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 latticeThere 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 latticeIn 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.
It is not individual atoms or molecules that enter into chemical interactions, but substances. Substances are classified according to the type of bond molecular and non-molecular buildings.
These are substances made up of molecules. The bonds between molecules in such substances are very weak, much weaker than between atoms inside the molecule, and even at relatively low temperatures they break - the substance turns into a liquid and then into a gas (sublimation of iodine). The melting and boiling points of substances consisting of molecules increase with increasing molecular weight. Molecular substances include substances with an atomic structure (C, Si, Li, Na, K, Cu, Fe, W), among them there are metals and non-metals.
Non-molecular structure of substances
To substances non-molecular structures include ionic compounds. Most compounds of metals with non-metals have this structure: all salts (NaCl, K 2 S0 4), some hydrides (LiH) and oxides (CaO, MgO, FeO), bases (NaOH, KOH). Ionic (non-molecular) substances have high melting and boiling points.
Solids: crystalline and amorphous
Amorphous substances they do not have a clear melting point - when heated, they gradually soften and turn into a fluid state. For example, plasticine and various resins are in an amorphous state.
Crystalline substances characterized by the correct arrangement of the particles of which they consist: atoms, molecules and ions - at strictly defined points in space. When these points are connected by straight lines, a spatial frame is formed, called crystal lattice. The points at which crystal particles are located are called lattice nodes.
Depending on the type of particles located at the nodes of the crystal lattice and the nature of the connection between them, four types of crystal lattices are distinguished: ionic, atomic, molecular and metallic .
Ionic crystal lattices
Ionic are called crystal lattices, in the nodes of which there are ions. They are formed by substances with ionic bonds, which can bind both simple ions Na +, Cl -, and complex S0 4 2-, OH -. Consequently, salts and some oxides and hydroxides of metals have ionic crystal lattices. For example, a sodium chloride crystal is built from alternating positive Na + and negative Cl - ions, forming a cube-shaped lattice.
Ionic crystal lattice of table salt
The bonds between ions in such a crystal are very stable. Therefore, substances with an ionic lattice are characterized by relatively high hardness and strength, they are refractory and non-volatile.
Atomic crystal lattices
Atomic are called crystal lattices, in the nodes of which there are individual atoms. In such lattices, the atoms are connected to each other by very strong covalent bonds. An example of substances with this type of crystal lattices is diamond, one of the allotropic modifications of carbon.
Atomic crystal lattice of diamond
Most substances with an atomic crystal lattice have very high melting points (for example, for diamond it is over 3500 ° C), they are strong and hard, and practically insoluble.
Molecular crystal lattices
Molecular called crystal lattices, in the nodes of which molecules are located.
Molecular crystal lattice of iodine
Chemical bonds in these molecules can be both polar (HCl, H 2 O) and non-polar (N 2, O 2). Despite the fact that the atoms inside the molecules are connected by very strong covalent bonds, weak intermolecular forces of attraction act between the molecules themselves. Therefore, substances with molecular crystal lattices have low hardness, low melting points, and are volatile. Most solid organic compounds have molecular crystal lattices (naphthalene, glucose, sugar).
Metal crystal lattices
Substances with metallic bonds have metal crystal lattices.
At the sites of such lattices there are atoms and ions (either atoms or ions, into which metal atoms easily transform, giving up their outer electrons “for common use”). This internal structure of metals determines their characteristic physical properties: malleability, ductility, electrical and thermal conductivity, characteristic metallic luster.
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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.
When carrying out many physical and chemical reactions, a substance passes into a solid state of aggregation. In this case, molecules and atoms tend to arrange themselves in such a spatial order in which the forces of interaction between particles of matter would be maximally balanced. This is how the strength of the solid substance is achieved. Atoms, once occupying a certain position, perform small oscillatory movements, the amplitude of which depends on temperature, but their position in space remains fixed. The forces of attraction and repulsion balance each other at a certain distance.
Modern ideas about the structure of matter
Modern science states that an atom consists of a charged nucleus, which carries a positive charge, and electrons, which carry negative charges. At a speed of several thousand trillion revolutions per second, electrons rotate in their orbits, creating an electron cloud around the nucleus. The positive charge of the nucleus is numerically equal to the negative charge of the electrons. Thus, the atom of the substance remains electrically neutral. Possible interactions with other atoms occur when electrons are detached from their parent atom, thereby disturbing the electrical balance. In one case, the atoms are arranged in a certain order, which is called a crystal lattice. In another, due to the complex interaction of nuclei and electrons, they are combined into molecules of various types and complexity.
Definition of crystal lattice
Taken together, various types of crystalline lattices of substances are networks with different spatial orientations, at the nodes of which ions, molecules or atoms are located. This stable geometric spatial position is called the crystal lattice of the substance. The distance between nodes of one crystal cell is called the identity period. The spatial angles at which the cell nodes are located are called parameters. According to the method of constructing bonds, crystal lattices can be simple, base-centered, face-centered, and body-centered. If the particles of matter are located only in the corners of the parallelepiped, such a lattice is called simple. An example of such a lattice is shown below:
If, in addition to the nodes, the particles of the substance are located in the middle of the spatial diagonals, then this arrangement of particles in the substance is called a body-centered crystal lattice. This type is clearly shown in the figure.
If, in addition to the nodes at the vertices of the lattice, there is a node at the place where the imaginary diagonals of the parallelepiped intersect, then you have a face-centered type of lattice.
Types of crystal lattices
The different microparticles that make up a substance determine the different types of crystal lattices. They can determine the principle of building connections between microparticles inside a crystal. Physical types of crystal lattices are ionic, atomic and molecular. This also includes various types of metal crystal lattices. Chemistry studies the principles of the internal structure of elements. The types of crystal lattices are presented in more detail below.
Ionic crystal lattices
These types of crystal lattices are present in compounds with an ionic type of bond. In this case, lattice sites contain ions with opposite electrical charges. Thanks to the electromagnetic field, the forces of interionic interaction are quite strong, and this determines the physical properties of the substance. Common characteristics are refractoriness, density, hardness and the ability to conduct electric current. Ionic types of crystal lattices are found in substances such as table salt, potassium nitrate and others.
Atomic crystal lattices
This type of structure of matter is inherent in elements whose structure is determined by covalent chemical bonds. Types of crystal lattices of this kind contain individual atoms at the nodes, connected to each other by strong covalent bonds. This type of bond occurs when two identical atoms “share” electrons, thereby forming a common pair of electrons for neighboring atoms. Thanks to this interaction, covalent bonds bind atoms evenly and strongly in a certain order. Chemical elements that contain atomic types of crystal lattices are hard, have a high melting point, are poor conductors of electricity, and are chemically inactive. Classic examples of elements with a similar internal structure include diamond, silicon, germanium, and boron.
Molecular crystal lattices
Substances that have a molecular type of crystal lattice are a system of stable, interacting, closely packed molecules that are located at the nodes of the crystal lattice. In such compounds, the molecules retain their spatial position in the gaseous, liquid and solid phases. At the nodes of the crystal, molecules are held together by weak van der Waals forces, which are tens of times weaker than the ionic interaction forces.
The molecules that form a crystal can be either polar or nonpolar. Due to the spontaneous movement of electrons and vibrations of nuclei in molecules, the electrical equilibrium can shift - this is how an instantaneous electric dipole moment arises. Appropriately oriented dipoles create attractive forces in the lattice. Carbon dioxide and paraffin are typical examples of elements with a molecular crystal lattice.
Metal crystal lattices
A metal bond is more flexible and ductile than an ionic bond, although it may seem that both are based on the same principle. The types of crystal lattices of metals explain their typical properties - such as mechanical strength, thermal and electrical conductivity, and fusibility.
A distinctive feature of a metal crystal lattice is the presence of positively charged metal ions (cations) at the sites of this lattice. Between the nodes there are electrons that are directly involved in creating an electric field around the lattice. The number of electrons moving around within this crystal lattice is called electron gas.
In the absence of an electric field, free electrons perform chaotic motion, randomly interacting with lattice ions. Each such interaction changes the momentum and direction of motion of the negatively charged particle. With their electric field, electrons attract cations to themselves, balancing their mutual repulsion. Although electrons are considered free, their energy is not enough to leave the crystal lattice, so these charged particles are constantly within its boundaries.
The presence of an electric field gives the electron gas additional energy. The connection with ions in the crystal lattice of metals is not strong, so electrons easily leave its boundaries. Electrons move along lines of force, leaving behind positively charged ions.
conclusions
Chemistry attaches great importance to the study of the internal structure of matter. The types of crystal lattices of various elements determine almost the entire range of their properties. By influencing crystals and changing their internal structure, it is possible to enhance the desired properties of a substance and remove undesirable ones and transform chemical elements. Thus, studying the internal structure of the surrounding world can help to understand the essence and principles of the structure of the universe.