties in groups of chemical elements in 1817, but at that time chemists were not yet well informed and understanding of the nature of the atom, after the English scientist John Dalton attempted in 1808 to establish Atomic theory and its explanation to the scientific community at that time.
Dalton proposed that chemical reactions occur when atoms separate or fuse, creating new substances, and he proposed that each element consists entirely of a single type of atom distinguished from others by its weight. For example, oxygen is eight times heavier than hydrogen atoms, and Dalton believed that carbon atoms are six times heavier than hydrogen. When these elements combine to form new substances, the quantities that have reacted can be calculated by knowing the atomic weights.
Perhaps Dalton was wrong about some of the weights, as oxygen is 16 times heavier than hydrogen, and carbon is 12 times heavier than hydrogen. In any case, the first introductions to atomic theory were an important motivation for chemists in the following decades to examine atomic weights more closely. Doberiner noticed that certain combinations of the three elements - which he called triads - have an interesting relationship. For example, the element bromine had an atomic weight located in the middle between that of chlorine and iodine, and these three elements show similar chemical behavior, and the same applies to the elements lithium, sodium, and potassium as well.
Other chemists realized that there were close connections between atomic weights and chemical properties, but due to the failure and limitations of techniques at the time, they were unable to adequately measure the atomic weights of elements.
Until the 1860s, distinctive names shone in the skies of Europe. In England, chemist John Newlands noticed that the arrangement of the known elements - which amounted to 56 elements at that time - could be achieved into 8 groups, and they were called “Newlands octets.”
With missing links, the previous arrangement did not last long, which later led Mendeleev to realize that the relationship between the properties of elements and atomic weights was more complex than he initially thought.
Putting the elements in perspective
At the beginning of his professional journey, Mendeleev moved around Europe to work in the most prominent university chemistry laboratories before settling and ending up in the city of Saint Petersburg in western Russia. At that time, he was good at writing, so he wrote a book on organic chemistry in order to receive a large cash prize. Then he worked as an editor, translator, and consultant in many chemical industries, and then finally returned to the educational field to obtain a doctorate and become a professor at the prestigious University of Saint Petersburg.
At the beginning of his educational career, Mendeleev found himself forced to teach a new field to him, which was inorganic chemistry, so he devoted most of his time to mastering this field, but he was not satisfied with the educational curriculum and university books available at that time. So he decided to write an entire curriculum himself, and here was the first spark; Organizing texts necessarily requires arranging chemical elements, so the question that came to mind was: What is the best way to arrange these elements?
By 1869 Mendeleev had made good progress in realizing that some groups of similar elements show regular increases in atomic weights, and other elements with approximately equal atomic weights share common properties. It seemed that arranging the elements according to their atomic weight was the key to their classification.
According to Mendeleev, he began his work by organizing his thoughts by writing down each property of the known elements, which numbered 63 at the time, on small note cards. Then arrange these cards in vertical arrays according to their atomic weights. From lowest to highest, he grouped elements with similar characteristics into horizontal rows. By then, Mendeleev's schedule had appeared, and he quickly sent it to the printing press and included it in his book to be published. Meanwhile, he began working on writing a scientific paper to submit to the Russian Chemical Society.
He stated in his paper that the elements arranged according to their atomic weights show clear periodic properties, and all of his conclusions point to one thing, which is that the atomic weight is the determinant of the nature and arrangement of the elements.
Ironically, the German chemist Luther Mayer was also working on similar and close work, but Mendeleev preceded him in publishing his schedule.
What distinguishes the periodic table that Mendeleev came up with is that it left empty spaces for new, undiscovered elements, which was actually achieved later. During the remainder of Mendeleev's life, scientists discovered three new chemical elements: gallium, scandium, and germanium.
In fact, Mendeleev not only predicted these elements, but also correctly described their properties in detail. For example, he said that gallium, which was discovered in 1875, had an atomic weight of 69.9, which matched experimental calculations.
Mendeleev's table has become an inspiration for all chemists, as it is like a great mosaic painting that contains the secrets of the universe, and that every piece is in the right place even before it is found. These correct and successful predictions earned him legendary status in scientific circles.
The periodic table organizes elements based on their atomic number, electron configuration, and recurring chemical properties. The rows of the table are usually called periods and the columns are called groups.
A deep mathematical map describing nature
In general, scientific predictions are made on a mathematical basis and on precise mathematical equations. For example, Einstein’s equations of relativity predict black holes and the curvature of the universe, which is proven by experiments, as well as the expansion of the universe, the existence of dark matter, and other scientific facts that were built on mathematical equations before they were discovered and verified. Of which.
However, interestingly, Mendeleev's predictions about the new chemical elements through his periodic table were not based on any innovative mathematical equations. Rather, the matter was much greater than that. What he achieved - without realizing it - was to draw a deep mathematical map of nature, because his table reflects the contents of the laws of quantum mechanics and the mathematical rules that govern atomic structure.
Mendeleev stated that differences within the atoms themselves could explain the recurring properties of the elements, but he did not investigate this matter, and merely pointed out the importance of atomic theory in relation to his periodic table.
In the midst of trying to uncover the secrets of the periodic table, in 1888 specifically, the German chemist Johannes Weslesnos arrived at an exciting fact: that the pattern of properties of elements arranged according to their atomic weight indicates that atoms are composed of smaller particles arranged regularly. This means that Mendeleev's table predicted the idea of the complex internal structure of atoms, and this was an additional motivation for chemists and physicists alike to understand the nature of the internal structure of the atom itself.
By the time of Mendeleev's death in 1907, scientists had identified some components of the atom, such as electrons that carried a negative electrical charge, in addition to some other positively charged component to make the atom electrically neutral.
The main clue to how these parts are arranged came in 1911, when physicist Ernest Rutherford discovered the existence of the nucleus inside the atom. Shortly thereafter, physicist Henry Moseley concluded that the amount of positive charge in the nucleus determines the correct arrangement of the elements in the periodic table, and the amount of positive charge in the nucleus is equal to the number of protons the atom contains, or its atomic number.
The atomic weight was closely related to the atomic number, which was recently discovered, to the point that the arrangement of elements according to their atomic weights differs only slightly from their arrangement according to their atomic numbers. Before his death, Mendeleev hinted that some of the atomic weights used at that time were incorrect and needed to be recalculated, and he was proven right in some cases. The discovered atomic number contributed to the effective rearrangement and organization of elements in the periodic table.
In approximately the same period, the Danish physicist Niels Bohr realized that the behavior of the electrons revolving around the nucleus of the atom is governed by quantum theory. He also realized that the outer electrons located in the outer shell far from the atom, known as “valence electrons,” play a decisive role in determining the chemical properties of the element. This insight was important because it contributed to understanding atomic structure and how elements interact with each other chemically.
Bohr also noticed that the outermost electrons of atoms had similar arrangements and were repeated periodically, and this helped him explain the specific patterns revealed by Mendeleev's periodic table. In 1922, Bohr developed his own version of the periodic table based on experimental measurements of electron energies, in addition to some properties of the periodic law.
Bohr's periodic table largely agreed with Mendeleev's arrangement, which reflects how precisely Mendeleev's initial table conformed to the laws of quantum and atomic physics without even having direct knowledge of them.
Bohr's periodic table was one of many versions that followed Mendeleev's original arrangement of the periodic table. Over the years, several periodic tables were introduced until it reached the final version that we see today, which is characterized by a horizontal arrangement, in contrast to Mendeleev's original vertical arrangement. The current arrangement is largely due to the efforts of American chemist Glenn Seaborg, who played an important role in its development.
The contribution of Seaborg and his colleagues included the creation of several new elements with atomic numbers higher than the atomic number of uranium, which was the last known natural element at that time. The new elements are called “transuranic elements,” and required Seaborg to create a new row in the periodic table, a development that Mendeleev had not previously predicted.
Seaborg included the new row of transuranic elements, placing them below a similar row of rare earth elements that included dysprosium, erbium, europium, and others. In an interview with Sporg in 1997, he pointed out that it would take a lot of courage and audacity to do something different from Mendeleev's table, even though the two tables were identical in many basic features.
Based on Seaborg’s contributions to the development of the periodic table, he was honored by naming one of the new discovered elements after him, which is the element seaborgium, while the element mendelevium, which Seaborg and his colleagues discovered in 1955, was named in memory of the Russian scientist who established the first features of the periodic table that we see today.