The molecular crystal lattice is characteristic of. Crystal lattices – Knowledge Hypermarket

Most substances are characterized by the ability, depending on conditions, to be in one of three states of aggregation: solid, liquid or gaseous.

For example, water at normal pressure in the temperature range 0-100 o C is a liquid, at temperatures above 100 o C it can only exist in a gaseous state, and at temperatures below 0 o C it is a solid.
Substances in the solid state are divided into amorphous and crystalline.

A characteristic feature of amorphous substances is the absence of a clear melting point: their fluidity gradually increases with increasing temperature. Amorphous substances include compounds such as wax, paraffin, most plastics, glass, etc.

Still, crystalline substances have a specific melting point, i.e. a substance with a crystalline structure passes from a solid to a liquid state not gradually, but abruptly, upon reaching a specific temperature. Examples of crystalline substances include table salt, sugar, and ice.

The difference in the physical properties of amorphous and crystalline solids is primarily due to the structural features of such substances. What is the difference between a substance in an amorphous and a crystalline state can be most easily understood from the following illustration:

As you can see, in an amorphous substance, unlike a crystalline one, there is no order in the arrangement of particles. If in a crystalline substance you mentally connect two atoms close to each other with a straight line, you can find that the same particles will lie on this line at strictly defined intervals:

Thus, in the case of crystalline substances, we can talk about such a concept as a crystal lattice.

Crystal lattice called a spatial framework connecting the points in space in which the particles that form the crystal are located.

The points in space at which the particles forming the crystal are located are called crystal lattice nodes .

Depending on which particles are located at the nodes of the crystal lattice, they are distinguished: molecular, atomic, ionic And metal crystal lattices .

In nodes molecular crystal lattice

Ice crystal lattice as an example of a molecular lattice

There are molecules within which the atoms are connected by strong covalent bonds, but the molecules themselves are held near each other by weak intermolecular forces. Due to such weak intermolecular interactions, crystals with a molecular lattice are fragile. Such substances differ from substances with other types of structure by significantly lower melting and boiling points, do not conduct electric current, and may or may not dissolve in various solvents. Solutions of such compounds may or may not conduct electric current, depending on the class of the compound. Compounds with a molecular crystal lattice include many simple substances - non-metals (hardened H 2, O 2, Cl 2, orthorhombic sulfur S 8, white phosphorus P 4), as well as many complex substances - hydrogen compounds of non-metals, acids, non-metal oxides, most organic substances. It should be noted that if a substance is in a gaseous or liquid state, it is inappropriate to talk about a molecular crystal lattice: it is more correct to use the term molecular type of structure.

Diamond crystal lattice as an example of an atomic lattice

In nodes atomic crystal lattice

there are atoms. Moreover, all the nodes of such a crystal lattice are “linked” together through strong covalent bonds into a single crystal. In fact, such a crystal is one giant molecule. Due to their structural features, all substances with an atomic crystal lattice are solid, have high melting points, are chemically inactive, insoluble in either water or organic solvents, and their melts do not conduct electric current. It should be remembered that substances with an atomic type of structure include boron B, carbon C (diamond and graphite), silicon Si from simple substances, and silicon dioxide SiO 2 (quartz), silicon carbide SiC, boron nitride BN from complex substances.

For substances with ionic crystal lattice

lattice sites contain ions connected to each other through ionic bonds.
Since ionic bonds are quite strong, substances with an ionic lattice have relatively high hardness and refractoriness. Most often, they are soluble in water, and their solutions, like melts, conduct electric current.
Substances with an ionic crystal lattice include metal and ammonium salts (NH 4 +), bases, and metal oxides. A sure sign of the ionic structure of a substance is the presence in its composition of both atoms of a typical metal and a non-metal.

Crystal lattice of sodium chloride as an example of an ionic lattice

observed in crystals of free metals, for example, sodium Na, iron Fe, magnesium Mg, etc. In the case of a metal crystal lattice, its nodes contain cations and metal atoms, between which electrons move. In this case, moving electrons periodically attach to cations, thus neutralizing their charge, and individual neutral metal atoms in return “release” some of their electrons, turning, in turn, into cations. In fact, “free” electrons do not belong to individual atoms, but to the entire crystal.

Such structural features lead to the fact that metals conduct heat and electric current well and often have high ductility (malleability).
The spread of melting temperatures of metals is very large. For example, the melting point of mercury is approximately minus 39 ° C (liquid under normal conditions), and tungsten is 3422 ° C. It should be noted that under normal conditions all metals except mercury are solids.

What is our resource for?

The main goal of our site is to help pupils and students who have difficulties solving a particular task, or who have missed any school topic. Our resource will also come to the aid of parents of students who are faced with difficulties in checking their children’s homework.

On our resource you can find ready-made homework for any grade from 1st to 11th grade in all academic subjects. For example, you can find GDZ in mathematics, foreign languages, physics, biology, literature, etc. To do this, you simply need to select the desired class, the required subject and the GDZ workbooks of suitable authors, after which you need to find the required section and get an answer to the assigned task. GDs allow you to quickly check the task assigned to the student at home, as well as prepare the child for the test.

How to get an A on your homework?

To do this, you need to go to our resource, where ready-made homework assignments for all disciplines of the school curriculum are posted. At the same time, you don’t need to worry about errors, typos and other shortcomings in the GDZ, because all the manuals posted with us were checked by experienced specialists. All answers to homework assignments are correct, so we can confidently say that for any of them you will receive an A! But you shouldn’t thoughtlessly copy everything into your notebook; on the contrary, you need to do the tasks yourself, then check them with the help of the GDZ and only after that rewrite them into a clean copy. This will allow you to gain the necessary knowledge and a high grade.

GDZ online

Now no one has problems accessing the GDZ, because our Internet resource is adapted for all modern devices: PCs, laptops, tablets and smartphones that have Internet access. Now, even during recess, you can go to our website from your phone and find out the answer to absolutely any task. Convenient navigation and fast loading of the site allows you to search and view GDZ as quickly and comfortably as possible. Access to our resource is free, and registration is very fast.

GDZ of the new program

The school curriculum changes periodically, so students constantly need new teaching aids, textbooks and GDs. Our specialists constantly monitor innovations and, after their implementation, immediately post new textbooks and GDs on the resource so that users have the latest editions available. Our resource is a kind of library for schoolchildren, which any student needs for successful studies. Almost every year the school curriculum becomes more complex, with new subjects and materials being introduced. Studying is becoming increasingly difficult, but our website makes life easier for parents and students.

Help for students

We do not forget about the complex, busy life of students. Each new academic year raises the bar in terms of knowledge, so not all students are able to cope with such a high load. Long classes, various abstracts, laboratory and dissertation work occupy almost all of the students’ free time. With the help of our website, any student can make their daily life easier. To do this, almost every day our specialists post new works on the portal. Now students can find cheat sheets for any task with us, and completely free of charge.

Now you don’t need to carry a huge number of textbooks to school every day

To take care of schoolchildren, our specialists have posted all school textbooks on the website in the public domain. Therefore, today any student or parent can use them, and students no longer have to strain their backs every day by carrying heavy textbooks to school. It is enough to download the necessary textbooks to your tablet, phone or other modern device, and the textbooks will always be with you anywhere. You can also read them online directly on the website - it’s very comfortable, fast and completely free.

Ready-made school essays

If you are suddenly required to write an essay about a book, then remember that on our website you can always find a huge number of ready-made school essays that were written by masters of words and approved by teachers. We are expanding the list of essays every day, writing new essays on many topics and taking into account user recommendations. This allows us to meet the daily needs of all schoolchildren.

For independent writing of essays, we have provided abbreviated works; they can also be viewed and downloaded on the website. They contain the main meaning of school literary works, which significantly reduces the study of books and saves the student’s energy, which he requires to study other subjects.

Presentations on various topics

If you urgently need to make a school presentation on a specific topic that you know nothing about, then with the help of our website you can do it. Now you don’t need to spend a lot of time searching for images, photographs, printed information and consulting on the topic with experts, etc., because our resource creates high-quality presentations with multimedia content on any topic. Our experts have posted a large number of author’s presentations on the website, which can be viewed and downloaded for free. Therefore, learning will be more educational and comfortable for you, because you will have more time for rest and other subjects.

Our advantages:

* large database of books and public records;

* materials are updated daily;

* access from any modern gadget;

* we take into account the wishes of users;

* we make the lives of pupils, students and parents freer and more joyful.

We are constantly improving our resource to make the lives of our users more comfortable and carefree. With the help of gdz.host you will be an excellent student, so great prospects will open up for you in adult life. As a result, your parents will be proud of you because you will be a good example for all people.

Page 1

Molecular crystal lattices and the corresponding molecular bonds are formed predominantly in crystals of those substances in whose molecules the bonds are covalent. When heated, the bonds between molecules are easily destroyed, which is why substances with molecular lattices have low melting points.

Molecular crystal lattices are formed from polar molecules, between which interaction forces arise, the so-called van der Waals forces, which are electrical in nature. In the molecular lattice they form a rather weak bond. Ice, natural sulfur and many organic compounds have a molecular crystal lattice.

The molecular crystal lattice of iodine is shown in Fig. 3.17. Most crystalline organic compounds have a molecular lattice.

The nodes of a molecular crystal lattice are formed by molecules. For example, crystals of hydrogen, oxygen, nitrogen, noble gases, carbon dioxide, and organic substances have a molecular lattice.

The presence of a molecular crystal lattice of the solid phase is the reason for the insignificant adsorption of ions from the mother liquor, and, consequently, for the much higher purity of the precipitates compared to precipitates characterized by an ionic crystal. Since precipitation in this case occurs in the optimal acidity region, which is different for the ions precipitated by this reagent, it depends on the value of the corresponding stability constants of the complexes. This fact allows, by adjusting the acidity of the solution, to achieve selective and sometimes even specific precipitation of certain ions. Similar results can often be obtained by appropriate modification of the donor groups in organic reagents, taking into account the characteristics of the complexing cations that are precipitated.

In molecular crystal lattices, local anisotropy of bonds is observed, namely: intramolecular forces are very large compared to intermolecular ones.

In molecular crystal lattices, molecules are located at lattice sites. Most substances with covalent bonds form crystals of this type. Molecular lattices form solid hydrogen, chlorine, carbon dioxide and other substances that are gaseous at ordinary temperatures. Crystals of most organic substances also belong to this type. Thus, a lot of substances with a molecular crystal lattice are known.

In molecular crystal lattices, the constituent molecules are connected to each other using relatively weak van der Waals forces, while the atoms within the molecule are connected by much stronger covalent bonds. Therefore, in such lattices the molecules retain their individuality and occupy one site of the crystal lattice. Substitution here is possible if the molecules are similar in shape and size. Since the forces connecting molecules are relatively weak, the boundaries of substitution here are much wider. As Nikitin showed, atoms of noble gases can isomorphically replace molecules of CO2, SO2, CH3COCH3 and others in the lattices of these substances. The similarity of the chemical formula is not necessary here.

In molecular crystal lattices, molecules are located at lattice sites. Most substances with covalent bonds form crystals of this type. Molecular lattices form solid hydrogen, chlorine, carbon dioxide and other substances that are gaseous at ordinary temperatures. Crystals of most organic substances also belong to this type. Thus, a lot of substances with a molecular crystal lattice are known. Molecules located at lattice sites are connected to each other by intermolecular forces (the nature of these forces was discussed above; see page. Since intermolecular forces are much weaker than chemical bonding forces, molecular crystals are low-melting, characterized by significant volatility, and their hardness is low. Particularly low the melting and boiling points of those substances whose molecules are non-polar. For example, paraffin crystals are very soft, although the C-C covalent bonds in the hydrocarbon molecules of which these crystals are composed are as strong as the bonds in diamond. Crystals formed by noble minerals gases, should also be classified as molecular, consisting of monatomic molecules, since valence forces do not play a role in the formation of these crystals, and the bonds between particles here are of the same nature as in other molecular crystals; this determines the relatively large interatomic distances in these crystals.

Debyegram registration scheme.

At the nodes of molecular crystal lattices there are molecules that are connected to each other by weak intermolecular forces. Such crystals form substances with covalent bonds in molecules. A lot of substances with a molecular crystal lattice are known. Molecular lattices contain solid hydrogen, chlorine, carbon dioxide and other substances that are gaseous at ordinary temperatures. Crystals of most organic substances also belong to this type.

Structure of matter.

It is not individual atoms or molecules that enter into chemical interactions, but substances.
Our task is to get acquainted with the structure of matter.

At low temperatures, substances are in a stable solid state.

☼ The hardest substance in nature is diamond. He is considered the king of all gems and precious stones. And its name itself means “indestructible” in Greek. Diamonds have long been looked upon as miraculous stones. It was believed that a person wearing diamonds does not know stomach diseases, is not affected by poison, retains his memory and a cheerful mood until old age, and enjoys royal favor.

☼ A diamond that has been subjected to jewelry processing - cutting, polishing - is called a diamond.

When melting as a result of thermal vibrations, the order of the particles is disrupted, they become mobile, while the nature of the chemical bond is not disrupted. Thus, there are no fundamental differences between solid and liquid states.
The liquid acquires fluidity (i.e., the ability to take the shape of a vessel).

Liquid crystals.

Liquid crystals were discovered at the end of the 19th century, but have been studied in the last 20-25 years. Many display devices of modern technology, for example, some electronic watches and mini-computers, operate on liquid crystals.

In general, the words “liquid crystals” sound no less unusual than “hot ice”. However, in reality, ice can also be hot, because... at a pressure of more than 10,000 atm. water ice melts at temperatures above 2000 C. The unusualness of the combination “liquid crystals” is that the liquid state indicates the mobility of the structure, and the crystal implies strict order.

If a substance consists of polyatomic molecules of an elongated or lamellar shape and having an asymmetrical structure, then when it melts, these molecules are oriented in a certain way relative to each other (their long axes are parallel). In this case, the molecules can move freely parallel to themselves, i.e. the system acquires the property of fluidity characteristic of a liquid. At the same time, the system retains an ordered structure, which determines the properties characteristic of crystals.

The high mobility of such a structure makes it possible to control it through very weak influences (thermal, electrical, etc.), i.e. purposefully change the properties of a substance, including optical ones, with very little energy expenditure, which is what is used in modern technology.

Types of crystal lattices.

Any chemical substance is formed by a large number of identical particles that are interconnected.
At low temperatures, when thermal movement is difficult, the particles are strictly oriented in space and form a crystal lattice.

Crystal cell is a structure with a geometrically correct arrangement of particles in space.

In the crystal lattice itself, nodes and internodal space are distinguished.
The same substance, depending on conditions (p, t,...), exists in different crystalline forms (i.e., they have different crystal lattices) - allotropic modifications that differ in properties.
For example, four modifications of carbon are known: graphite, diamond, carbyne and lonsdaleite.

☼ The fourth variety of crystalline carbon, “lonsdaleite,” is little known. It was discovered in meteorites and obtained artificially, and its structure is still being studied.

☼ Soot, coke, and charcoal were classified as amorphous polymers of carbon. However, it has now become known that these are also crystalline substances.

☼ By the way, shiny black particles were found in the soot, which were called “mirror carbon”. Mirror carbon is chemically inert, heat-resistant, impervious to gases and liquids, has a smooth surface and is absolutely compatible with living tissues.

☼ The name graphite comes from the Italian “graffito” - I write, I draw. Graphite is dark gray crystals with a weak metallic luster and has a layered lattice. Individual layers of atoms in a graphite crystal, connected to each other relatively weakly, are easily separated from each other.

TYPES OF CRYSTAL LATTICES

Properties of substances with different crystal lattices (table)

If the rate of crystal growth is low upon cooling, a glassy state (amorphous) is formed.

The relationship between the position of an element in the Periodic Table and the crystal lattice of its simple substance.

There is a close relationship between the position of an element in the periodic table and the crystal lattice of its corresponding elemental substance.

The simple substances of the remaining elements have a metallic crystal lattice.

FIXING

Study the lecture material and answer the following questions in writing in your notebook:
- What is a crystal lattice?
- What types of crystal lattices exist?
- Describe each type of crystal lattice according to the plan:

What is in the nodes of the crystal lattice, structural unit → Type of chemical bond between the particles of the node → Interaction forces between the particles of the crystal → Physical properties determined by the crystal lattice → Aggregate state of the substance under normal conditions → Examples

Complete tasks on this topic:

- What type of crystal lattice does the following substances widely used in everyday life have: water, acetic acid (CH3 COOH), sugar (C12 H22 O11), potassium fertilizer (KCl), river sand (SiO2) - melting point 1710 0C, ammonia (NH3) , salt? Make a general conclusion: by what properties of a substance can one determine the type of its crystal lattice?
Using the formulas of the given substances: SiC, CS2, NaBr, C2 H2 - determine the type of crystal lattice (ionic, molecular) of each compound and, based on this, describe the physical properties of each of the four substances.
Trainer No. 1. "Crystal lattices"
Trainer No. 2. "Test tasks"
Test (self-control):

1) Substances that have a molecular crystal lattice, as a rule:
a). refractory and highly soluble in water
b). fusible and volatile
V). Solid and electrically conductive
G). Thermally conductive and plastic

2) The concept of “molecule” is not applicable to the structural unit of a substance:

b). oxygen

V). diamond

3) The atomic crystal lattice is characteristic of:

a). aluminum and graphite

b). sulfur and iodine

V). silicon oxide and sodium chloride

G). diamond and boron

4) If a substance is highly soluble in water, has a high melting point, and is electrically conductive, then its crystal lattice is:

A). molecular

b). atomic

V). ionic

G). metal

Instructions

As you can easily guess from the name itself, the metal type of lattice is found in metals. These substances are usually characterized by a high melting point, metallic luster, hardness, and are good conductors of electric current. Remember that lattice sites of this type contain either neutral atoms or positively charged ions. In the spaces between the nodes there are electrons, the migration of which ensures the high electrical conductivity of such substances.

Ionic type of crystal lattice. It should be remembered that it is also inherent in salts. Characteristic - crystals of the well-known table salt, sodium chloride. Positively and negatively charged ions alternate alternately at the sites of such lattices. Such substances are usually refractory and have low volatility. As you might guess, they are of the ionic type.

The atomic type of crystal lattice is inherent in simple substances - nonmetals, which under normal conditions are solids. For example, sulfur, phosphorus,... At the sites of such lattices there are neutral atoms connected to each other by covalent chemical bonds. Such substances are characterized by refractoriness and insolubility in water. Some (for example, carbon in the form) have exceptionally high hardness.

Finally, the last type of lattice is molecular. It is found in substances that are under normal conditions in liquid or gaseous form. As again can be easily understood from, at the nodes of such lattices there are molecules. They can be either non-polar (for simple gases such as Cl2, O2) or polar (the most famous example is water H2O). Substances with this type of lattice do not conduct current, are volatile, and have low melting points.

Sources:

• grating type

Temperature melting of a solid is measured to determine its purity. Impurities in a pure substance usually lower the temperature melting or increase the interval over which the compound melts. The capillary method is a classic method for controlling impurities.

You will need

• - test substance;
• - glass capillary, sealed at one end (diameter 1 mm);
• - glass tube with a diameter of 6-8 mm and a length of at least 50 cm;
• - heated block.

Instructions

Place the glass tube vertically on a hard surface and drop the capillary through it several times, sealed end down. This helps compact the substance. To determine the temperature, the column of the substance in the capillary should be about 2-5 mm.

Place the capillary thermometer in the heated block and observe the changes in the test substance as the temperature increases. Before and during heating, the thermometer should not touch the walls of the block or other very hot surfaces, otherwise it may burst.

Note the temperature at which the first drops appear in the capillary (beginning melting), and the temperature at which the last substances disappear (end melting). In this interval, the substance begins to decrease until it completely transforms into a liquid state. When performing the analysis, also look for changes or decomposition of the substance.

Repeat measurements 1-2 more times. Present the results of each measurement in the form of the corresponding temperature interval during which the substance passes from solid to liquid. At the end of the analysis, make a conclusion about the purity of the test substance.

Video on the topic

In crystals, chemical particles (molecules, atoms and ions) are arranged in a certain order; under certain conditions they form regular symmetrical polyhedra. There are four types of crystal lattices - ionic, atomic, molecular and metallic.

Crystals

The crystalline state is characterized by the presence of long-range order in the arrangement of particles, as well as the symmetry of the crystal lattice. Solid crystals are three-dimensional formations in which the same structural element is repeated in all directions.

The correct shape of crystals is determined by their internal structure. If you replace molecules, atoms and ions in them with points instead of the centers of gravity of these particles, you get a three-dimensional regular distribution - . The repeating elements of its structure are called elementary cells, and the points are called nodes of the crystal lattice. There are several types of crystals depending on the particles that form them, as well as the nature of the chemical bond between them.

Ionic crystal lattices

Ionic crystals form anions and cations, between which there are. This type of crystal includes salts of most metals. Each cation is attracted to the anion and repelled by other cations, so it is impossible to isolate single molecules in an ionic crystal. The crystal can be considered as one huge one, and its size is not limited; it is capable of attaching new ions.

Atomic crystal lattices

In atomic crystals, individual atoms are united by covalent bonds. Like ionic crystals, they can also be thought of as huge molecules. At the same time, atomic crystals are very hard and durable, and do not conduct electricity and heat well. They are practically insoluble and are characterized by low reactivity. Substances with atomic lattices melt at very high temperatures.

Molecular crystals

Molecular crystal lattices are formed from molecules whose atoms are united by covalent bonds. Because of this, weak molecular forces act between molecules. Such crystals are characterized by low hardness, low melting point and high fluidity. The substances that they form, as well as their melts and solutions, do not conduct electric current well.

Metal crystal lattices

In metal crystal lattices, atoms are arranged with maximum density, their bonds are delocalized, and they extend throughout the entire crystal. Such crystals are opaque, have a metallic luster, are easily deformed, and are good conductors of electricity and heat.

This classification describes only limiting cases; most crystals of inorganic substances belong to intermediate types - molecular-covalent, covalent, etc. An example is a graphite crystal, inside each layer it has covalent-metallic bonds, and between the layers there are molecular ones.

Sources:

• alhimik.ru, Solids

Diamond is a mineral that belongs to one of the allotropic modifications of carbon. Its distinctive feature is its high hardness, which rightfully earns it the title of the hardest substance. Diamond is a fairly rare mineral, but at the same time it is the most widespread. Its exceptional hardness finds its application in mechanical engineering and industry.

Instructions

Diamond has an atomic crystal lattice. The carbon atoms that form the basis of the molecule are arranged in the form of a tetrahedron, which is why diamond has such high strength. All atoms are connected by strong covalent bonds, which are formed based on the electronic structure of the molecule.

The carbon atom has sp3 hybridized orbitals that are at an angle of 109 degrees and 28 minutes. The overlap of hybrid orbitals occurs in a straight line in the horizontal plane.

Thus, when the orbitals overlap at such an angle, a centered one is formed, which belongs to the cubic system, so we can say that diamond has a cubic structure. This structure is considered one of the strongest in nature. All tetrahedra form a three-dimensional network of layers of six-membered rings of atoms. Such a stable network of covalent bonds and their three-dimensional distribution leads to additional strength of the crystal lattice.