Vol. 12, no. 1 (2020)

Man has always striven to know the world around him, nature and the entire universe. But whereas for the ancient Mesopotamian and Egyptian civilisations knowledge was guaranteed by the deities, the man of seventh-century-BC Greece began systematically to observe the physical, plant and animal world in search of a rational explanation of the principle that had generated it. At the beginning of this evolution, which would lead to the framing of the scientific method, we find Thales of Miletus (624-546 BC), remembered as one of the seven sages of the archaic age and whom Aristotle regards as “the initiator of the philosophy that seeks the reason and the principle from which all reality springs in its manifold manifestations”. Thales considered water to be the “first principle” from which all things came, and his philosophy was called “monist” or also “reductionist”. “Monism” was upheld by various philosophers until the fifth century, when the search for the first principle took a new path: the origin of all things had to be explained by the intervention of several principles. The thinkers who followed this idea were called “pluralist physicists” because they dealt with nature and held that it was formed from a multiplicity of generating substances. Among them we find Leucippus of Miletus (of the school of Thales), who, together with his pupil Democritus of Abdera, gave rise to what would later be called Atomism. Theirs was defined as a “mechanistic corpuscular theory” in which atoms, eternal, solid and impenetrable, endowed with a whirling motion, could group together as a result of collisions and give rise to existing things. The two researchers worked roughly between 450 and 370 BC and thought that any portion of matter could be divided into ever smaller parts until reaching a minimal, indivisible fragment called “atomos”, where the privative “a” indicated precisely that this fragment could no longer be divided. This term of Democritus’s has entered modern science as “atom”: the smallest part of a chemical element that retains the properties of the element itself. Only two millennia later did the experiments of physics demonstrate that the atom is made up of smaller “subatomic” particles. When it first appeared, the idea of the Democritean atom would have deserved closer consideration, but the Greek philosophers, Plato and Aristotle in particular, opposed it harshly; the former for the lack of a creator of the atomic particles, and the latter because he considered it impossible that a fragment of substance could not be divided to infinity, and also because Aristotle did not admit the existence of the void. Because of these authoritative dissents, atomism remained “in the drawer” for a long time, even though some Greek and Roman philosophers took the idea up again a few centuries after its appearance. In particular, around 300 BC Epicurus revived Democritus’s philosophy in Greece and introduced the possibility of a spontaneous deviation of the atoms from their rectilinear motion, so as to give rise to the aggregations that formed physical reality. In Rome, in the Republican age, atomism was taken up and spread by Lucretius (95-55 BC), who set the theory out again in the poem “On the Nature of Things”, later taken up by the post-Aristotelian philosophers in the sixteenth century. Lucretius’s poem is not merely a restatement of atomist philosophy, but a hymn to Epicurus who, according to the Roman poet and philosopher, made the understanding of the world possible. “You I follow, glory of the Greek race, and… I set my footsteps in the prints of your feet… because… full of love I mean to imitate you…. You, father, have found the truth; you lavish fatherly precepts upon us. …. As soon as your reason began to proclaim…. what the nature of all things may be, behold the terrors of the heart vanish, the barriers of the world melt away (1). Naturally he accepted the spontaneous deviation introduced by Epicurus and, in Latin, called it “clinamen”. Atomism was fought against in the Greek and Roman period and above all in the Middle Ages, for philosophical and religious reasons; in particular it was accused of materialism and atheism. Nevertheless, in the first half of the sixteenth century the Copernican revolution dealt the first blow to Aristotelian astronomy, and subsequently the attack on the Greek philosopher shifted towards the constitution of matter, of space and of the void; the “corpuscularist” philosophers and the “atomist” philosophers appeared, and from that moment atomism began to have an influence on scientific research concerning the universe. The task of combating Aristotle’s physics and metaphysics was not, however, easy or free of trouble; some atomists who intended to organise a debate to refute certain Aristotelian principles were banished from Paris and threatened with corporal punishment. Nevertheless, in the seventeenth century atomism began to find admirers among prominent philosophers and scientists, and a new philosophy and a new physics were born with Descartes, Galileo, Bacon, Giordano Bruno, Boyle, Hobbes, Gassendi and others. Among these, Gassendi, a Christian priest and theologian, is remembered because he worked systematically to remove the atheistic implications from Epicurean atomism: God is the creator of the universe and therefore also of the atoms. The physicist Newton too, in the first decades of the eighteenth century, gave rise to a theory that regarded matter as made up of primordial particles called “minima”. All the scholars mentioned, however, share the merit of having created in Europe the scientific-experimental philosophy that would later lead to modern science. At the end of the eighteenth and the beginning of the nineteenth century, this placed in the hands of scientists tools suited to experimentation, which led to the overcoming of mere speculation. In 1808 the chemist and physicist John Dalton presented his composition of matter based on atoms, obtained from empirical evidence. It can be summed up in five points:

• Every element is made up of atoms; 2) all the atoms of an element are alike; 3) the atoms of different elements are different; 4) in chemical reactions atoms cannot be created, destroyed, changed or transmuted, and they transfer whole from one compound to another; 5) the atoms of an element combine, to form a compound, only in whole numbers of atoms of other elements.

Twenty-two centuries had passed since Democritus, and the first scientific evidence for the existence of atoms had been obtained; yet some sixty years more would have to pass before arriving at Mendeleev’s Periodic Table, so dear to Primo Levi. In Dalton’s day, in fact, the existence of the molecule (an aggregation of atoms) was still not clear, and Dalton himself, failing to recognise that certain gases (e.g. hydrogen and oxygen) were made up not of atoms but of diatomic molecules, misinterpreted certain experimental data. Ten years after Dalton’s publication, the Italian chemist Amedeo Avogadro, through his studies in the gaseous phase, using a balance, a thermometer, a manometer and volume-calibrated flasks, was able to demonstrate that one litre of oxygen, at constant temperature and pressure, weighed sixteen times more than one litre of hydrogen under the same experimental conditions; and since it had been shown that the molecules of the two elements were each made up of two atoms, he was able to obtain a relative scale of weight for the atoms of the chemical elements. This was done first of all for the gaseous elements and then, by modifying the experimental procedures, for the other elements as well. The clear and precise definition of atom and molecule was given by Stanislao Cannizzaro, of the University of Genoa, at a chemistry congress in Karlsruhe in 1860: “molecule is the quantity of matter that comes into play in a … reaction ….chemical ….. taking atom to mean the minimum quantity of a substance contained in a molecule”. Also present at that congress was Dmitri Mendeleev, a guest of a German laboratory, who on returning to Russia obtained the chair of General Chemistry. He had it in mind to give a course that would teach his students, clearly and simply, the fundamentals of chemistry that had emerged in recent years; he therefore felt the need to write a book that he regarded as an outline for students, and he called it “Principles of Chemistry”. He had the idea of arranging in order of increasing atomic weight, a numerical value that can be determined experimentally with precision, the 62 elements then known, specifying the properties and characteristics of each. The important discovery he made was that the properties of the elements recurred periodically as the atomic weight increased, and he came to define groups of elements of different atomic weight with similar properties. He also predicted the existence of elements not yet discovered, to which he gave provisional names. The table he compiled took the name of Mendeleev’s Periodic Table (from here on simply the Table), and it still keeps the same name today, even though it has been modified several times. Two main changes have been introduced: the addition of new elements and the replacement, in the listing, of atomic weight by atomic number. The increase in the number of elements was due first of all to the discovery, among the rocks of our planet, of 30 new natural elements, in part foreseen by Mendeleev, and so in total they became 92, the last number being assigned to uranium. But then several artificial elements were added, produced by synthesis carried out in the laboratory; they had an atomic number greater than 92 and were created through reactions involving the nucleus, so that at present the elements of the Table number 118 (the last was registered in 2018). The arrangement by atomic number came about when it became possible to acquire this parameter experimentally through new discoveries and new instruments. In 1913, in fact, the number of protons was determined by means of X-rays; it corresponded to the atomic number, which thus entered the Table. Until 1897 the atom had remained indivisible, like that of Democritus, but in that year the particles that composed it began to be identified. The first was the electron, with a negative charge, then the proton, with a positive charge, and finally, in 1932, the neutron, without charge and with a mass almost equal to that of the proton. It was also shown that the mass of protons was 1,835 times greater than that of electrons, and that the latter move in the void at a great distance from the nucleus, as in a miniature solar system, in orbitals of different energy and shape. (Figure 1). To build an atom in ideal terms, at ambient temperature and pressure, we must introduce the electrons available according to the atomic number into the orbitals arranged around the nucleus, beginning with those of lowest energy, closest to the nucleus, until we fill the outermost ones, bearing in mind that each orbital can contain at most two electrons. Those in the outermost orbitals are called “valence electrons” and are responsible for the bonds between atoms. These, in fact, have the extraordinary capacity to bond stably with atoms of the same element or of different elements, forming larger and heavier aggregates of matter called molecules. The orbitals containing the valence electrons have well-defined spatial shapes, and they condition the structure of the molecules that form (see Figure 2). The triangular structure of the three atoms of water is explained by the spatial arrangement of the valence orbitals of oxygen. To give an idea of the size of the atom, which is the smallest particle of matter that retains the characteristics of the element, I will quote a sentence from Massimo Teodorani’s book “The Atom and the Elementary Particles”: ..a small object such as a drop of water contains as many atoms as there are drops of water in the Mediterranean Sea. With the definitive ascertainment of the structure of the atom and the understanding of the origin of molecules, a new phase of chemical research begins, the one lived through by Primo Levi, who graduated in 1941. He writes that he had enrolled in Chemistry in order to understand matter, and that understanding matter was necessary in order to understand the Universe. This was also the aim of Thales, who lived some 2,300 years earlier and had imagined water as the principle of everything. In the twentieth century, however, research had been enriched with new theories, methodologies and instruments better suited to guiding the human mind towards knowledge of the universe, and so it is justified to think that in Levi’s day the truth was closer. One tool for the understanding of matter is Mendeleev’s Table, which contains the symbol, name and properties of all the known elements; these are simple, pure substances composed of atoms all identical in the number and arrangement of their electrons, but which may differ in atomic weight. The atoms of the elements and the molecules they form constitute all the matter existing in the Universe (2). This tells us that if we carry out a qualitative analysis on a small particle of matter from our planet, or hypothetically from any celestial body of any galaxy, we will find only atoms of elements that occupy a cell of the Table. Information on the composition of celestial bodies extremely far from us can be obtained by analysing the light emitted by the atoms of the elements that compose them, through spectroscopic methods. The blue light coming from interstellar space certifies the presence of hydrogen, the first element formed after the Big Bang (see below) and the first element of the Table.https://www.focus.it/scienza/spazio/idrogeno-il-bagliore-invisibile-delluniverso.

(Readers who wish to explore the chemical characteristics of Mendeleev’s Table in greater depth can find popular-science information in the Appendix).

With the completion of the periodic table and the acquisition of the concept of the chemical bond, responsible for the formation of molecular structures, research takes the path of synthesising new chemical substances, not existing in nature but suited to technological applications. In step with this, new instruments are devised that help chemists to acquire rapid information about atoms and molecules and to carry out appropriate qualitative and quantitative analyses.

Levi too undertakes “the work of a chemist who weighs, separates, measures and judges on the basis of sure proofs, and strives to answer the whys”. Initially he finds work in a firm that recovers nickel (the element in cell 28 of the Table) from waste materials, and then he works in a pharmaceutical firm where he will surely have built new molecules, purified them, carried out their elemental analysis (analysis of all the elements that make up the molecule) and checked their purity. In that period there is an enormous development of chemical synthesis in every field, and above all in that of plastics, which in 1963 culminates in the Nobel Prize for Chemistry awarded to Giulio Natta and Karl Ziegler for their research on polypropylene.

But where do all the atoms and molecules (the matter) of our planet of stone and of the entire universe come from? This question intrigued physicists and drove them to thorough research. We have already seen that they had obtained important results in the study of the atom before chemists began to build their molecules. The scholars of the physics of the universe were interested in understanding matter and its origin; indeed, around 1930 a theory was born, called the Big Bang (BB), which described the genesis of the universe in a very remote time. The theory was the work of the Belgian physicist and priest Georges Lemaître; it had a difficult life for many decades. But when it began to receive experimental confirmation from research with particle accelerators, where the behaviour of matter under extreme conditions was studied, the world scientific community accepted it by a large majority. Guido Tonelli (2), a physicist at CERN in Geneva and a professor at the University of Pisa, published in 2019 a book that describes what happened after the BB (which took place 13.8 billion years ago), the origin of all the elements of the universe and therefore also of those we can touch every day on our planet. The first stable particle that came into being a few instants after the BB is the proton, the nucleus of the hydrogen atom, and Tonelli says “a proton is forever”, utterly stable, “immortal”. One may say that protons are still today the principal component of the matter of the universe. Then the neutron forms, neutral, as its name says, which has practically the same mass as the proton and which is stable when packed into an atomic nucleus. Indeed, given the temperature conditions existing at the moment of the formation of protons and neutrons, they can aggregate to form the nuclei of the light elements, but not atoms. A proton and a neutron form the nucleus of deuterium (hydrogen with mass 2, contained in the molecule of heavy water), and two deuterium nuclei form the nucleus of helium (no. 2 in the Table). As the temperature falls (with increasing distance in time from the BB), the third particle needed to form the atoms of the elements comes into play: the electron, extremely light and with a negative charge; under favourable energy conditions the electron is then captured by the nuclei and the first atoms form. Tonelli says that at that moment 380,000 years had passed since the BB: “The electrons will be able to bond stably with the protons and the first atoms will be born”. The first elements to appear in the universe, about 9 billion years before the formation of the solar system, are number 1 and number 2 of the Periodic Table: hydrogen and helium. These are the lightest elements (mass 1 and 4) and make up, at 75% and 24% respectively, the overwhelming majority of the universe. These percentages may astonish an inhabitant of a planet in a solar system where there are four rocky planets (Mercury, Venus, Earth and Mars), but they are certified by science, and the clouds of hydrogen can be seen shining in the universe by their characteristic emission lines. The blue emission of hydrogen can be observed on this website:

https://www.media.inaf.it/2018/10/02/splendido-splendente-ecco-luniverso-primordiale/.

And fortunately there is hydrogen in abundance, because this is the fuel that keeps the nuclear furnaces burning (all the stars of the galaxies, including our Sun), producing helium. The hydrogen of the Sun, according to physicists, is enough to keep it burning for about ten billion years, and this sets our minds at rest. The 1% of elements heavier than helium are scattered through interstellar space; they are also present in the rocky planets and are of particular interest to the inhabitants of the Earth. A certain number, from lithium (no. 3) to iron (no. 26), were produced by nucleosynthesis through the collision of protons and neutrons at very high temperatures and speeds, with the consequent formation of new nuclei; these then became atoms by capturing electrons. The process was possible in the megastars formed just after the BB, which had internal temperatures suited to the synthesis of nuclei; the stars then exploded, dispersing the synthesised materials through cosmic space. The elements heavier still in the Table were likewise produced by nucleosynthesis, but under conditions of catastrophic collisions of stars. Recently a group of astronomers recorded a collision between neutron stars that acted as a “cosmic factory of heavy metals”:

http://www.blueplanetheart.it/2019/06/collisione-cosmica-creato-della-terra/.

Nine billion years after the BB, a gathering of matter that directly concerns our planet begins to form: the solar system takes shape. The explosions of stars in the preceding millennia had scattered through sidereal space dusts and gases containing hydrogen and helium, but also all the elements of the Table. As the temperature falls, the force of gravity begins to dominate and to give shape to a number of centres where matter collects. There form the sun, which begins to radiate energy by burning hydrogen, and its gaseous and rocky satellites. Among the latter is also the planet Earth, which raked together from the interstellar dusts all the elements present today, with some addition of material coming from meteorites. “All the nuclei that make up our body, the calcium of the bones, the oxygen of water, the iron of haemoglobin, have passed through this stormy and terrible past. Now the atoms they have formed submit meekly to the chemical and biological reactions that guarantee our existence. If only they could tell us something of their infancy…… first produced under extreme conditions of temperature and pressure in the heart of a star, and then hurled ….. into absolute emptiness for billions of years, waiting for a new aggregation to be created” (2). Primo Levi had grown fascinated by the wandering of atoms in the universe, and he imagined a journey, which he described in his book “The Periodic Table”, undergone by a “certain atom” of carbon. At the outset Levi’s carbon lay on the surface of a rock made of calcium carbonate, where it had been for hundreds of millions of years, having arrived “from a very long cosmic history” that Levi knows but does not intend to recount. His interest, rather, is to show the transformations the atom underwent after its departure from an inanimate object, such as a rock, to enter into a line of life. Indeed, after leaving the rock to be fired in a lime kiln, “it went out through the chimney and took to the air” as gaseous carbon dioxide. After a few adventurous flights it chanced upon a vine leaf and, by means of sunlight, entered a molecule of sugar that ended up in a bottle of wine. It then passes into the body of a drinker who in the end expels it once more as carbon dioxide. The carbon returns, with the help of light, into a life cycle of a cedar of Lebanon and then of a woodworm, only to become once again a molecule of carbon dioxide. Levi concludes that, on average, every two hundred years a carbon atom not immobilised in the depths of a rock “enters and re-enters the cycle of life”. It is painful to read that the idea of writing the adventure of the carbon atom came to Levi at a most sorrowful moment of his life. He writes: “To carbon, the element of life, was directed my first literary dream, insistently dreamed at an hour and in a place where my life was not worth much”. He is clearly referring to his imprisonment in the camp of Auschwitz where, instead of the chimney of the lime kiln, he saw other wicked, evil and sacrilegious chimneys.

These articles were written in memory of Primo Levi on the centenary of his birth, as a commentary on his thought as set down in “The Periodic Table”:

The nobility of Man, acquired in a hundred centuries of trial and error, had lain in making himself the master of matter, and I had enrolled in Chemistry because I wished to remain faithful to this nobility. To conquer matter is to understand it, and to understand matter is necessary in order to understand the universe and ourselves, and therefore the Periodic Table of Mendeleev.

References

• Lucretius. On the Nature of Things (Italian Edition). Feltrinelli Editore. Kindle Edition.
• Guido Tonelli, Genesis, Feltrinelli, 2019.
• Massimo Teodorani, The Atom and the Elementary Particles, Macro Edizioni, 2007.

Appendix

The complete Table (118 elements) is shown in figure 3. An interactive Table can be seen on the website: https://www.ptable.com/?lang=it (Control + click): selecting an element with the cursor enlarges the cell, and one can see all the characteristics of the element, including the numbers that describe the electron distribution. On the website: https://www.studenti.it/tavola-periodica-spiegazione-riassunto.html (Control+click) further information about the Table can be found. It is formed of cells containing the names and symbols of the elements, arranged in 18 columns, or groups, and 7 rows, or periods. At the top left of each cell (Figure 4) the atomic number is given, while at the bottom is the total mass (weight), the sum of that of the protons and the neutrons. On the right is a small column of numbers giving the distribution of the electrons in the various electron shells. The value we see at the foot of the little column is that of the valence orbitals, and it is important because it determines the bonding potential and therefore the chemical properties of each element. The atoms placed in the cells of each column have an equal number of valence electrons (see figure 4 and the websites indicated) and therefore have similar chemical characteristics. The atoms arranged in each row, or period, on the other hand, have properties that differ from one another, because each has one electron more than the preceding one in the valence orbitals. If we look at the cell of lithium in the second row, we see three electrons in all: two complete the lowest orbital and play no part in the formation of bonds (the configuration is like that of helium, a noble, inert gas), and a single electron in the outermost valence shell (the little column of numbers on the right); then we find beryllium, which has two, and the following ones three, four, etc. up to neon, which has eight (Figure 3). With this arrangement of electrons the so-called valence “octet” is completed, and neon is a noble gas that forms bonds with no one; it exists only in atomic form (my professor used to say it is like a stone). In first place in the third period we find sodium, which has the same properties as lithium (as does potassium, which lies below it) and which returns to having a single electron in the valence shell, and another octet begins to fill, leading to argon, once again a noble gas unwilling to form bonds. So, periodically, along the rows, after the addition of the eight electrons from left to right, we find the same chemical properties again. The same happens for the other periods, even though the appearance of the elements in the pink part of the table in figure 3 (columns IIIB, IVB, etc.) would make the explanation too long and complex for the purpose of this work. In any case, the last number at the bottom of the little column on the right in the cell always indicates the valence electrons; one starts from 1 and arrives at 8, at the unreactive noble gas. This is the meaning of periodic: a return, as the atomic number increases, to the same chemical properties after the introduction of another 8 electrons. The number of bonds that the atom of each element can form is not definable by a simple, precise rule; many elements give rise to different compounds by varying the number and type of bond according to their chemical properties. For example, iron (no. 26 in the Table) can create in its compounds 2, 3 or 6 bonds with other atoms; a multiplicity of bonding, even greater, and with different numbers than in iron, is followed by the majority of the other elements. Ruthenium and osmium, in their tetroxides, achieve 8 bonds with 4 molecules of oxygen, the maximum observed among all the elements of the Table.

Gian Gaetano Aloisi was full professor of Physical Chemistry at the University of Perugia. He is the author of numerous publications in international scientific journals. Many of his works present research concerning chemical reactions induced by the absorption of light. He also took an interest in the photoinduced reactivity of antimalarial drugs. For example: “DNA cleavage induced by photoexcited antimalarial drugs”, published in Photochemistry and Photobiology, 83, 664, 2007. Having reached retirement in 2008, he then devoted himself to the study of ancient history and prehistory.