Why can atoms combine with each other to form molecules? What is the reason for the possible existence of substances, which include atoms of completely different chemical elements? These are global issues affecting the fundamental concepts of modern physical and chemical science. You can answer them, having an idea about the electronic structure of atoms and knowing the characteristics of the covalent bond, which is the basic basis for most classes of compounds. The purpose of our article is to get acquainted with the mechanisms of formation of various types of chemical bonds and the features of the properties of compounds containing them in their molecules.
Electronic structure of the atom
Electroneutral particles of matter, which are its structural elements, have a structure that mirrors the structure of the solar system. As the planets revolve around the central star - the Sun, so the electrons in the atom move around the positively charged nucleus. To characterizeIn a covalent bond, the electrons located at the last energy level and the most distant from the nucleus will be significant. Since their connection with the center of their own atom is minimal, they are able to be easily attracted by the nuclei of other atoms. This is very important for the occurrence of interatomic interactions leading to the formation of molecules. Why is the molecular form the main type of existence of matter on our planet? Let's find out.
Basic property of atoms
The ability of electrically neutral particles to interact, leading to a gain in energy, is their most important feature. Indeed, under normal conditions, the molecular state of matter is more stable than the atomic state. The main provisions of modern atomic and molecular theory explain both the principles of the formation of molecules and the characteristics of a covalent bond. Recall that the outer energy level of an atom can contain from 1 to 8 electrons, in the latter case the layer will be complete, which means it will be very stable. Atoms of noble gases have such an external level structure: argon, krypton, xenon - inert elements that complete each period in the system of D. I. Mendeleev. The exception here is helium, which has not 8, but only 2 electrons in the last level. The reason is simple: in the first period there are only two elements whose atoms have a single electron layer. All other chemical elements have from 1 to 7 electrons on the last, incomplete layer. In the process of interacting with each other, the atoms willstrive to be filled with electrons up to an octet and restore the configuration of an atom of an inert element. Such a state can be achieved in two ways: by the loss of one's own or by the acceptance of foreign negatively charged particles. These forms of interaction explain how to determine whether an ionic or covalent bond will form between the reacting atoms.
Mechanisms for the formation of a stable electronic configuration
Let's imagine that two simple substances enter into the reaction of the compound: metallic sodium and gaseous chlorine. A substance of the class of s alts is formed - sodium chloride. It has an ionic type of chemical bond. Why and how did it come about? Let us turn again to the structure of the atoms of the initial substances. Sodium has only one electron on the last layer, weakly bound to the nucleus due to the large radius of the atom. The ionization energy of all alkali metals, including sodium, is low. Therefore, the electron of the outer level leaves the energy level, is attracted by the nucleus of the chlorine atom and remains in its space. This creates a precedent for the transition of the Cl atom into the form of a negatively charged ion. Now we are no longer dealing with electrically neutral particles, but with charged sodium cations and chlorine anions. In accordance with the laws of physics, electrostatic attraction forces arise between them, and the compound forms an ionic crystal lattice. The mechanism of formation of the ionic type of a chemical bond considered by us will help to clarify the specifics and main characteristics of a covalent bond more clearly.
Shared electron pairs
If an ionic bond occurs between atoms of elements that differ greatly in electronegativity, i.e., metals and non-metals, then the covalent type appears when atoms of the same or different non-metallic elements interact. In the first case, it is customary to speak of a non-polar, and in the other, of a polar form of a covalent bond. The mechanism of their formation is common: each of the atoms partially gives electrons for common use, which are combined in pairs. But the spatial arrangement of electron pairs relative to the nuclei of atoms will be different. On this basis, the types of covalent bonds are distinguished - non-polar and polar. Most often, in chemical compounds consisting of atoms of non-metallic elements, there are pairs consisting of electrons with opposite spins, i.e., rotating around their nuclei in opposite directions. Since the movement of negatively charged particles in space leads to the formation of electron clouds, which ultimately ends in their mutual overlap. What are the consequences of this process for atoms and what does it lead to?
Physical properties of a covalent bond
It turns out that between the centers of two interacting atoms there is a two-electron cloud with a high density. The electrostatic forces of attraction between the negatively charged cloud itself and the nuclei of atoms increase. A portion of energy is released and the distances between atomic centers decrease. For example, at the beginning of the formation of a molecule H2 the distance between the nuclei of hydrogen atomsis 1.06 A, after the overlap of clouds and the formation of a common electron pair - 0.74 A. Examples of a covalent bond formed according to the above mechanism can be found both among simple and complex inorganic substances. Its main distinguishing feature is the presence of common electron pairs. As a result, after the emergence of a covalent bond between atoms, for example, hydrogen, each of them acquires the electronic configuration of inert helium, and the resulting molecule has a stable structure.
Spatial shape of a molecule
Another very important physical property of a covalent bond is directionality. It depends on the spatial configuration of the substance molecule. For example, when two electrons overlap with a spherical cloud, the appearance of the molecule is linear (hydrogen chloride or hydrogen bromide). The shape of water molecules, in which s- and p-clouds hybridize, is angular, and very strong particles of gaseous nitrogen look like a pyramid.
Structure of simple substances - non-metals
Having found out what kind of bond is called covalent, what signs it has, now it's time to deal with its varieties. If atoms of the same non-metal - chlorine, nitrogen, oxygen, bromine, etc., interact with each other, then the corresponding simple substances are formed. Their common electron pairs are located at the same distance from the centers of atoms, without shifting. For compounds with a non-polar type of covalent bond, the following features are inherent: low boiling points andmelting, insolubility in water, dielectric properties. Next, we will find out which substances are characterized by a covalent bond, in which a shift of common electron pairs occurs.
Electronegativity and its effect on the type of chemical bond
The property of a certain element to attract electrons from an atom of another element in chemistry is called electronegativity. The scale of values for this parameter, proposed by L. Pauling, can be found in all textbooks on inorganic and general chemistry. Its highest value - 4.1 eV - has fluorine, the smaller one - other active non-metals, and the lowest indicator is typical for alkali metals. If elements differing in their electronegativity react with each other, then inevitably one, more active, will attract negatively charged particles of an atom of a more passive element to its nucleus. Thus, the physical properties of a covalent bond directly depend on the ability of the elements to donate electrons for common use. The resulting common pairs are no longer located symmetrically with respect to the nuclei, but are shifted towards the more active element.
Features of compounds with a polar bond
Substances in molecules of which joint electron pairs are asymmetric with respect to the nuclei of atoms include hydrogen halides, acids, compounds of chalcogens with hydrogen and acid oxides. These are sulfate and nitrate acids, oxides of sulfur and phosphorus, hydrogen sulfide, etc. For example, a hydrogen chloride molecule contains one common electron pair,formed by unpaired electrons of hydrogen and chlorine. It is shifted closer to the center of the Cl atom, which is a more electronegative element. All substances with a polar bond in aqueous solutions dissociate into ions and conduct an electric current. Compounds that have a polar covalent bond, examples of which we have given, also have higher melting and boiling points compared to simple non-metal substances.
Methods for breaking chemical bonds
In organic chemistry, the substitution reactions of saturated hydrocarbons with halogens follow a radical mechanism. A mixture of methane and chlorine in the light and at ordinary temperature reacts in such a way that the chlorine molecules begin to split into particles carrying unpaired electrons. In other words, the destruction of the common electron pair and the formation of very active radicals -Cl are observed. They are able to influence methane molecules in such a way that they break the covalent bond between carbon and hydrogen atoms. An active particle –H is formed, and the free valency of the carbon atom takes on a chlorine radical, and chloromethane becomes the first product of the reaction. Such a mechanism for the splitting of molecules is called homolytic. If the common pair of electrons completely passes into the possession of one of the atoms, then they speak of a heterolytic mechanism characteristic of reactions taking place in aqueous solutions. In this case, polar water molecules will increase the rate of destruction of the chemical bonds of the dissolved compound.
Double and triplelinks
The vast majority of organic substances and some inorganic compounds contain in their molecules not one, but several common electron pairs. The multiplicity of the covalent bond reduces the distance between atoms and increases the stability of compounds. They are usually referred to as chemically resistant. For example, in a nitrogen molecule there are three pairs of electrons, they are indicated in the structural formula by three dashes and determine its strength. The simple substance nitrogen is chemically inert and can react with other compounds, such as hydrogen, oxygen or metals, only when heated or at elevated pressure, as well as in the presence of catalysts.
Double and triple bonds are inherent in such classes of organic compounds as unsaturated diene hydrocarbons, as well as substances of the ethylene or acetylene series. Multiple bonds determine the main chemical properties: addition and polymerization reactions occurring at the points of their break.
In our article, we gave a general description of the covalent bond and examined its main types.
