The chemical bond is the interaction of particles (ions or atoms), which is carried out in the process of exchange of electrons at the last electronic level. There are several types of such a bond: covalent (it is divided into non-polar and polar) and ionic. In this article we will dwell on the first kind of chemical bonds - covalent. And to be more precise, then on its polar form.
A covalent polar bond is a chemical bond between the valence electron clouds of neighboring atoms. The prefix "ko-" means in this case "jointly", and the basis of "valenta" is translated as strength or ability. Those two electrons that are connected to each other are called the electron pair.
For the first time this term was used in the scientific context by the Nobel Prize-winning chemist Irving Lenngryum. It happened in 1919. In his work, the scientist explained that the bond, 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 V. Heitler, taking the hydrogen molecule as the most simple model as an example of a hydrogen molecule, described a covalent bond. They got down to business from the other end, and justified their observations using quantum mechanics.
Essence of the reaction
The process of converting atomic hydrogen to molecular is a typical chemical reaction, a qualitative sign of which is a large heat release when two electrons combine. It looks like this: two helium atoms approach each other, each having an electron in its orbit. Then these two clouds come together and form a new, similar to the shell of helium, in which two electrons already rotate.
Completed electron shells are more stable than incomplete ones, therefore their energy is much lower than that of two separate atoms. When a molecule is formed, excess heat is dissipated in the environment.
In chemistry, there are two types of covalent bonds:
- Covalent non-polar bond formed between two atoms of the same non-metallic 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 also a kind of covalent bond. It lies in the fact that one atom of a pair provides both electrons, becoming a donor, and the host atom, respectively, is considered an acceptor. When a bond is formed between atoms, the donor charge increases by one, and the acceptor charge decreases.
Semi-polar connection - e e can be considered a subspecies donor-acceptor. Only in this case, atoms are combined, one of which has a complete electron orbital (halogens, phosphorus, nitrogen), and the second has two unpaired electrons (oxygen). The formation of communication takes place in two stages:
- at first, one electron separates from the lonely pair and joins the unpaired ones;
- the union of the remaining unpaired electrodes, that is, a covalent polar bond is formed.
The polar covalent bond has its own physicochemical properties, such as directionality, saturability, polarity, polarizability. They determine the characteristics of the resulting molecules.
The directionality of a bond depends on the future molecular structure of the substance formed, namely, on the geometric shape that two atoms form upon attachment.
Saturation indicates how many covalent bonds a single atom of a substance can form. This number is limited by the number of external atomic orbitals.
The polarity of the molecule arises because the electron cloud, which is formed from two different electrons, is uneven throughout its entire circumference. This is due to the difference in negative charge in each of them. It is this property that determines the polar connection or non-polar. When two atoms of one element are combined, the electron cloud is symmetrical, which means that the covalent bond is non-polar. And if atoms of different elements are combined, then an asymmetric electron cloud is formed, the so-called dipole moment of the molecule.
Polarizability reflects how actively the electrons in a molecule shift under the action of external physical or chemical agents, such as an electric or magnetic field, of other particles.
The last two properties of the resulting molecule determine its ability to react with other polar reagents.
Sigma bond and pi bond
The formation of these bonds depends on the density of the distribution of electrons in the electron cloud during the formation of the molecule.
The presence of a dense electron cluster along the axis connecting the atomic nuclei, that is, in the horizontal plane, is characteristic of a sigma bond.
A pi bond is characterized by the condensation of electron clouds at their intersection, that is, above and below the nucleus of an atom.
Visualization of the relationship in the formula records
For example, we can take a chlorine atom. Its outer electronic level contains seven electrons. In the formula, they have three pairs and one unpaired electron around the designation of the element in the form of points.
If we write 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. In addition, each of them received eight electrons.
Octet doublet rule
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 to produce an electron configuration that repeats like atoms of noble elements.
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.
The covalent polar bond, among other things, is characterized by a certain distance between the nuclei of the 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 as much as possible. There is a directly proportional pattern between the size of atoms and a long bond. The larger the atom, the longer the bond between the nuclei.
It is possible that an atom forms not one, but several covalent polar bonds. Then the so-called valent angles are formed between the cores. They can be from ninety to one hundred and eighty degrees. They determine the geometric formula of the molecule.