Molecular substances are substances, the smallest structural particles of which are molecules
Molecules are the smallest particle of a molecular substance that can exist independently and retain its chemical properties.
Molecular substances have low melting and boiling points and are in a solid, liquid or gaseous state under standard conditions.
For example: Water H 2 O - liquid, t pl \u003d 0 ° C; t bale \u003d 100 ° C; Oxygen O 2 - gas, t pl \u003d -219 ° C; t bale \u003d -183 ° C; Nitric oxide (V) N 2 O 5 - solid, t pl \u003d 30.3 ° C; t bale \u003d 45 ° C;
Molecular substances include:
most simple substances non-metals: O 2, S 8, P 4, H 2, N 2, Cl 2, F 2, Br 2, I 2;
connections of non-metals with each other (binary and multi-element): NH 3, CO 2, H 2 SO 4.
Non-molecular substances
Nonmolecular substances are substances, the smallest structural particles of which are atoms or ions.
An ion is an atom or group of atoms with a positive or negative charge.
For example: Na +, Cl -.
Nonmolecular substances are in standard conditions in a solid state of aggregation and have high melting and boiling points.
For example: sodium chloride NaCl - solid, t pl \u003d 801 ° C; t bale \u003d 1465 ° C; copper Cu - solid, t pl \u003d 1083 ° C; t bale \u003d 2573 ° C; silicon Si - solid, t pl \u003d 1420 ° C; t bale \u003d 3250 ° C;
Non-molecular substances include:
simple substances (metals): Na, Cu, Fe, ...;
alloys and compounds of metals with non-metals: NaH, Na 2 SO 4, CuCl 2, Fe 2 O 3;
non-metals: boron, silicon, carbon (diamond), phosphorus (black and red);
some binary compounds of non-metals: SiC, SiO 2.
The modern doctrine of the properties of organic is a development of A.M.Butlerov's idea of \u200b\u200bdependence on its structure. The expressive structure gives an idea of \u200b\u200ball the diversity, although the predictions are not the result of strict mathematical laws, but are only of a qualitative nature and provide much more to the talent and intuition of the experimental chemist.
The characteristics of the physical properties of compounds are often expressed as the sum of several terms related to the corresponding elements that make up a given compound. Application of such additive schemesto find any physicochemical characteristic of a compound by the formula of its structure is tantamount, therefore, to the assumption that an element, being a part of various compounds, always brings the same share of such a characteristic.
In the simplest cases, this assumption in relation turns out to be very close to the truth (for example, the values \u200b\u200bof molecular volumes and
Most substances are characterized by the ability, depending on the 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 exist only in a gaseous state, and at a temperature of less than 0 o C is a solid.
Substances in the solid state are distinguished between amorphous and crystalline.
The characteristic features of amorphous substances are 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.
However, 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, when a specific temperature is reached. Examples of crystalline substances include table salt, sugar, 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 crystalline state is easiest to understand from the following illustration:
As you can see, in an amorphous substance, in contrast to a crystalline one, there is no order in the arrangement of particles. If, however, in a crystalline substance, mentally connect two atoms closely spaced to each other by a straight line, then one 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 crystal cell.
Crystal lattice is called a spatial framework that connects points in space in which the particles that form a crystal are located.
The points in space in which the particles forming the crystal are located are called lattice nodes .
Depending on which particles are in the nodes of the crystal lattice, they are distinguished: molecular, atomic, ionic and metal crystal lattice .
In knots molecular crystal lattice
The crystal lattice of ice as an example of a molecular latticethere are molecules, inside which the atoms are bound firmly covalent bonds, however, the molecules themselves are held near each other by weak intermolecular forces. Due to these 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 carry out electricity, may or may not dissolve in various solvents. Solutions of such compounds can either conduct or 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, rhombic sulfur S 8, white phosphorus P 4), as well as many complex substances - hydrogen compounds of non-metals, acids, oxides of non-metals, most organic matter. 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.
The crystal lattice of diamond as an example of an atomic latticeIn knots atomic crystal lattice
are atoms. Moreover, all the nodes of such a crystal lattice are "cross-linked" to each other by means of strong covalent bonds into a single crystal. In fact, such a crystal is one giant molecule. Due to the structural features, all substances with an atomic crystal lattice are solid, have high melting points, are chemically weak, 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 from simple substances include boron B, carbon C (diamond and graphite), silicon Si, from complex substances - silicon dioxide SiO2 (quartz), silicon carbide SiC, boron nitride BN.
For substances with ionic crystal lattice
lattice sites contain ions connected to each other through ionic bonds.
Since ionic bonds are strong enough, substances with an ionic lattice have relatively high hardness and refractoriness. Most often they are soluble in water, and their solutions, like melts, conduct an electric current.
Substances with an ionic type of crystal lattice include metal and ammonium salts (NH 4 +), bases, metal oxides. A sure sign of the ionic structure of a substance is the presence in its composition of both typical metal and non-metal atoms.
The 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 metallic crystal lattice, its sites 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 instead "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, often have high ductility (ductility).
The spread in the values \u200b\u200bof the 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 - 3422 ° C. It should be noted that under normal conditions all metals, except mercury, are solids.
Electronegativity is the property of a chemical element to attract electrons to its atom from atoms of other elements with which this element forms a chemical bond in compounds.
In education chemical bond Between the atoms of different elements, the common electron cloud shifts to a more electronegative atom, due to which the bond becomes covalently polar, and with a large difference in electronegativity, ionic.
Electronegativity is taken into account when writing chemical formulas: in binary compounds, the symbol of the most electronegative element is written behind.
Electronegativity increases from left to right for elements of each period and decreases from top to bottom for elements of the same PS group.
Valence element is the property of its atoms to combine with a certain number of other atoms.
Distinguish between stoichiometric, electronic valence and coordination number. We will only consider stoichiometric valence.
Stoichiometric valence shows how many atoms of another element are attached by an atom of this element. The valence unit is taken as the hydrogen valence, since hydrogen is always monovalent. For example, in compounds HCl, H 2 O, NH 3 ( correct writing ammonia H 3 N is already used in modern manuals), CH 4 chlorine is monovalent, oxygen is divalent, nitrogen is trivalent and carbon is tetravalent.
The stoichiometric valence of oxygen is usually 2. Since almost all elements form compounds with oxygen, it is convenient to use it as a standard for determining the valence of another element. For example, in compounds Na 2 O, CoO, Fe 2 O 3, SO 3 sodium is monovalent, cobalt is divalent, iron is trivalent, sulfur is hexavalent.
In redox reactions, it will be important for us to determine the oxidation states of elements.
Oxidation state element in a substance is called its stoichiometric valence, taken with a plus or minus sign.
Chemical elements are subdivided into elements of constant valence elements of variable valence.
1.3.3. Substances of molecular and non-molecular structure. Crystal lattice type. The dependence of the properties of substances on their composition and structure.
Depending on the state of the compounds in nature, they are divided into molecular and non-molecular. AT molecular substances The smallest structural particles are molecules. These substances have a molecular crystal lattice. In non-molecular substances, the smallest structural particles are atoms or ions. Their crystal lattice is atomic, ionic or metallic.
The type of crystal lattice largely determines the properties of substances. For example, metals with metal type of crystal lattice, differ from all other elements high plasticity, electrical and thermal conductivity... These properties, as well as many others - malleability, metallic luster, etc. due to a special type of bond between metal atoms - metal connection. It should be noted that the properties inherent in metals appear only in the condensed state. For example, silver in a gaseous state does not have physical properties metals.
A special type of bond in metals - metallic - is due to a deficiency of valence electrons, therefore they are common to the entire structure of the metal. The simplest model of the structure of metals assumed that the crystal lattice of metals consists of positive ionssurrounded by free electrons, the movement of electrons occurs chaotically, like gas molecules. However, such a model, while qualitatively explaining many properties of metals, turns out to be insufficient in quantitative verification. Further development of the theory of the metallic state led to the creation zone theory of metals, which is based on the concepts of quantum mechanics.
In the nodes of the crystal lattice there are cations and metal atoms, and the electrons freely move along the crystal lattice.
A characteristic mechanical property of metals is plastic, due to the peculiarities of the internal structure of their crystals. Plasticity is understood as the ability of bodies under the influence of external forces to undergo deformation, which remains even after the termination of external influence. This property of metals allows them to be shaped into different shapes during forging, rolled into sheets or drawn into wire.
The plasticity of metals is due to the fact that, under external influence, the layers of ions that form the crystal lattice shift relative to each other without rupture. This occurs as a result of the fact that the displaced electrons, due to free redistribution, continue to communicate between the ionic layers. Under mechanical action on a solid with an atomic lattice, its individual layers are displaced and the adhesion between them is broken due to the breaking of covalent bonds.
ions, then these substances form ionic type of crystal lattice.
These are salts, as well as oxides and hydroxides of typical metals. These are hard, fragile substances, but their main quality : solutions and melts of these compounds conduct electric current.
If the sites of the crystal lattice are atoms, then these substances form atomic type of crystal lattice(diamond, boron, silicon oxides of aluminum and silicon). Very hard and refractory in properties, insoluble in water.
If the sites of the crystal lattice are molecules, then these substances form (under normal conditions gases and liquids: О 2, HCl; I 2 organic matter).
It is interesting to note the metal gallium, which melts at a temperature of 30 o C. This anomaly is explained by the fact that Ga 2 molecules are located in the nodes of the crystal lattice, and its properties in which become similar to substances with a molecular crystal lattice.
Example.All non-metals of the group have a non-molecular structure:
1) carbon, boron, silicon; 2) fluorine, bromine, iodine;
3) oxygen, sulfur, nitrogen; 4) chlorine, phosphorus, selenium.
In non-molecular substances, the smallest structural particles are atoms or ions. Their crystal lattice is atomic, ionic or metallic
When decision this question is easier to go from the opposite. If the sites of the crystal lattice are molecules, then these substances form molecular type of crystal lattice(under normal conditions gases and liquids: О 2, HCl; also I 2, rhombic sulfur S 8, white phosphorus Р 4, organic substances). According to their properties, these are fragile low-melting compounds.
In the second answer there is fluorine gas, in the third - oxygen and nitrogen gases, in the fourth - chlorine gas. This means that these substances have a molecular crystal lattice and molecular structure.
AT first The answer is that all substances are solid compounds under normal conditions and form an atomic lattice, which means they have a non-molecular structure.
Correct answer:1) carbon, boron, silicon
Lecture 7 The dependence of the properties of substances on their structure. Chemical bond. The main types of chemical bonds. Questions to be considered: 1. Levels of substance organization. Structure hierarchy. 2. Substances of molecular and non-molecular structure. 3. Variety chemical structures... 4. Reasons for the appearance of a chemical bond. 5. Covalent bond: mechanisms of formation, methods of overlapping atomic orbitals, polarity, dipole moment of the molecule. 6. Ionic bond. 7. Comparison of covalent polar and ionic bonds. 8. Comparison of the properties of substances with covalent polar and ionic bonds. 9. Metallic bond. 10. Intermolecular interactions.
Substance (over 70 million) What you need to know about each substance? 1. 2. 3. 4. 5. Formula (what it consists of) Structure (how it works) Physical properties Chemical properties Production methods (lab. And industrial) 6. Practical application
Hierarchy of the structure of matter All substances are made of atoms, but not all are made of molecules. Atom Molecule For all substances Only for substances of molecular structure Nanolevel For all substances Volumetric (macro) level For all substances All 4 levels are an object of study of chemistry
Substances Molecular structure Non-molecular structure Consist of molecules Consist of atoms or ions H 2 O, CO 2, HNO 3, C 60, almost all org. substances Diamond, graphite, Si. O 2, metals, salts The formula reflects the composition of the molecule The formula reflects the composition of the formula unit
Substances Silicon dioxide Formula unit Si. O 2 The Fersman Mineralogical Museum is located near the entrance to the Neskuchny Garden. Address: Moscow, Leninsky prospect, house 18, building 2.
Variety of chemical structures. propellane C 5 H 6 coronene (superbenzene) C 24 H 12 cavitand C 36 H 32 O 8
A molecule is a stable system consisting of several atomic nuclei and electrons. Atoms combine into molecules through the formation of chemical bonds. The main driving force behind the formation of a molecule from atoms is a decrease in total energy. Molecules have a geometric shape characterized by distances between nuclei and angles between bonds.
The main types of chemical bonds: 1. Ionic 2. Covalent 3. Metallic Main intermolecular interactions: 1. Hydrogen bonds 2. Van der Waals bonds
Ionic bond If the bond is formed by atoms with sharply differing values \u200b\u200bof electronegativity (ΔEEO ≥ 1, 7), the total electron pair is almost completely displaced towards the more electronegative atom. Na Cl OEO 0, 9 3, 16 ∆ 2, 26 + Na Anion: Cl. Cation The chemical bond between ions, resulting from their electrostatic attraction, is called ionic.
Ionic bond The Coulomb potential is spherically symmetric, directed in all directions, therefore the ionic bond is not directed. The Coulomb potential has no restrictions on the number of added counterions - therefore, the ionic bond is unsaturated.
Ionic bond Compounds with an ionic type of bond are solid, readily soluble in polar solvents, and have high melting and boiling points.
Ionic bond Curve I: attraction of ions, if they were point charges. Curve II: repulsion of nuclei in case of strong approach of ions. Curve III: the minimum energy E 0 on the curve corresponds to equilibrium state ion pair, in which the forces of attraction of electrons to the nuclei are compensated by the forces of repulsion of the nuclei among themselves at a distance r 0,
Chemical bond in molecules Chemical bond in molecules can be described from the standpoint of two methods: - the method of valence bonds, MBC - the method of molecular orbitals, MMO
The method of valence bonds Geitler-London theory The main provisions of the VS method: 1. A bond is formed by two electrons with opposite spins, while the wave functions overlap and the electron density between nuclei increases. 2. The bond is localized in the direction of the maximum overlap of the Ψ-functions of electrons. The stronger the overlap, the stronger the bond.
Formation of a hydrogen molecule: H · + · H → H: H When two atoms approach each other, forces of attraction and repulsion arise: 1) attraction: "electron-nucleus" of neighboring atoms; 2) repulsion: "nucleus-nucleus", "electron-electron" of neighboring atoms.
The chemical bond carried out by shared electron pairs is called covalent. A common electron pair can be formed in two ways: 1) as a result of the union of two unpaired electrons; 2) as a result of the socialization of the lone electron pair of one atom (donor) and the empty orbital of another (acceptor). Two mechanisms of covalent bond formation: exchange and donor-acceptor.
Methods of overlapping atomic orbitals during the formation of a covalent bond If the formation of the maximum electron density of a bond occurs along a line connecting the centers of atoms (nuclei), then such an overlap is called a σ-bond:
Methods of overlapping atomic orbitals during the formation of a covalent bond If the formation of the maximum electron density of a bond occurs on both sides of the line connecting the centers of atoms (nuclei), then such an overlap is called a π-bond:
Polar and non-polar covalent bond 1) If the bond is formed by the same atoms, the two-electron bond cloud is distributed in space symmetrically between their nuclei - such a bond is called non-polar: H2, Cl 2, N 2. 2) if the bond is formed by different atoms, the bond cloud is shifted to side of a more electronegative atom - this bond is called polar: HCl, NH 3, CO 2.
Polar covalent bond Dipole moment of the bond Dipole H + δCl-δ or H + 0, 18 Cl-0, 18 + δ -δ Where ± δ is the effective charge of the atom, the fraction of the absolute charge of the electron. Not to be confused with the oxidation state! l The product of the effective charge and the length of the dipole is called the electric moment of the dipole: μ \u003d δl This is a vector quantity: directed from a positive charge to a negative one.
Polar covalent bond The dipole moment of a molecule is equal to the sum of the vectors of the dipole moments of bonds, taking into account the lone electron pairs. The unit of measurement of the dipole moment is Debye: 1 D \u003d 3, 3 · 10 -30 C · m.
Polar covalent bond Dipole moment of the molecule In the product μ \u003d δl, both quantities are oppositely directed. Therefore, it is necessary to carefully monitor the reason for the change in μ. For example, Cs. F Cs. Cl 24 31 δ “lost” l Cs. I HF HCl HBr HI 37 5, 73 3, 24 2, 97 1, 14 vice versa
Polar covalent bond Dipole moment of a molecule Can a molecule be non-polar if all bonds in it are polar? Type AB molecules are always polar. Molecules of the AB 2 type can be both polar and non-polar. ... ... Н 2 О О Н СО 2 μ\u003e 0 Н О С μ \u003d 0 О
Polar covalent bond Molecules consisting of three or more atoms (AB 2, AB 3, AB 4, AB 5, AB 6) can be non-polar if they are symmetrical. What does the presence of the dipole moment of the molecule affect? There are intermolecular interactions, and, therefore, the density of the substance, melting point and boiling point increase.
Comparison of ionic and covalent polar bonds General: formation of a common electron pair. Difference: the degree of displacement of the common electron pair (bond polarization). The ionic bond should be considered as an extreme case of the covalent polar bond.
Comparison of characteristics of ionic and covalent polar bonds Covalent bond: saturated and directed Saturation (maximum valence) - is determined by the ability of an atom to form a limited number of bonds (taking into account both mechanisms of formation). The bond direction sets the bond angle, which depends on the type of hybridization of the central atom orbitals. Ionic bond: unsaturated and undirected.
Comparison of characteristics of ionic and covalent polar bonds The bond direction is determined by the bond angles. The bond angles are determined experimentally or predicted on the basis of L. Polling's theory of hybridization of atomic orbitals or Gillespie's theory. More on this at seminars.
Comparison of properties of substances with ionic and covalent bonds Covalent bonds Atomic crystals Between atoms in the crystal itself High hardness high melting temperature, boiling temperature poor heat and electrical conductivity Molecular crystals Between atoms in a molecule Moderate softness rather low melting temperature, boiling temperature poor heat and electrical conductivity Insoluble in water
Comparison of properties of substances with ionic and covalent bonds Atomic covalent crystal Melting point ≈ 3700 ° С
Comparison of properties of substances with ionic and covalent bonds Ionic bonds between ions in a crystal hardness and fragility high melting point poor thermal and electrical conductivity Soluble in water
The metallic bond is carried out by electrons belonging to all atoms at the same time. The electron density is delocalized "electron gas". Characteristic metallic luster Ductility Ductility High thermal and electrical conductivity Melting points are very different.
Intermolecular bonds. 1. Hydrogen bond The attraction between the hydrogen atom (+) of one molecule and the F, O, N (-) atom of another molecule Polymer (HF) n Acetic acid dimer Hydrogen bonds are weak individually, but strong collectively
Intermolecular bonds. 5. Van der Waals bonds Even if there are no hydrogen bonds between molecules, molecules are always attracted to each other. The attraction between molecular dipoles is called the Wonder Waals bond. In-d-c attraction is the stronger, the more: 1) polarity; 2) the size of the molecules. Example: methane (CH 4) - gas, benzene (C 6 H 6) - liquid One of the most weak in-d-in bonds - between H 2 molecules (m.p. - 259 o. C, boiling point - 253 o. C). The interaction between molecules is many times weaker than the bond between atoms: Ecov (Cl – Cl) \u003d 244 k J / mol, Evdv (Cl 2 – Cl 2) \u003d 25 k J / mol, precisely ensures the existence of liquid and solid state substances
The lecture used materials of the professor of the Faculty of Chemistry of Moscow State University. Lomonosov Eremina Vadim Vladimirovich Thank you for your attention!