Trurl began to catch atoms, scrape electrons from them, knead protons until only his fingers flickered, prepared proton dough, laid out electrons around it and - for the next atom; Not even five minutes had passed before he was holding a block of pure gold in his hands: he handed it to his muzzle, and she, having tried the block on her tooth and nodded her head, said:
- And indeed it’s gold, but I can’t chase atoms like that. I'm too big.
- It’s okay, we’ll give you a special device! - Trurl persuaded him.
Stanislaw Lem, Cyberiad
Is it possible, using a microscope, to see an atom, distinguish it from another atom, observe the destruction or formation of a chemical bond, and see how one molecule transforms into another? Yes, if it is not a simple microscope, but an atomic force one. And you don’t have to limit yourself to observation. We live in a time when the atomic force microscope is no longer just a window into the microworld. Today, the instrument can be used to move atoms, break chemical bonds, study the stretching limit of single molecules - and even study the human genome.
Letters made from xenon pixels
Looking at atoms wasn't always so easy. The history of the atomic force microscope began in 1979, when Gerd Karl Binnig and Heinrich Rohrer, working at the IBM Research Center in Zurich, began creating an instrument that would allow the study of surfaces at atomic resolution. To come up with such a device, the researchers decided to use the tunneling effect - the ability of electrons to overcome seemingly impenetrable barriers. The idea was to determine the position of atoms in the sample by measuring the strength of the tunneling current arising between the scanning probe and the surface under study.
Binnig and Rohrer succeeded, and they went down in history as the inventors of the scanning tunneling microscope (STM), and in 1986 they received the Nobel Prize in Physics. The scanning tunneling microscope has made a real revolution in physics and chemistry.
In 1990, Don Eigler and Erhard Schweitzer, working at the IBM Research Center in California, showed that STM can be used not only to observe atoms, but to manipulate them. Using a scanning tunneling microscope probe, they created perhaps the most popular image symbolizing the transition of chemists to working with individual atoms - they painted three letters on a nickel surface with 35 xenon atoms (Fig. 1).
Binnig did not rest on his laurels - in the year he received the Nobel Prize, together with Christopher Gerber and Kelvin Quaite, who also worked at the IBM Zurich Research Center, he began work on another device for studying the microworld, devoid of the disadvantages inherent in STM. The fact is that with the help of a scanning tunneling microscope it was impossible to study dielectric surfaces, but only conductors and semiconductors, and to analyze the latter, it was necessary to create a significant vacuum between them and the microscope probe. Realizing that creating a new device was easier than upgrading an existing one, Binnig, Gerber and Quaite invented the atomic force microscope, or AFM. The principle of its operation is radically different: to obtain information about the surface, they measure not the current strength that arises between the microscope probe and the sample being studied, but the value of the attractive forces that arise between them, that is, weak non-chemical interactions - van der Waals forces.
The first working model of AFM was relatively simple. The researchers moved a diamond probe over the surface of the sample, connected to a flexible micromechanical sensor - a cantilever made of gold foil (attraction arises between the probe and the atom, the cantilever bends depending on the force of attraction and deforms the piezoelectric). The degree of bending of the cantilever was determined using piezoelectric sensors - in a similar way that the grooves and ridges of a vinyl record are converted into an audio recording. The design of the atomic force microscope allowed it to detect attractive forces of up to 10–18 newtons. A year after creating a working prototype, the researchers were able to obtain an image of the graphite surface topography with a resolution of 2.5 angstroms.
Over the three decades that have passed since then, AFM has been used to study almost any chemical object - from the surface of a ceramic material to living cells and individual molecules, both in a static and dynamic state. Atomic force microscopy has become the workhorse of chemists and materials scientists, and the number of studies using this method is constantly growing (Fig. 2).
Over the years, researchers have selected conditions for both contact and non-contact study of objects using atomic force microscopy. The contact method is described above and is based on van der Waals interaction between the cantilever and the surface. When operating in non-contact mode, the piezovibrator excites oscillations of the probe at a certain frequency (most often resonant). The force exerted by the surface causes both the amplitude and phase of the probe's oscillations to change. Despite some disadvantages of the non-contact method (primarily sensitivity to external noise), it eliminates the influence of the probe on the object under study, and therefore is more interesting for chemists.
Lively on probes, in pursuit of connections
Atomic force microscopy became non-contact in 1998 thanks to the work of Binnig’s student, Franz Josef Gissibl. It was he who proposed using a quartz reference oscillator of a stable frequency as a cantilever. 11 years later, researchers from the IBM laboratory in Zurich undertook another modification of non-contact AFM: the role of a sensor probe was not played by a sharp diamond crystal, but by a single molecule - carbon monoxide. This made it possible to move to subatomic resolution, as demonstrated by Leo Gross from the Zurich department of IBM. In 2009, using AFM, he made visible not atoms, but chemical bonds, obtaining a fairly clear and unambiguously readable “picture” for the pentacene molecule (Fig. 3; Science, 2009, 325, 5944, 1110–1114, doi: 10.1126/science.1176210).
Convinced that chemical bonds could be seen using AFM, Leo Gross decided to go further and use an atomic force microscope to measure bond lengths and orders - key parameters for understanding the chemical structure, and therefore the properties of substances.
Recall that differences in bond orders indicate different electron densities and different interatomic distances between two atoms (simply put, a double bond is shorter than a single bond). In ethane the carbon-carbon bond order is one, in ethylene it is two, and in the classical aromatic molecule benzene the carbon-carbon bond order is greater than one but less than two, and is considered to be 1.5.
Determining the bond order is much more difficult when moving from simple aromatic systems to planar or bulk polycondensed cyclic systems. Thus, the order of bonds in fullerenes, consisting of condensed five- and six-membered carbon rings, can take any value from one to two. The same uncertainty is theoretically inherent in polycyclic aromatic compounds.
In 2012, Leo Gross, together with Fabian Mohn, showed that an atomic force microscope with a non-contact metal probe modified with carbon monoxide can measure differences in the charge distribution of atoms and interatomic distances - that is, parameters associated with bond order ( Science, 2012, 337, 6100, 1326–1329, doi: 10.1126/science.1225621).
To do this, they studied two types of chemical bonds in fullerene - a carbon-carbon bond, common to the two six-membered carbon-containing rings of the C60 fullerene, and a carbon-carbon bond, common to the five- and six-membered rings. An atomic force microscope has shown that the condensation of six-membered rings produces a bond that is shorter and of greater order than the condensation of cyclic fragments C 6 and C 5 . The study of the features of chemical bonding in hexabenzocoronene, where six more C 6 rings are symmetrically located around the central C 6 ring, confirmed the results of quantum chemical modeling, according to which the order of the C-C bonds of the central ring (in Fig. 4, the letter i) must be greater than the bonds connecting this ring with peripheral cycles (in Fig. 4 the letter j). Similar results were obtained for a more complex polycyclic aromatic hydrocarbon containing nine six-membered rings.
Bond orders and interatomic distances were, of course, of interest to organic chemists, but it was more important to those who studied the theory of chemical bonds, predicting reactivity, and studying the mechanisms of chemical reactions. However, both synthetic chemists and specialists in studying the structure of natural compounds were in for a surprise: it turned out that the atomic force microscope can be used to determine the structure of molecules in the same way as NMR or IR spectroscopy. Moreover, it provides a clear answer to questions that these methods cannot handle.
From photography to cinema
In 2010, the same Leo Gross and Rainer Ebel were able to unambiguously establish the structure of a natural compound - cephalandol A, isolated from a bacterium Dermacoccus abyssi(Nature Chemistry, 2010, 2, 821–825, doi: 10.1038/nchem.765). The composition of cephalandol A was previously established using mass spectrometry, but analysis of the NMR spectra of this compound did not give a clear answer to the question of its structure: four options were possible. Using an atomic force microscope, the researchers immediately eliminated two of the four structures, and made the correct choice of the remaining two by comparing the results obtained using AFM and quantum chemical modeling. The task turned out to be difficult: unlike pentacene, fullerene and coronenes, cephalandol A contains not only carbon and hydrogen atoms, in addition, this molecule does not have a plane of symmetry (Fig. 5) - but this problem was also solved.
Further confirmation that the atomic force microscope can be used as an analytical tool was obtained in the group of Oscar Kustanza, who at that time worked at the School of Engineering at Osaka University. He showed how to use AFM to distinguish atoms that differ from each other much less than carbon and hydrogen ( Nature, 2007, 446, 64–67, doi: 10.1038/nature05530). Kustants examined the surface of an alloy consisting of silicon, tin and lead with a known content of each element. As a result of numerous experiments, he found that the force generated between the tip of the AFM probe and different atoms differs (Fig. 6). For example, the strongest interaction was observed when probing silicon, and the weakest interaction was observed when probing lead.
It is assumed that in the future, the results of atomic force microscopy for the recognition of individual atoms will be processed in the same way as the results of NMR - by comparing relative values. Since the exact composition of the sensor tip is difficult to control, the absolute value of the force between the sensor and various surface atoms depends on the experimental conditions and the brand of the device, but the ratio of these forces for any composition and shape of the sensor remains constant for each chemical element.
In 2013, the first examples of using AFM to obtain images of individual molecules before and after chemical reactions appeared: a “photoset” of reaction products and intermediates is created, which can then be edited into a kind of documentary film ( Science, 2013, 340, 6139, 1434–1437; doi: 10.1126/science.1238187 ).
Felix Fischer and Michael Crommie from the University of California at Berkeley applied silver to the surface 1,2-bis[(2-ethynylphenyl)ethynyl]benzene, imaged the molecules and heated the surface to initiate cyclization. Half of the original molecules turned into polycyclic aromatic structures consisting of fused five six-membered and two five-membered rings. Another quarter of the molecules formed structures consisting of four six-membered rings connected through one four-membered ring, and two five-membered rings (Fig. 7). The remaining products were oligomeric structures and, in minor quantities, polycyclic isomers.
These results surprised the researchers twice. Firstly, only two main products were formed during the reaction. Secondly, their structure was surprising. Fisher notes that chemical intuition and experience made it possible to draw dozens of possible reaction products, but none of them corresponded to the compounds that formed on the surface. It is possible that the occurrence of atypical chemical processes was facilitated by the interaction of the starting substances with the substrate.
Naturally, after the first serious successes in the study of chemical bonds, some researchers decided to use AFM to observe weaker and less studied intermolecular interactions, in particular hydrogen bonding. However, work in this area is just beginning, and the results are contradictory. Thus, some publications report that atomic force microscopy made it possible to observe hydrogen bonding ( Science, 2013, 342, 6158, 611–614, doi: 10.1126/science.1242603), others argue that these are just artifacts due to the design features of the device, and the experimental results need to be interpreted more carefully ( Physical Review Letters, 2014, 113, 186102, doi: 10.1103/PhysRevLett.113.186102). Perhaps the final answer to the question of whether hydrogen and other intermolecular interactions can be observed using atomic force microscopy will be obtained already in this decade. To do this, it is necessary to increase the AFM resolution at least several times more and learn to obtain images without interference ( Physical Review B, 2014, 90, 085421, doi: 10.1103/PhysRevB.90.085421).
Single molecule synthesis
In skillful hands, both STM and AFM transform from devices capable of studying matter into devices capable of purposefully changing the structure of matter. With the help of these devices, it has already been possible to obtain “the smallest chemical laboratories”, in which a substrate is used instead of a flask, and individual molecules are used instead of moles or millimoles of reacting substances.
For example, in 2016, an international team of scientists led by Takashi Kumagai used non-contact atomic force microscopy to convert the porphycene molecule from one form to another ( Nature Chemistry, 2016, 8, 935–940, doi: 10.1038/nchem.2552). Porphycene can be considered a modification of porphyrin, the internal ring of which contains four nitrogen atoms and two hydrogen atoms. The vibrations of the AFM probe transferred enough energy to the porphycene molecule to transfer these hydrogens from one nitrogen atom to another, and the result was a “mirror image” of this molecule (Fig. 8).
The team led by the indefatigable Leo Gross also showed that it was possible to initiate the reaction of a single molecule - they converted dibromomanthracene into a ten-membered cyclic diyne (Fig. 9; Nature Chemistry, 2015, 7, 623–628, doi: 10.1038/nchem.2300 ). Unlike Kumagai et al., they used a scanning tunneling microscope to activate the molecule, and the result of the reaction was monitored using an atomic force microscope.
The combined use of a scanning tunneling microscope and an atomic force microscope has even made it possible to obtain a molecule that cannot be synthesized using classical techniques and methods ( Nature Nanotechnology, 2017, 12, 308–311, doi: 10.1038/nnano.2016.305 ). This is triangulene, an unstable aromatic diradical whose existence was predicted six decades ago, but all attempts at synthesis failed (Fig. 10). Chemists from Niko Pavlicek's group obtained the desired compound by removing two hydrogen atoms from its precursor using STM and confirming the synthetic result using AFM.
It is expected that the number of works devoted to the use of atomic force microscopy in organic chemistry will continue to grow. Currently, more and more scientists are trying to replicate on the surface reactions that are well known in “solution chemistry.” But perhaps synthetic chemists will begin to reproduce in solution the reactions that were originally carried out on the surface using AFM.
From nonliving to living
Cantilevers and probes of atomic force microscopes can be used not only for analytical studies or the synthesis of exotic molecules, but also for solving applied problems. There are already known cases of using AFM in medicine, for example, for the early diagnosis of cancer, and here the pioneer is the same Christopher Gerber, who had a hand in developing the principle of atomic force microscopy and the creation of AFM.
Thus, Gerber was able to teach AFM to detect point mutations in ribonucleic acid in melanoma (on material obtained as a result of a biopsy). To do this, the gold cantilever of an atomic force microscope was modified with oligonucleotides that can enter into intermolecular interaction with RNA, and the strength of this interaction can also be measured due to the piezoelectric effect. The sensitivity of the AFM sensor is so high that they are already trying to use it to study the effectiveness of the popular genome editing method CRISPR-Cas9. Technologies created by different generations of researchers come together here.
To paraphrase a classic of one of the political theories, we can say that we already see the limitless possibilities and inexhaustibility of atomic force microscopy and are hardly able to imagine what lies ahead in connection with the further development of these technologies. But today, scanning tunneling microscopes and atomic force microscopes give us the opportunity to see and touch atoms. We can say that this is not only an extension of our eyes, allowing us to look into the microcosm of atoms and molecules, but also new eyes, new fingers, capable of touching and controlling this microcosm.
But they are too small to be seen with an optical or even electron microscope. However, it is easy to observe some phenomena that indicate their existence.
When a charged particle, such as a nucleus or helium, passes through a moist gas, it leaves behind a trail of vapor, similar to that left by an airplane high in the sky. We can see or photograph this trace even without a microscope.
Connecting atoms into molecules
Although until recently it was not possible to see an atom, you can see a molecule, which is a chemical combination of atoms. Occasionally there are molecules so large that they can already be observed using an electron microscope, although they are too small to reflect the longer wavelengths of visible light and therefore cannot be seen using a simple microscope.
How many atoms does a molecule contain?
A virus is a huge molecule, one of the largest known molecules. The polio virus, which has caused so much trouble, is a spherical molecule containing many thousands of atoms. It can be seen using an electron microscope at 180,000x magnification.
In 1957, University of Pennsylvania scientist Erwin Mueller took the first real photograph of individual atoms. On the left are tungsten atoms arranged in a crystal lattice on the surface of a very thin metal needle. They were observed by Muller in the field of an ion microscope. Each small dot is an individual atom, the bright dots are groups of several atoms. The increase here is about 2,000,000.
At the Massachusetts Institute of Technology, Dr. Martin Burger used X-rays to record the arrangement of individual atoms in a pyrite crystal. Pyrite, iron disulfide, is a compound of iron and sulfur. Each pyrite cell contains 1 iron atom and 2 sulfur atoms.
Atoms are, of course, extremely small. One iron atom has a diameter of less than three hundred millionths of a centimeter. The photograph cannot examine the atom in detail, but it clearly shows the position of individual atoms in the crystal.
How are atoms connected in molecules?
Now let's find out the question, how to make different atoms, for example iron and sulfur, combine and form molecules?
If we thoroughly mix iron filings with sulfur, the mixture will still remain iron filings and sulfur. They are easy to separate again. All you need is a magnet. Thus, iron and sulfur do not form a chemical compound when mixed. They form a mixture of individual iron and sulfur atoms just sitting next to each other. However, if we place the mixture in a crucible and heat it, the iron and sulfur atoms form a chemical compound.
Heated together, they lose their individuality and become a compound, namely iron sulfide, which differs in all its properties from both iron and sulfur. Each molecule of this iron sulfide, or FeS, has 1 iron atom and 1 sulfur atom. This substance is very similar to iron disulfide, whose formula is FeS2. It is quite easy to prepare a magnesium compound because it requires only a small amount of magnesium metal and heat. The necessary oxygen will come from the air.
You can weigh magnesium and make sure that it gains weight when burned, rather than losing it. This happens because magnesium combines with oxygen in the air, forming a new compound - magnesium oxide MgO. Magnesium oxide, of course, is heavier than the original magnesium, since we must add to the weight of the magnesium atom the weight of the oxygen atom. The resulting magnesium oxide is neither magnesium nor oxygen.
The process of combining iron and sulfur or magnesium and oxygen is called a chemical reaction. The chemical formulas for both reactions are very simple:
- Fe + S -> FeS (iron plus sulfur forms iron sulfide)
- 2Mg + 0 2 -> 2MgO (magnesium plus oxygen forms magnesium oxide).
So far we have seen how a chemist prepares a compound of two elements, but we have not clarified the question of why the reaction occurs. There are many, many ways in which atoms can join together to form molecules, but any of them always involves the rearrangement of atomic electrons in orbits.
The rearrangement of electrons in atoms actually determines chemical processes.
The first horizontal row of the periodic table contains only two elements - hydrogen and helium. Each of them has one electron shell. The second horizontal row already consists of eight elements with two electron shells. We can say that each of the elements has an “eight-place” outer shell, with one or more “places” on it occupied by electrons.
As we have already seen, the first element of this series - lithium - has only one electron in its outer shell, beryllium - two, etc., up to neon, in which all eight places are occupied by electrons.
A similar situation occurs in the other five horizontal rows. The filling of a new shell begins from the first element of each row.
Each vertical column of the periodic table contains elements that have the same number of electrons in their outer shell. Hydrogen, lithium, sodium, potassium, rubidium, cesium, francium - they all have a single electron in their outer shell.
On the other, right edge of the periodic table there are elements whose outer shells are filled - helium, neon, argon, krypton, xenon and radon.
The elements of each vertical column are members of the same family. And since they all have the same number of electrons in their outer shells, they have similar chemical properties.
Apart from hydrogen itself, the elements in the first column - chemical relatives of hydrogen - are called alkali metals. Each of them has a single electron that is capable of moving in chemical reactions.
Such a reaction occurs, for example, when sodium combines with chlorine, forming molecules of the well-known table salt. In a two-dimensional model of the sodium atom, you can see that it has 11 protons in the nucleus and 11 electrons that balance the positive charge of the nucleus: two electrons in the first shell, 8 in the second and 1 in the third.
A chlorine atom has 17 electrons - 2, 8 and 7 in each shell. Sodium has one outer electron, while chlorine is one electron short of filling its shell. They form a chemical compound when a single electron from the outer shell of sodium jumps and fills the outer shell of the chlorine atom. Sodium dissolved without an electron becomes positively charged. One of its negative charges (electron) was added to chlorine. The two atoms now have opposite charges and are thus held together by a strong electrical bond.
Actually, it is more correct to talk not about atoms, but about ions that have opposite charges, since atoms that have lost their neutrality by gaining or losing electrons are called ions.
To put this experiment into practice, we must take a bottle of chlorine, which is a yellowish-green poisonous gas, and throw into it a piece of a soft poison - sodium. Soon, when sodium and chlorine combine, table salt NaCl is formed. Of course, only a very large number of molecules of salt or some other compound grouped together can form appreciable quantities of it.
Sodium and chlorine grow together as crystals. Sodium ions and chlorine ions, alternating with each other, form a cubic lattice. Salt crystals are usually imperfect and have various defects. However, crystals grown without disturbance have a distinct cubic structure. The complex crystal shown on the previous page contains a huge number of atoms - approximately 1025 sodium atoms and the same number of chlorine atoms.
This number looks very impressive: 10,000,000,000,000,000,000,000,000.
Table salt illustrates only one of the ways atoms combine to form molecules. This method, however, is by no means the main one. Another example would be . Let us have two hydrogen atoms, each of which has one electron, and one oxygen atom. Oxygen, which contains eight protons in its nucleus, also has 8 electrons. Two electrons are on the inner shell and six on the outer shell, leaving two unfilled spaces on it, which can probably be occupied by the electrons of two hydrogen atoms.
But in this case, electrons are not transferred, as was the case with sodium and chlorine. Instead, two hydrogen atoms move closer to the oxygen atom and share their electrons with it. The combination of atoms into molecules in this way is called differently: either joint possession of electrons, or communication using electron pairs, or, finally, . A huge number of molecules are formed precisely according to this principle.
In such molecules, tiny electric currents arise that constantly change direction. This circumstance causes molecules to be attracted to each other and stick together, forming visible quantities of water, sugar or other substances. In the absence of forces holding molecules together, all molecules would move independently, as in air, and any substance would be gaseous.
Atoms in molecules are connected to each other due to the electromagnetic interactions of their constituent electrons and nuclei. This connection is not very “hard”.
A model of a molecule, constructed from balls called atoms held together by rigid rods, is not very similar to a real molecule. In molecules, atoms are in continuous motion - they vibrate or rotate. But this picture is also inaccurate.
It would be more correct to say that it is not atoms that move in a molecule, but their constituent nuclei and electrons.
When combining into molecules, atoms do not leave all their electrons around. They either produce a “redistribution” of electrons, in which one of the atoms gives up part of its electrons to the other, positive and negative ions are formed, which “hold on to each other” due to Coulomb forces (ionic bond).
Or the atoms in the molecule begin to share some of their electrons (covalent bond). In both cases, the atoms in the molecule cease to exist as such, they “lose their face.” But this picture is not entirely correct.
After all, molecules, atoms, electrons and nuclei obey the laws of the microworld. This means that you cannot say about them: “They move this way or that way, they are there and there.” Their states need to be described in the language of quantum mechanics, and this is the “language of probability”.
Therefore, you can only draw the distribution densities of the particles that make up the molecule. And in these pictures both general electron clouds and individual ions will indeed be visible.
It does not have an exact solution for the simplest molecule - the hydrogen molecule H2, consisting of four particles - two protons and two electrons. An exact solution is possible only for the two-body problem.
Therefore, for molecules, the Schrödinger equation is solved by approximate methods, and all calculations are carried out using computers. As an example, we show the results of such calculations performed for molecules of lithium fluoride LiF (ionic bond) and hydrogen H2 (covalent bond).
The figure shows a graph of the dependence of the energy of the system E on the distance R between the Li and F nuclei. In configuration b at R = 8 A? (1 A? = 10-10 m) the outer electron of the lithium atom went to fluorine. This means that the state of two ions turned out to be energetically more favorable than the state of two atoms.
In state g at R = 1.5 A? the energy of the system takes on a minimum value; this is the energetically most favorable state. The figure shows the results of similar calculations for H atoms and the H2 molecule. The process of formation of a common electron shell around two H nuclei is clearly visible.
Substances. Molecules. Atoms
Continuation. See No. 6–14, 15, 16, 17/2003
Coming home from school, Sasha asked not to be disturbed and locked herself in the room.
“Their class is preparing a performance for the last bell,” Masha explained. – They were probably asked to make congratulations to the graduates and costumes for the concert.
An hour later, the mother decided to look at her daughter. She expected to find the girl drawing or sewing, but Sasha simply sat at the table and thoughtfully looked at a glass of water, which, apparently, was prepared for watercolors.
Hearing a rustling sound, Sasha raised her eyes and asked:
– Is a glass of water water?
“Of course,” the mother answered automatically, not quite understanding what her daughter meant.
– Is half a glass also water?
- Why not? - Mom was surprised.
“And a drop of water is also water, and half a drop...” Sasha continued. – How many parts can a drop of water be divided into? What is the smallest piece of water?
“The smallest piece of water is a water molecule,” said mom.
“The molecule is probably so small that it can only be seen under a microscope,” Sasha suggested.
– No, you can’t even see a molecule under a microscope. She is very, very small. And a huge number of molecules make up the water that stands in front of you.
- What quantity? – Sasha immediately asked.
“It’s so big that it’s hard to even imagine.” Someone calculated that in one glass of water there are more molecules than the number of glasses of water in all the seas, oceans, rivers and lakes of the Earth.
“Wow!..” Sasha suddenly spoke in a whisper. - Marvelous!
“The most amazing thing,” my mother said calmly, “is that even a single molecule of water behaves in chemical reactions in the same way as any amount of water.”
Sasha looked around.
– So, every substance has its own molecule? – she asked. – And they are all just as tiny?
– Among the tiny molecules there are different ones: larger and smaller. But all of them, of course, are very small compared to the objects that surround us. True, it cannot be said that all substances consist of molecules - there are other particles of matter. But you will learn about this in high school, but now let’s get to work, otherwise your high school students will be left without a holiday.
Mom left, and Sasha began to think where she should start. I had to draw a greeting card, inflate two balloons, and sew glitter onto a costume for a concert.
After some thought, she decided to tackle the balloons first. Taking in more air, the girl began to inflate the first balloon. At first, the ball filled with air easily, but the further it went, the more the ball increased in size, and it became more and more difficult to inflate it. Finally he became huge. Sasha, with a ball in her teeth, walked up to her mother and mumbled:
- Mmmm, pmmmmmmmm...
Mom quickly took out a strong thread and helped tie the ball. Taking it in her hands, Sasha began to examine it from all sides. It seemed to her that the balloon was not inflated enough, and she tried to lightly press on it. The ball was very elastic, but still gave a little under Sasha’s hand.
– Mom, look, I’m shrinking the air molecules!
“You’re wrong,” said mom. – Firstly, air has no molecules. Air is a mixture of gases, and each of them has its own molecules. Secondly, you are not reducing the molecules, but the spaces between them.
– Are there gaps between molecules? – Sasha was surprised.
- How could you inflate your balloon? After all, with each portion of air you blow more and more gas molecules into it. You probably noticed that the gas in the ball is slightly compressed compared to the surrounding air. Calculate how many exhalations you need to make to inflate the balloon.
Sasha took another ball. Soon it became as big as the first one. She could not speak, but from her gestures my mother understood that she blew twice ten times.
– At one time a person exhales about one liter of air. But the volume of your ball, of course, is less than twenty liters - after all, it’s about two buckets.
Sasha began to nod her head as a sign that she agreed with her mother. At that moment, the ball jumped out of her mouth and began to rush wildly throughout the room.
- Molecules run out of the ball! – Sasha screamed. - They tickle me!
Mom laughed. Sasha picked up the fallen ball and sat down on the floor.
“But in the floor there are certainly no distances between molecules,” she said. - He doesn’t shrink.
“Although solid and liquid substances hardly compress, they also have gaps between the molecules, just not as large as in gases,” said mom.
– And if a gas is compressed very strongly, will it become solid? – suggested Sasha.
- Certainly. This is how dry ice is obtained from carbon dioxide, which is placed in boxes of ice cream. And if you put a piece of dry ice on the table, after some time it will evaporate and turn back into gas.
“Then why doesn’t the table turn into gas?” – Sasha asked sarcastically.
“Molecules attract each other and repel each other at the same time,” said mom.
Noticing that Sasha was about to ask another question, her mother continued:
– Why this is happening, I cannot explain to you yet. Even many students do not immediately understand this. But if the attraction is stronger than the repulsion, the substance is liquid or solid, and if it is weaker, it turns into a gas. It depends on the substance itself and on the temperature: when heated, the attraction becomes weaker.
“Now I understand,” said Sasha, “why the water boils.” By the way, let's have some tea.
“Okay,” Mom agreed. – By the way, Masha is baking a pie. And, in my opinion, he is already ready. Do you feel how delicious it smells?
- But Masha is baking a pie in the kitchen, why did the smell reach the room?
– These are the molecules of substances that were released during baking that came to us. All molecules are moving all the time. In solids they move slightly in one place, in liquids they move from place to place, and in gases they move quite quickly.
Maxim came, and Sasha began to tell him about molecules.
– And I know what our class is like when we sit at our desks during the lesson. Me and riddle I remembered the right one:
– Do you mean frozen water that floats in ordinary liquid water?
- Certainly! And when we walk hand in hand into the dining room, it looks like moving water, as if we were swimming,” Maxim explained.
– When classes end, we run to the schoolyard, and then it turns out, as in another riddle:
After drinking tea and pie, Sasha and Maxim went to paint. Sasha dipped the brush into a glass of water, then scooped some paint onto it. A bright drop fell on the table, Sasha wiped it with a rag. Then she dropped the same drop into the water. The droplet sank to the bottom and began to slowly blur.
“Probably water molecules move in the glass and push paint molecules apart,” Sasha suggested. - Wow, molecules cannot be seen, but what they do is noticeable...
She opened her chemistry notebook and showed Maxim the notes on what her mother told her:
Then the girl took a piece of paper from the folder and folded it in half. She decided to draw baby Fantik with a bouquet of flowers on the greeting card. It didn't take her long. Having decided that she would make a festive costume tomorrow, Sasha began to draw new pictures about the bear cub. And Maxim, without wasting time, began to work on another crossword puzzle.
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Crossword. Particle name
If you enter the names of substances from left to right, then from top to bottom you get the name of the smallest particle of the substance.
Answers to riddles. Ice and liquid water; dew and water vapor.
Crossword answers.
Horizontally: 1. Soap. 2. Water. 3. Chlorine.
4. Protein. 5. Oxygen. 6. Vinegar. 7. Ice. 8. Nitrogen.
Vertically: 1. Molecule.
1. Basic concepts, definitions and laws of chemistry
1.2. Atom. Chemical element. Simple substance
The atom is a central concept in chemistry. All substances are made up of atoms. An atom is the limit of crushing a substance by chemical means, i.e. an atom is the smallest chemically indivisible particle of matter. Atomic fission is possible only in physical processes - nuclear reactions and radioactive transformations.
Modern definition of an atom: an atom is the smallest chemically indivisible electrically neutral particle, consisting of a positively charged nucleus and negatively charged electrons.
In nature, atoms exist both in a free (individual, isolated) form (for example, noble gases are made up of individual atoms) and as part of various simple and complex substances. It is clear that in the composition of complex substances, atoms are not electrically neutral, but have an excess positive or negative charge (for example, Na + Cl −, Ca 2+ O 2−), i.e. In complex substances, atoms can be found in the form of monoatomic ions. Atoms and the monoatomic ions formed from them are called atomic particles.
The total number of atoms in nature cannot be counted, but they can be classified into narrower types, just as, for example, all the trees in a forest are divided into birch, oak, spruce, pine, etc., according to their characteristic features. The basis for classifying atoms into certain types is the charge of the nucleus, i.e. the number of protons in the nucleus of an atom, since it is this characteristic that is preserved, regardless of whether the atom is in a free or chemically bound form.
Chemical element- This is a type of atomic particles with the same nuclear charge.
For example, we mean the chemical element sodium, regardless of whether free sodium atoms or Na + ions in the composition of salts are considered.
The concepts of atom should not be confused, chemical element And simple substance. An atom is a concrete concept, atoms really exist, but a chemical element is an abstract, collective concept. For example, in nature there are specific copper atoms with rounded relative atomic masses of 63 and 65. But the chemical element copper is characterized by an averaged relative atomic mass, given in the periodic table of chemical elements by D.I. Mendeleev, which, taking into account the content of isotopes, is equal to 63.54 (in nature, there are no copper atoms with such an A r value). An atom in chemistry is traditionally understood as an electrically neutral particle, while a chemical element in nature can be represented by both electrically neutral and charged particles - monatomic ions: , , , .
A simple substance is one of the forms of existence of a chemical element in nature (another form is a chemical element in the composition of complex substances). For example, the chemical element oxygen in nature exists in the form of a simple substance O 2 and as part of a number of complex substances (H 2 O, Na 2 SO 4 ⋅ 10H 2 O, Fe 3 O 4). Often the same chemical element forms several simple substances. In this case, they talk about allotropy - the phenomenon of the existence of an element in nature in the form of several simple substances. The simple substances themselves are called allotropic modifications ( modifications) . A number of allotropic modifications are known for carbon (diamond, graphite, carbyne, fullerene, graphene, tubulenes), phosphorus (white, red and black phosphorus), oxygen (oxygen and ozone). Due to the phenomenon of allotropy, there are approximately 5 times more known simple substances than chemical elements.
Causes of allotropy:
- differences in the quantitative composition of molecules (O 2 and O 3);
- differences in the structure of the crystal lattice (diamond and graphite).
Allotropic modifications of a given element always differ in physical properties and chemical activity. For example, ozone is more active than oxygen, and the melting point of diamond is higher than fullerene. Allotropic modifications under certain conditions (changes in pressure, temperature) can transform into each other.
In most cases, the names of a chemical element and a simple substance are the same (copper, oxygen, iron, nitrogen, etc.), so it is necessary to distinguish between the properties (characteristics) of a simple substance as a collection of particles and the properties of a chemical element as a type of atom with the same nuclear charge.
A simple substance is characterized by structure (molecular or non-molecular), density, a certain state of aggregation under given conditions, color and odor, electrical and thermal conductivity, solubility, hardness, boiling and melting points (t boil and t pl), viscosity, optical and magnetic properties , molar (relative molecular) mass, chemical formula, chemical properties, methods of preparation and use. We can say that the properties of a substance are the properties of a collection of chemically related particles, i.e. physical body, since one atom or molecule has no taste, odor, solubility, melting and boiling points, color, electrical and thermal conductivity.
Properties (characteristics) chemical element: atomic number, chemical sign, relative atomic mass, atomic mass, isotopic composition, occurrence in nature, position in the periodic table, atomic structure, ionization energy, electron affinity, electronegativity, oxidation states, valence, allotropy phenomenon, mass and mole fraction in the composition of a complex substance, absorption and emission spectra. We can say that the properties of a chemical element are the properties of one particle or isolated particles.
The differences between the concepts of “chemical element” and “simple substance” are shown in table. 1.2 using nitrogen as an example.
Table 1.2
Differences between the concepts of “chemical element” and “simple substance” for nitrogen
| Nitrogen - chemical element | Nitrogen is a simple substance |
|---|---|
| 1. Atomic number 7. | 1. Gas (n.o.) is colorless, odorless and tasteless, non-toxic. |
| 2. Chemical symbol N. | 2. Nitrogen has a molecular structure, formula N 2, the molecule consists of two atoms. |
| 3. Relative atomic mass 14. | 3. Molar mass 28 g/mol. |
| 4. In nature it is represented by nuclides 14 N and 15 N. | 4. Poorly soluble in water. |
| 5. Mass fraction in the earth's crust 0.030% (16th place in prevalence). | 5. Density (n.s.) 1.25 g/dm3, slightly lighter than air, relative density for helium 7. |
| 6. Does not have allotropic modifications. | 6. Dielectric, conducts heat poorly. |
| 7. Part of various salts - nitrates (KNO 3, NaNO 3, Ca(NO 3) 2). | 7. t boil = −195.8 °C; t pl = −210.0 °C. |
| 8. Mass fraction in ammonia is 82.35%, it is part of proteins, amines, and DNA. | 8. Dielectric constant 1.00. |
| 9. The mass of an atom is (for 14 N) 14u or 2.324 10 −23 g. | 9. The dipole moment is 0. |
| 10. Atomic structure: 7p,7e,7n (for 14 N), electronic configuration 1s 2 2s 2 2p 3, two electron layers, five valence electrons, etc. | 10. Has a molecular crystal lattice (in the solid state). |
| 11. In the periodic table it is in the 2nd period and the VA group, belongs to the family of p-elements. | 11. In the atmosphere the volume fraction is 78%. |
| 12. Ionization energy 1402.3 kJ/mol, electron affinity −20 kJ/mol, electronegativity 3.07. | 12. World production 44 · 10 6 tons per year. |
| 13. Exhibits covalences I, II, III, IV and oxidation states −3, −2, −1, 0, +1, +2, +3, +4, +5. | 13. Obtained: in the laboratory - by heating NH 4 NO 2; in industry - by heating liquefied air. |
| 14. Atomic radius (orbital) 0.052 nm. | 14. Chemically inactive, when heated it interacts with oxygen and metals. |
| 15. The main line in the spectrum is 399.5 nm. | 15. Used to create an inert atmosphere when drying explosives, when storing valuable works of painting and manuscripts, to create low temperatures (liquid nitrogen). |
| 16. The body of an average person (body weight 70.0 kg) contains 1.8 kg of nitrogen. | |
| 17. As part of ammonia, it participates in the formation of hydrogen bonds. |
Example 1.2. Indicate which of the following statements mention oxygen as a chemical element:
- a) the mass of the atom is 16u;
- b) forms two allotropic modifications;
- c) molar mass is 32 g/mol;
- d) poorly soluble in water.
Solution. Statements c), d) refer to a simple substance, and statements a), b) refer to the chemical element oxygen.
Answer: 3).
Each chemical element has its own symbol - chemical sign (symbol): K, Na, O, N, Cu, etc.
A chemical symbol can also express the composition of a simple substance. For example, the symbol of the chemical element Fe also reflects the composition of the simple substance iron. However, the chemical symbols O, H, N, Cl denote only chemical elements; simple substances have the formulas O 2, H 2, N 2, Cl 2.
As already noted, in most cases the names of chemical elements and simple substances are the same. Exceptions are the names of allotropic modifications of carbon (diamond, graphite, carbyne, fullerene) and one of the modifications of oxygen (oxygen and ozone). For example, when we use the word “graphite,” we mean only the simple substance (but not the chemical element) carbon.
The abundance of chemical elements in nature is expressed in mass and mole fractions. Mass fraction w is the ratio of the mass of atoms of a given element to the total mass of atoms of all elements. Mole fraction χ is the ratio of the number of atoms of a given element to the total number of atoms of all elements.
In the earth's crust (a layer about 16 km thick), oxygen atoms have the largest mass (49.13%) and molar (55%) shares, followed by silicon atoms (w (Si) = 26%, χ(Si) = 16 .35%). In the Galaxy, almost 92% of the total number of atoms are hydrogen atoms, and 7.9% are helium atoms. Mass fractions of atoms of the main elements in the human body: O - 65%, C - 18%, H - 10%, N - 3%, Ca - 1.5%, P - 1.2%.
The absolute values of the atomic masses are extremely small (for example, the mass of an oxygen atom is about 2.7 ⋅ 10 −23 g) and are inconvenient for calculations. For this reason, a scale of relative atomic masses of elements was developed. Currently, the unit of measurement for relative atomic masses is 1/12 of the mass of an atom of the C-12 nuclide. This quantity is called constant atomic mass or atomic mass unit(a.u.m.) and has the international designation u:
m u = 1 a. e.m. = 1 u = 1 / 12 (m a 12 C) =
1.66 ⋅ 10 − 24 g = 1.66 ⋅ 10 − 27 kg.
It is easy to show that the numerical value of u is equal to 1/N A:
1 u = 1 12 m a (12 C) = 1 12 M (C) N A = 1 12 12 N A = 1 N A =
1 6.02 ⋅ 10 23 = 1.66 ⋅ 10 − 24 (g).
Relative atomic mass of an element A r (E) is a physical dimensionless quantity that shows how many times the mass of an atom or the average mass of an atom (respectively for isotopically pure and isotopically mixed elements) is greater than 1/12 of the mass of an atom of the C-12 nuclide:
A r (E) = m a (E) 1 a. e.m. = m a (E) 1 u. (1.1)
Knowing the relative atomic mass, you can easily calculate the mass of an atom:
m a (E) = A r (E)u = A r (E) ⋅ 1.66 ⋅ 10 −24 (g) =
A r (E) ⋅ 1.66 ⋅ 10 −27 (kg).
Molecule. And he. Substances of molecular and non-molecular structure. Chemical equation
When atoms interact, more complex particles are formed - molecules.
A molecule is the smallest electrically neutral isolated collection of atoms, capable of independent existence and being the bearer of the chemical properties of a substance.
Molecules have the same qualitative and quantitative composition as the substance they form. The chemical bond between atoms in a molecule is much stronger than the interaction forces between molecules (which is why a molecule can be considered as a separate, isolated particle). In chemical reactions, molecules, unlike atoms, are not preserved (destroyed). Like an atom, an individual molecule does not possess such physical properties of a substance as color and odor, melting and boiling points, solubility, thermal and electrical conductivity, etc.
Let us emphasize that the molecule is precisely the carrier of the chemical properties of a substance; it cannot be said that the molecule retains (has exactly the same) chemical properties of the substance, since the chemical properties of the substance are significantly influenced by intermolecular interaction, which is absent for an individual molecule. For example, the substance trinitroglycerin has the ability to explode, but not an individual molecule of trinitroglycerin.
An ion is an atom or group of atoms that has a positive or negative charge.
Positively charged ions are called cations, and negatively charged ones are called anions. Ions can be simple, i.e. monoatomic (K +, Cl −), and complex (NH 4 +, NO 3 −), single-charged (Na +, Cl −) and multi-charged (Fe 3+, PO 4 3 −).
1. For a given element, a simple ion and a neutral atom have the same number of protons and neutrons, but differ in the number of electrons: the cation has fewer and the anion has more than the electrically neutral atom.
2. The mass of a simple or complex ion is the same as the mass of the corresponding electrically neutral particle.
It should be borne in mind that not all substances are composed of molecules.
Substances made up of molecules are called substances of molecular structure. These can be either simple (argon, oxygen, fullerene) or complex (water, methane, ammonia, benzene) substances.
All gases and almost all liquids (with the exception of mercury) have a molecular structure; solids can have both a molecular (sucrose, fructose, iodine, white phosphorus, phosphoric acid) and non-molecular structure (diamond, black and red phosphorus, carborundum SiC, table salt NaCl). In substances with a molecular structure, the bonds between molecules (intermolecular interaction) are weak. When heated, they are easily destroyed. It is for this reason that substances of molecular structure have relatively low melting and boiling points and are volatile (as a result of which they often have an odor).
Substances of non-molecular structure consist of electrically neutral atoms or simple or complex ions. For example, diamond, graphite, black phosphorus, silicon, boron are made of electrically neutral atoms, and salts are made of simple and complex ions, for example KF and NH 4 NO 3. Metals are made up of positively charged atoms (cations). Carborundum SiC, silicon oxide (IV) SiO 2, alkalis (KOH, NaOH), most salts (KCl, CaCO 3), binary compounds of metals with non-metals (basic and amphoteric oxides, hydrides, carbides, silicides, nitrides, phosphides), intermetallic compounds (compounds of metals with each other). In substances of non-molecular structure, individual atoms or ions are connected to each other by strong chemical bonds, therefore, under normal conditions, these substances are solid, non-volatile, and have high melting points.
For example, sucrose (molecular structure) melts at 185 °C, and sodium chloride (non-molecular structure) melts at 801 °C.
In the gas phase, all substances consist of molecules, and even those that at ordinary temperatures have a non-molecular structure. For example, at high temperatures, molecules of NaCl, K 2 , and SiO 2 were found in the gas phase.
For substances that decompose when heated (CaCO 3, KNO 3, NaHCO 3), molecules cannot be obtained by heating the substance
Molecular substances form the basis of the organic world, and non-molecular substances form the basis of the inorganic (mineral) world.
Chemical formula. Formula unit. Chemical equation
The composition of any substance is expressed using a chemical formula. Chemical formula is an image of the qualitative and quantitative composition of a substance using symbols of chemical elements, as well as numerical, alphabetic and other signs.
For simple substances of non-molecular structure, the chemical formula coincides with the sign of the chemical element (for example, Cu, Al, B, P). In the formula of a simple substance of molecular structure, indicate (if necessary) the number of atoms in the molecule: O 3, P 4, S 8, C 60, C 70, C 80, etc. The formulas of noble gases are always written with one atom: He, Ne, Ar, Xe, Kr, Rn. When writing equations of chemical reactions, the chemical formulas of some polyatomic molecules of simple substances can (unless specifically stated) be written in the form of symbols of elements (single atoms): P 4 → P, S 8 → S, C 60 → C (this cannot be done for ozone O 3, oxygen O 2, nitrogen N 2, halogens, hydrogen).
For complex substances of molecular structure, empirical (simple) and molecular (true) formulas are distinguished. Empirical formula shows the smallest integer ratio of the numbers of atoms in a molecule, and molecular formula- true integer ratio of atoms. For example, the true formula of ethane is C 2 H 6, and the simplest is CH 3. The simplest formula is obtained by dividing (reducing) the numbers of atoms of the elements in the true formula by some suitable number. For example, the simplest formula for ethane was obtained by dividing the numbers of C and H atoms by 2.
The simplest and true formulas can either coincide (methane CH 4, ammonia NH 3, water H 2 O) or not coincide (phosphorus oxide (V) P 4 O 10, benzene C 6 H 6, hydrogen peroxide H 2 O 2, glucose C 6 H 12 O 6).
Chemical formulas allow you to calculate the mass fractions of atoms of elements in a substance.
The mass fraction w of atoms of the element E in a substance is determined by the formula
w (E) = A r (E) ⋅ N (E) M r (V) , (1.2)
where N (E) is the number of atoms of the element in the formula of the substance; M r (B) - relative molecular (formula) mass of a substance.
For example, for sulfuric acid M r (H 2 SO 4) = 98, then the mass fraction of oxygen atoms in this acid
w (O) = A r (O) ⋅ N (O) M r (H 2 SO 4) = 16 ⋅ 4 98 ≈ 0.653 (65.3%).
Using formula (1.2), the number of atoms of an element in a molecule or formula unit is found:
N (E) = M r (V) ⋅ w (E) A r (E) (1.3)
or molar (relative molecular or formula) mass of a substance:
M r (V) = A r (E) ⋅ N (E) w (E) . (1.4)
In formulas 1.2–1.4, the values of w (E) are given in fractions of unity.
Example 1.3. In a certain substance, the mass fraction of sulfur atoms is 36.78%, and the number of sulfur atoms in one formula unit is two. Specify the molar mass (g/mol) of the substance:
Solution . Using formula 1.4, we find
M r = A r (S) ⋅ N (S) w (S) = 32 ⋅ 2 0.3678 = 174 ,
M = 174 g/mol.
Answer: 2).
The following example shows a method for finding the simplest formula of a substance based on the mass fractions of elements.
Example 1.4. In some chlorine oxide, the mass fraction of chlorine atoms is 38.8%. Find the formula of the oxide.
Solution . Since w (Cl) + w (O) = 100%, then
w(O) = 100% − 38.8% = 61.2%.
If the mass of the substance is 100 g, then m (Cl) = 38.8 g and m (O) = 61.2 g.
Let's imagine the oxide formula as Cl x O y. We have
x : y = n (Cl) : n (O) = m (Cl) M (Cl) : m (O) M (O) ;
x: y = 38.8 35.5: 61.2 16 = 1.093: 3.825.
Dividing the resulting numbers by the smallest of them (1.093), we find that x: y = 1: 3.5 or, multiplying by 2, we get x: y = 2: 7. Therefore, the formula of the oxide is Cl 2 O 7.
Answer: Cl 2 O 7.
For all complex substances of non-molecular structure, chemical formulas are empirical and reflect the composition not of molecules, but of the so-called formula units.
Formula unit(FE) - a group of atoms corresponding to the simplest formula of a substance of non-molecular structure.
Thus, the chemical formulas of substances of non-molecular structure are formula units. Examples of formula units: KOH, NaCl, CaCO 3, Fe 3 C, SiO 2, SiC, KNa 2, CuZn 3, Al 2 O 3, NaH, Ca 2 Si, Mg 3 N 2, Na 2 SO 4, K 3 PO 4, etc.
Formula units can be considered as structural units of substances of non-molecular structure. For substances with a molecular structure, these are obviously actually existing molecules.
Using chemical formulas, the equations of chemical reactions are written.
Chemical equation is a conventional notation of a chemical reaction using chemical formulas and other signs (equals, plus, minus, arrows, etc.).
A chemical equation is a consequence of the law of conservation of mass, so it is composed so that the numbers of atoms of each element in both its sides are equal.
The numbers before the formulas are called stoichiometric coefficients, in this case the unit is not written down, but is implied (!) and taken into account when calculating the total sum of stoichiometric coefficients. Stoichiometric coefficients show in what molar ratios the starting substances react and the reaction products are formed. For example, for a reaction whose equation is
3Fe 3 O 4 + 8Al = 9Fe + 4Al 2 O 3
n (Fe 3 O 4) n (Al) = 3 8; n (Al) n (Fe) = 8 9 etc.
In reaction schemes, coefficients are not placed and an arrow is used instead of an equal sign:
FeS 2 + O 2 → Fe 2 O 3 + SO 2
The arrow is also used when writing equations of chemical reactions involving organic substances (so as not to confuse the equals sign with a double bond):
CH 2 =CH 2 + Br 2 → CH 2 Br–CH 2 Br,
as well as equations for the electrochemical dissociation of strong electrolytes:
NaCl → Na + + Cl − .
Law of Constancy of Composition
For substances of molecular structure this is true law of constancy of composition(J. Proust, 1808): every substance of molecular structure, regardless of the method and conditions of production, has a constant qualitative and quantitative composition.
From the law of constancy of composition it follows that in molecular compounds the elements must be in strictly defined mass proportions, i.e. have a constant mass fraction. This is true if the isotopic composition of the element does not change. For example, the mass fraction of hydrogen atoms in water, regardless of the method of its preparation from natural substances (synthesis from simple substances, heating of copper sulfate CuSO 4 · 5H 2 O, etc.) will always be equal to 11.1%. However, in water obtained by the interaction of deuterium molecules (hydrogen nuclide with A r ≈ 2) and natural oxygen (A r = 16), the mass fraction of hydrogen atoms
w (H) = 2 ⋅ 2 2 ⋅ 2 + 16 = 0.2 (20%).
Substances that obey the law of constancy of composition, i.e. substances of molecular structure are called stoichiometric.
Substances of non-molecular structure (especially carbides, hydrides, nitrides, oxides and sulfides of d-family metals) do not obey the law of constant composition, which is why they are called non-stoichiometric. For example, depending on the production conditions (temperature, pressure), the composition of titanium(II) oxide is variable and ranges from TiO 0.7 –TiO 1.3, i.e. in a crystal of this oxide, for every 10 titanium atoms there can be from 7 to 13 oxygen atoms. However, for many substances of non-molecular structure (KCl, NaOH, CuSO 4), deviations from a constant composition are very insignificant, so we can assume that their composition is practically independent of the method of preparation.
Relative molecular and formula weight
To characterize substances of molecular and non-molecular structure, respectively, the concepts of “relative molecular mass” and “relative formula mass” are introduced, which are denoted by the same symbol - M r
Relative molecular weight- a dimensionless physical quantity that shows how many times the mass of a molecule is greater than 1/12 of the mass of an atom of the C-12 nuclide:
M r (B) = m mol (B) u . (1.5)
Relative formula mass- a dimensionless physical quantity that shows how many times the mass of a formula unit is greater than 1/12 of the mass of an atom of the C-12 nuclide:
M r (B) = m PU (B) u . (1.6)
Formulas (1.5) and (1.6) allow you to find the mass of a molecule or physical unit:
m (mol, FU) = uM r . (1.7)
In practice, M r values are found by summing the relative atomic masses of the elements forming a molecule or formula unit, taking into account the number of individual atoms. For example:
M r (H 3 PO 4) = 3A r (H) + A r (P) + 4A r (O) =
3 ⋅ 1 + 31 + 4 ⋅ 16 = 98.