Trurl began to catch atoms, scrape electrons off them, knead protons, so that only fingers flickered, prepared a proton dough, put 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, she tasted the block on the tooth and nodded her head, said:
- And indeed gold, only I cannot chase atoms like that. I'm too big.
- Nothing, we will give you a special apparatus! Trurl persuaded him.
Stanislav Lem, "Cyberiada"
Is it possible with a microscope to see an atom, distinguish it from another atom, trace 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. Or you may not be limited to observation. We live at a time when the atomic force microscope is no longer just a window into the microcosm. Today, this device can be used to move atoms, break chemical bonds, study the tensile limit of single molecules - and even study the human genome.
Xenon pixel letters
It hasn't always been easy to look at atoms. The history of the atomic force microscope began in 1979, when Gerd Karl Binnig and Heinrich Rohrer, who worked at the IBM Research Center in Zurich, began to create an instrument that would allow the study of surfaces with atomic resolution. To come up with such a device, the researchers decided to use the tunnel junction effect - the ability of electrons to overcome seemingly impassable barriers. The idea was to determine the position of atoms in the sample by measuring the strength of the tunneling current that occurs 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 revolutionized physics and chemistry.
In 1990, Don Eigler and Erhard Schweizer, who worked 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 what is arguably the most popular image of chemists' transition to working with individual atoms — they painted three letters with 35 xenon atoms on a nickel surface (Figure 1).
Binnig did not rest on his laurels - in the year of receiving the Nobel Prize, together with Christopher Gerber and Calvin Quait, who also worked at the IBM Zurich Research Center, he began work on another device for studying the microcosm, devoid of the shortcomings 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, a significant rarefaction had to be created between them and the microscope probe. Realizing that creating a new device was easier than upgrading an existing one, Binnig, Gerber, and Quait invented the atomic force microscope, or AFM. The principle of its operation is fundamentally different: to obtain information about the surface, it is not the current that arises between the microscope probe and the studied sample that is measured, but the value of the attractive forces arising between them, that is, weak non-chemical interactions - the van der Waals forces.
The first working model of the AFM was relatively simple. The researchers moved a diamond probe connected to a flexible micromechanical sensor - a cantilever made of gold foil over the surface of the sample (an attraction arises between the probe and the atom, the cantilever bends depending on the force of attraction and deforms the piezoelectric). The degree of cantilever bending was determined using piezoelectric sensors - in a similar way, the grooves and ridges of a vinyl record are transformed into an audio recording. The design of the atomic force microscope allowed it to detect attractive forces up to 10 –18 Newtons. A year after the creation of a working prototype, the researchers managed to obtain an image of the graphite surface relief with a resolution of 2.5 angstroms.
Over the three decades that have passed since then, AFM has been used to study practically 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 works in which this method is applied is constantly growing (Fig. 2).
Over the years, researchers have found the conditions for both contact and non-contact study of objects using atomic force microscopy. The contact method is described above and is based on the van der Waals interaction between the cantilever and the surface. When operating in a non-contact mode, the piezo vibrator excites the oscillations of the probe at a certain frequency (most often resonant). The force acting from the surface causes both the amplitude and the phase of the probe to change. Despite some drawbacks of the non-contact method (first of all, sensitivity to external noise), it is this method that excludes the influence of the probe on the object under study, which means that it is more interesting for chemists.
Alive by probes, chasing connections
Non-contact atomic force microscopy became in 1998 thanks to the work of Binnig's student, Franz Josef Gissible. It was he who suggested 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 the non-contact AFM: the role of the probe-sensor was played not by a sharp diamond crystal, but by one molecule - carbon monoxide. This allowed the transition to subatomic resolution, as demonstrated by Leo Gross from the Zurich division of IBM. In 2009, with the help of AFM, he made visible no longer atoms, but chemical bonds, having obtained 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 AFM could see a chemical bond, 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 the difference in bond orders indicates different values of the electron density and different interatomic distances between two atoms (more simply, a double bond is shorter than a single one). In ethane, the order of the carbon-carbon bond is one, in ethylene - two, and in the classical aromatic molecule - benzene - the order of the carbon-carbon bond is greater than one, but less than two, and is considered equal to 1.5.
It is much more difficult to determine the bond order when going from simple aromatic systems to planar or bulky polycondensed ring 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 Monn, showed that an atomic force microscope with a metal non-contact probe modified with carbon monoxide can measure differences in the charge distribution of atoms and interatomic distances - that is, parameters associated with the order of the bond ( 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 for two six-membered carbon-containing rings of C 60 fullerene, and a carbon-carbon bond, common for five- and six-membered rings. The atomic force microscope showed that the condensation of six-membered rings results in a bond that is shorter and with a higher 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 cycles are symmetrically located around the central C 6 cycle, confirmed the results of quantum chemical modeling, according to which the order of 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.
The bond orders and interatomic distances, of course, were of interest to organic chemists, but it was more important for those who were engaged in the theory of chemical bonds, predicting reactivity, and studying the mechanisms of chemical reactions. Nevertheless, both synthetic chemists and specialists in the study of the structure of natural compounds were in for a surprise: it turned out that an atomic force microscope can be used to determine the structure of molecules in the same way as NMR or IR spectroscopy. Moreover, it provides an unambiguous answer to questions that these methods are unable to cope with.
From photography to cinematography
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; however, analysis of the NMR spectra of this compound did not give an unambiguous answer to the question of its structure: four variants were possible. Using an atomic force microscope, the researchers immediately excluded two of the four structures, and made the right choice of the remaining two by comparing the results obtained from 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, moreover, this molecule does not have a plane of symmetry (Fig. 5) - but such a problem was also solved.
Another confirmation that the atomic force microscope can be used as an analytical tool came from the group of Oskar Kustanz, 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). Kostanz investigated 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 out that the force arising between the tip of the AFM probe and different atoms is different (Fig. 6). For example, the strongest interaction was observed when probing silicon, and the weakest 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 comparison of relative values. Since the exact composition of the probe needle is difficult to control, the absolute value of the force between the probe 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 probe 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" is created from the products and intermediates of the reaction, which can then be edited as a kind of documentary film ( Science, 2013, 340, 6139, 1434-1437; doi: 10.1126 / science.1238187).
Felix Fischer and Michael Crommey 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 condensed five six-membered and two five-membered rings. Another quarter of the molecules formed structures consisting of four six-membered rings linked through one four-membered cycle and two five-membered rings (Fig. 7). The rest of the products were oligomeric structures and, in small amounts, polycyclic isomers.
These results surprised researchers twice. First, only two main products were formed during the reaction. Secondly, their structure was surprising. Fischer notes that chemical intuition and experience allowed him to draw dozens of possible reaction products, but none of them corresponded to the compounds that formed on the surface. Possibly, the course of atypical chemical processes was facilitated by the interaction of the initial substances with the substrate.
Naturally, after the first serious advances in the study of chemical bonds, some researchers decided to use AFM to observe weaker and less studied intermolecular interactions, in particular, hydrogen bonds. However, work in this area is just beginning, and the results are contradictory. So, in some publications it is reported that atomic force microscopy made it possible to observe the hydrogen bond ( Science, 2013, 342, 6158, 611–614, doi: 10.1126 / science.1242603), in others they argue that these are just artifacts due to the design features of the device, and the experimental results need to be interpreted more accurately ( Physical Review Letters, 2014, 113, 186102, doi: 10.1103 / PhysRevLett.113.186102). Perhaps the final answer to the question of whether it is possible to observe hydrogen and other intermolecular interactions using atomic force microscopy will be obtained already in this decade. To do this, it is necessary to at least several times increase the AFM resolution and learn how to obtain images without interference ( Physical Review B, 2014, 90, 085421, doi: 10.1103 / PhysRevB.90.085421).
Synthesis of one molecule
In skillful hands, both STM and AFM are transformed from devices capable of studying matter into devices capable of directionally 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 instead of moles or millimoles of reactants.
For example, in 2016, an international team of scientists led by Takashi Kumagai used non-contact atomic force microscopy to transfer a porphysene molecule from one form to another ( Nature chemistry, 2016, 8, 935-940, doi: 10.1038 / nchem.2552). Porphycene can be regarded as a modification of porphyrin, the internal cycle 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).
A group led by the indefatigable Leo Gross also showed that it is possible to initiate the reaction of a single molecule - they converted dibromoanthracene 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 with an atomic force microscope.
The combined use of a scanning tunneling microscope and an atomic force microscope 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 a triangulene, an unstable aromatic biradical, the existence of which was predicted six decades ago, but all attempts at synthesis were unsuccessful (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 assumed 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 repeat on the surface the reactions that are well known to "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 non-living to living
Cantilevers and probes of atomic force microscopes can be used not only for analytical research or synthesis of exotic molecules, but also for solving applied problems. There are already known cases of the use of 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 the development of the principle of atomic force microscopy and the creation of AFM.
So, Gerber was able to teach AFM to determine the point mutation of ribonucleic acid in melanoma (on the material obtained as a result of biopsy). For this, the gold cantilever of the atomic force microscope was modified with oligonucleotides, which can enter into intermolecular interaction with RNA, and the strength of this interaction can still be measured due to the piezoelectric effect. The sensitivity of the AFM sensor is so great that they are already trying to use it to study the efficiency of the popular CRISPR-Cas9 genome editing method. It brings together technologies created by different generations of researchers.
Paraphrasing the classic of one of the political theories, we can say that we already see the endless possibilities and inexhaustibility of atomic force microscopy and are hardly able to imagine what awaits us ahead in connection with the further development of these technologies. But already today, the scanning tunneling microscope and the atomic force microscope enable us 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 that can touch and control this microcosm.
An atom (from the Greek "indivisible") is once the smallest particle of a substance of microscopic size, the smallest part of a chemical element that carries its properties. The constituents of an atom - protons, neutrons, electrons - no longer have these properties and form them together. Covalent atoms form molecules. Scientists study the features of the atom, and although they are already quite well studied, they do not miss the opportunity to find something new - in particular, in the field of creating new materials and new atoms (continuing the periodic table). 99.9% of the mass of an atom falls on the nucleus.
Don't be intimidated by the headline. The black hole, accidentally created by the staff of the SLAC National Accelerator Laboratory, turned out to be only one atom in size, so nothing threatens us. And the name "black hole" only vaguely describes the phenomenon observed by researchers. We have repeatedly told you about the world's most powerful X-ray laser, called
Indeed, the author of RTCh in his "reflections" went so far as to provoke heavy counter-arguments, namely, the data of the experiment of Japanese scientists on photographing the hydrogen atom, which became known on November 4, 2010. The image clearly shows the atomic shape, which confirms both discreteness and roundness of atoms: “A group of scientists and specialists from the University of Tokyo for the first time in the world photographed a single hydrogen atom - the lightest and smallest of all atoms, according to news agencies.
The picture was taken using one of the latest technologies - a special scanning electron microscope. With the help of this device, together with the hydrogen atom, a single vanadium atom was photographed.
The diameter of a hydrogen atom is one ten-billionth of a meter. Previously it was believed that it was almost impossible to photograph it with modern equipment. Hydrogen is the most abundant substance. Its part in the entire Universe is approximately 90%.
According to scientists, other elementary particles can be captured in the same way. “Now we can see all the atoms that make up our world,” said Professor Yuichi Ikuhara. "This is a breakthrough to new forms of production, when in the future it will be possible to make decisions at the level of individual atoms and molecules."
Hydrogen atom, conventional colors
http://prl.aps.org/abstract/PRL/v110/i21/e213001
A group of scientists from Germany, Greece, the Netherlands, USA and France took pictures of the hydrogen atom. These images, obtained with a photoionization microscope, show the electron density distribution, which completely coincides with the results of theoretical calculations. The work of the international group is presented on the pages of Physical Review Letters.
The essence of the photoionization method consists in the sequential ionization of hydrogen atoms, that is, in the detachment of an electron from them due to electromagnetic irradiation. The separated electrons are directed to the sensitive matrix through a positively charged ring, and the position of the electron at the moment of collision with the matrix reflects the position of the electron at the moment of ionization of the atom. The charged ring, which deflects electrons to the side, acts as a lens and with its help the image is magnified millions of times.
This method, described in 2004, was already used to take “photographs” of individual molecules, but physicists went further and used a photoionization microscope to study hydrogen atoms. Since the impact of one electron gives only one point, the researchers accumulated about 20 thousand individual electrons from different atoms and compiled an average image of the electron shells.
According to the laws of quantum mechanics, an electron in an atom does not have any definite position by itself. Only when an atom interacts with the external environment, an electron with some probability is manifested in a certain vicinity of the atomic nucleus: the region in which the probability of detecting an electron is maximum is called the electron shell. The new images show the differences between atoms of different energy states; scientists were able to visually demonstrate the shape of the electron shells predicted by quantum mechanics.
With other instruments, scanning tunneling microscopes, individual atoms can not only be seen, but also moved to the desired location. About a month ago, this technique allowed IBM engineers to draw a cartoon, each frame of which is composed of atoms: such artistic experiments do not have any practical effect, but demonstrate the fundamental possibility of manipulating atoms. For applied purposes, it is no longer an atomic assembly that is used, but chemical processes with self-organization of nanostructures or self-limitation of the growth of monatomic layers on a substrate.
As you know, everything material in the Universe consists of atoms. An atom is the smallest unit of matter that carries its properties. In turn, the structure of the atom is made up of the magic trinity of microparticles: protons, neutrons and electrons.
Moreover, each of the microparticles is universal. That is, you cannot find two different protons, neutrons or electrons in the world. They are all absolutely alike. And the properties of the atom will depend only on the quantitative composition of these microparticles in the general structure of the atom.
For example, the structure of a hydrogen atom consists of one proton and one electron. Next in complexity, the helium atom is made up of two protons, two neutrons, and two electrons. The lithium atom is made up of three protons, four neutrons and three electrons, etc.
Atomic structure (from left to right): hydrogen, helium, lithium
Atoms combine into molecules, and molecules - into substances, minerals and organisms. The DNA molecule, which is the basis of all living things, is a structure assembled from the same three magic bricks of the universe as a stone lying on the road. Although this structure is much more complex.
Even more surprising facts are revealed when we try to take a closer look at the proportions and structure of the atomic system. It is known that an atom consists of a nucleus and electrons moving around it along a trajectory that describes a sphere. That is, it cannot even be called a movement in the usual sense of the word. The electron is rather found everywhere and immediately within this sphere, creating an electron cloud around the nucleus and forming an electromagnetic field.
Schematic representations of the structure of the atom
The nucleus of an atom consists of protons and neutrons, and almost all the mass of the system is concentrated in it. But at the same time, the nucleus itself is so small that if you increase its radius to a scale of 1 cm, then the radius of the entire atomic structure will reach hundreds of meters. Thus, everything that we perceive as dense matter consists of more than 99% of energy connections between physical particles and less than 1% of the physical forms themselves.
But what are these physical forms? What are they made of, and how material are they? To answer these questions, let's take a closer look at the structures of protons, neutrons and electrons. So, we descend one more step into the depths of the microworld - to the level of subatomic particles.
What does an electron consist of?
The smallest particle in an atom is an electron. An electron has mass, but it has no volume. In the scientific view, the electron does not consist of anything, but is a structureless point.
An electron cannot be seen under a microscope. It is observed only in the form of an electron cloud, which looks like a blurry sphere around the atomic nucleus. At the same time, it is impossible to say with accuracy where the electron is at the moment of time. Devices are able to capture not the particle itself, but only its energy trace. The essence of the electron is not embedded in the concept of matter. Rather, it is like a kind of empty form that exists only in motion and due to motion.
So far, no structure has been found in the electron. It is the same pointlike particle as a quantum of energy. In fact, the electron is energy, however, it is a more stable form of it than that which is represented by the photons of light.
At the moment, the electron is considered indivisible. This is understandable, because it is impossible to divide what has no volume. However, in theory there are already developments, according to which the composition of the electron contains the triunity of such quasiparticles as:
- Orbiton - contains information about the orbital position of the electron;
- Spinon is responsible for spin or torque;
- Holon - carries information about the charge of an electron.
However, as we can see, quasiparticles with matter no longer have absolutely nothing in common, and carry only one information.
Photos of atoms of different substances in an electron microscope
Interestingly, an electron can absorb quanta of energy, such as light or heat. In this case, the atom moves to a new energy level, and the boundaries of the electron cloud expand. It also happens that the energy absorbed by the electron is so great that it can jump out of the atomic system, and then continue its motion as an independent particle. At the same time, it behaves like a photon of light, that is, it seems to cease to be a particle and begins to manifest the properties of a wave. This has been proven experimentally.
Jung's experiment
In the course of the experiment, a stream of electrons was directed onto a screen with two slits cut through it. Passing through these slots, electrons collided with the surface of another - projection - screen, leaving their mark on it. As a result of this "bombardment" with electrons, an interference pattern appeared on the projection screen, similar to that which would appear if waves, but not particles, would pass through the two slits.
Such a pattern arises due to the fact that a wave, passing between two slots, is divided into two waves. As a result of further movement, the waves overlap each other, and in some areas their mutual damping occurs. As a result, we get many stripes on the projection screen, instead of one as it would be if the electron behaved like a particle.
The structure of the nucleus of an atom: protons and neutrons
Protons and neutrons make up the nucleus of an atom. And despite the fact that the core occupies less than 1% of the total volume, it is in this structure that almost the entire mass of the system is concentrated. But at the expense of the structure of protons and neutrons, physicists were divided, and at the moment there are two theories at once.
- Theory # 1 - Standard
The Standard Model says that protons and neutrons are made up of three quarks connected by a cloud of gluons. Quarks are point particles, just like quanta and electrons. And gluons are virtual particles that ensure the interaction of quarks. However, neither quarks nor gluons have been found in nature, therefore this model lends itself to severe criticism.
- Theory # 2 - Alternative
But according to the alternative theory of a unified field, developed by Einstein, a proton, like a neutron, like any other particle of the physical world, is an electromagnetic field rotating at the speed of light.
Human and planet electromagnetic fields
What are the principles of the structure of the atom?
Everything in the world - thin and dense, liquid, solid and gaseous - is only the energy states of countless fields that permeate the space of the Universe. The higher the level of energy in the field, the thinner and less perceptible it is. The lower the energy level, the more stable and tangible it is. In the structure of the atom, as in the structure of any other unit of the Universe, there is the interaction of such fields - different in energy density. It turns out that matter is only an illusion of the mind.
Physicists from the United States managed to photograph individual atoms with a record resolution, Day.Az reports with reference to Vesti.ru
Scientists from Cornell University in the United States managed to capture individual atoms in a photo with a record resolution - less than half an angstrom (0.39 Å). Previous photographs had twice the low resolution - 0.98 Å.
Powerful electron microscopes that can see atoms have been around for half a century, but their resolution is limited by the wavelength of visible light, which is larger than the average diameter of an atom.
Therefore, scientists use a kind of analogue of lenses that focus and magnify the image in electron microscopes - they are a magnetic field. However, fluctuations in the magnetic field distort the result. To remove distortions, additional devices are used that correct the magnetic field, but at the same time increase the complexity of the electron microscope design.
Earlier, physicists at Cornell University developed the Electron Microscope Pixel Array Detector (EMPAD) device, replacing a complex system of generators that focus incoming electrons with one small matrix with a resolution of 128x128 pixels, sensitive to individual electrons. Each pixel records the angle of reflection of an electron; Knowing it, scientists, using the ptycographic technique, reconstruct the characteristics of electrons, including the coordinates of the point from where it was released.
Atoms in the highest resolution
David A. Muller et al. Nature, 2018.
In the summer of 2018, physicists decided to improve the quality of the resulting images to a record resolution to date. The scientists attached a sheet of 2D material, molybdenum sulfide MoS2, to a movable beam, and released beams of electrons, turning the beam at different angles to the electron source. Using EMPAD and ptycography, the scientists determined the distances between individual molybdenum atoms and obtained an image with a record resolution of 0.39 Å.
“In fact, we have created the world's smallest line,” explains Sol Gruner, one of the authors of the experiment. In the resulting image, it was possible to discern sulfur atoms with a record resolution of 0.39 Å. Moreover, it was even possible to discern a place where one such atom is missing (indicated by an arrow).
Sulfur atoms in record resolution