A chemical bond is the interaction of particles (ions or atoms), which occurs during the exchange of electrons that are at the last electronic level. There are several types of this connection: covalent (it is divided into non-polar and polar) and ionic. In this article we will dwell in detail on the first form of chemical bonds, covalent. And to be more precise, it is in its polar form.
A covalent polar bond is a chemical bond between the valence electron clouds of neighboring atoms. The prefix "co-" means in this case "jointly", and the basis of "valence" is translated as force or ability. Those two electrons that are connected together are called an electron pair.
For the first time this term was used in the scientific context by the Nobel prize laureate chemist Irving Lengring. It happened in 1919. In his work, the scientist explained that the connection in which electrons common to two atoms are observed differs from metallic or ionic. So, it requires a separate name.
Later, already in 1927, F. London and W. Heitler, taking as an example the hydrogen molecule as the chemically and physically most simple model, described a covalent bond. They got down to business from the other end, and their observations were justified using quantum mechanics.
The essence of the reaction
The process of converting atomic hydrogen to molecular hydrogen is a typical chemical reaction, the qualitative feature of which is a large heat release when two electrons are combined. It looks like this: two helium atoms approach each other, having one electron in their orbit. Then these two clouds approach each other and form a new one, similar to the helium shell, in which two electrons rotate already.
Completed electronic shells are more stable than unfinished ones, therefore their energy is significantly lower than that of two separate atoms. When a molecule is formed, excess heat dissipates in the environment.
Two types of covalent bond are distinguished in chemistry:
- Covalent nonpolar bond formed between two atoms of one nonmetallic element, for example, oxygen, hydrogen, nitrogen, carbon.
- Polar covalent bond, occurs between atoms of different nonmetals. A good example is a molecule of hydrogen chloride. When the atoms of two elements are connected to each other, an unpaired electron from the hydrogen is partially transferred to the electronic level of the chlorine atom. Thus, the hydrogen atom forms a positive charge and the chlorine atom is negative.
Donor-acceptor bond is also a kind of covalent bond. It consists in the fact that one atom from a pair provides both electrons, becoming a donor, and the atom receiving them, respectively, is considered an acceptor. When a bond is formed between atoms, the donor's charge increases by one, and the charge of the acceptor decreases.
Semi-polar communication - e e can be regarded as a donor-acceptor subspecies. Only in this case atoms are combined, one of which has a completed electron orbital (halogens, phosphorus, nitrogen), and the second - two unpaired electrons (oxygen). The formation of communication takes place in two stages:
- first, from an unshared pair, it detaches one electron and joins the unpaired pair;
- unification of the remaining unpaired electrodes, that is, a covalent polar bond is formed.
The polar covalent bond has its own physico-chemical properties, such as directionality, saturation, polarity, polarizability. They determine the characteristics of the molecules formed.
The direction of the bond depends on the future molecular structure of the material being formed, namely, on the geometric shape that two atoms form when they are attached.
Saturation shows how many covalent bonds can form one atom of matter. This number is limited by the number of external atomic orbitals.
The polarity of the molecule arises because the electron cloud, formed from two different electrons, is uneven across its entire circumference. This is due to the difference in the negative charge in each of them. It is this property that determines whether the polar connection is non-polar. When two atoms of one element are combined, the electron cloud is symmetrical, so the covalent bond is nonpolar. And if atoms of different elements are combined, an asymmetric electron cloud is formed, the so-called dipole moment of the molecule.
Polarizability reflects how actively the electrons in the molecule are displaced by external physical or chemical agents, for example, electric or magnetic fields, other particles.
The last two properties of the resulting molecule determine its ability to react with other polar reagents.
Sigma-link and p-bond
The formation of these bonds depends on the density of the electron distribution in the electron cloud during the formation of the molecule.
For the sigma-connection, a dense cluster of electrons is characteristic along the axis joining the nuclei of the atoms, that is, in the horizontal plane.
Pi-coupling is characterized by the condensation of electron clouds at the point of their intersection, that is, above and below the nucleus of the atom.
Visualization of the connection in the record of the formula
For example, we can take a chlorine atom. At its external electronic level contains seven electrons. In the formula, they are arranged with three pairs and one unpaired electron around the element designation in the form of dots.
If we also record the chlorine molecule in the same way, it will be seen that two unpaired electrons formed a pair common to two atoms, it is called divided. Each of them received eight electrons.
Rule octet doublet
Chemist Lewis, who suggested how a covalent polar bond is formed, was the first of his colleagues to formulate a rule explaining the stability of atoms when they are combined into molecules. Its essence lies in the fact that chemical bonds between atoms are formed in the case when a sufficient number of electrons are socialized in order to obtain an electronic configuration that repeats the noble elements similar to atoms.
That is, in the formation of molecules for their stabilization requires that all atoms have a complete outer electron level. For example, hydrogen atoms together in a molecule, a repeating electron shells helium, chlorine atoms acquire the similarity in electronic level with an atom of argon.
Length of connection
Covalent polar bond, among other things, is characterized by a certain distance between the nuclei of atoms forming the molecule. They are at such a distance from each other, at which the energy of the molecule is minimal. In order to achieve this, it is necessary that the electron clouds of atoms overlap each other. There is a directly proportional law between the size of atoms and the long bond. The larger the atom, the longer the connection between the nuclei.
A variant is possible when the atom forms not one but several covalent polar bonds. Then between the nuclei formed the so-called valence angles. They can be from ninety to one hundred and eighty degrees. They also determine the geometric formula of the molecule.