A chemical bond is a bond of two or more atoms (molecules) in an organic or inorganic compound. It is formed under the condition of a decrease in the total energy in the system.
Do all elements form chemical bonds?
All elements of the periodic system have different ability to form bonds. The most stable and, as a result, chemically inactive are atoms of noble (inert) gases, since they contain two or eight electrons on the outer electron shell. They form a small number of bonds. For example, neon, helium and argon do not form chemical bonds with any element, while xenon, krypton and radon are able to react with fluorine and water molecules.
The atoms of other elements have external levels that are not complete and have from one to seven electrons; therefore, to increase the stability of the shell, they form chemical bonds.
Types of Chemical Bonding
There are several types of communication:
- Covalent.
- Ionic.
- Metal.
- Hydrogen.
Covalent bond
This type of bond is formed between atoms in a molecule as a result of socialization or overlap of a valence electron pair. Accordingly, there are exchange (a) and donor-acceptor (b) mechanisms of covalent bond formation. A separate case is a native connection, which will be discussed below.
Covalent bond: exchange mechanism
Atoms on the external level have unpaired electrons. During the interaction, the outer shells overlap. The antiparallel spins of single electrons contained at external levels pair with the formation of an electron pair common to both atoms. This pair of electrons is, in fact, a covalent bond, which is formed by the exchange mechanism, for example, in a hydrogen molecule.
Covalent bond: donor-acceptor mechanism
This mechanism consists in the socialization by two atoms of two electrons located on an external level. In this case, one of the atoms acts as a donor (provides two electrons), and the other as an acceptor (has a vacant orbital for electrons). Atoms of s- and p-elements can be either acceptors or electron donors. Atoms of d-elements can be both donors and acceptors.
To understand what a donor-acceptor mechanism is, we consider two simplest examples - the formation of hydroxonium cations H 3 O + and ammonium NH 4 + .
An example of a donor-acceptor mechanism is ammonium cation
Schematically, the reaction of the formation of an ammonium particle is as follows:
NH 3 + H + = NH 4 +
Electrons in the N atom are distributed in the following order: 1s 2 2s 2 2p 3 .
Electronic structure of cation H: 1s 0 .
The nitrogen atom at the external level contains two s and three p electrons. Three p-electrons are involved in the formation of three covalent exchange type nitrogen – hydrogen NH bonds. As a result of this, an ammonia molecule NH 3 with a covalent bond type is formed. Since the nitrogen atom N at the external level also has a pair of electrons s, the NH 3 molecule can also attach a hydrogen cation. The ammonia molecule is a donor, and the hydrogen cation H + is an acceptor that accepts donor electrons from nitrogen to its own free s-orbital.
An example of a donor-acceptor mechanism is H3O (hydroxonium ion)
The electrons in the oxygen atom are distributed in the following order: 1s 2 2s 2 2p 4 .
The oxygen atom at the external level has two s and four p-electrons. Based on this, two free p-electrons and two s-electrons from two H atoms take part in the formation of HO bonds. That is, the 2 available bonds in the H 2 O molecule are covalent, formed by the exchange mechanism.
Electronic structure of hydrogen cation: 1s 0 .
Since the oxygen atom at the external level has two more electrons (s-type), it can form a third covalent bond by the donor-acceptor mechanism. The acceptor may be an atom having a free orbital, in this example it is a H + particle. The free s-orbital of the H + cation is occupied by two electrons (s) of the oxygen atom.
Donor-acceptor mechanism for the formation of covalent bonds between inorganic molecules
The donor-acceptor covalent bond mechanism is possible not only in atom-atom or molecule-atom interactions, but also in reactions between molecules. The only condition for donor – acceptor interaction of kinetically independent molecules is a decrease in entropy, in other words, an increase in the ordering of the chemical structure.
Consider the first example - the formation of aprotic acid ( Lewis acid ) NH 3 BF 3 . This inorganic complex is formed in the reaction of the addition of an ammonia molecule and boron fluoride.
NH 3 + BF 3 = NH 3 BF 3
The electrons in the boron atom are distributed in the following order: 1s 2 2s 2 2p 1 .
Upon excitation of the B atom, one s-type electron passes to the p-sublevel (1s 2 2s 1 2p 2 ). Thus, at the external level of the excited boron atom, there are two s and two p electrons.
In the BF 3 molecule, three covalent bonds of boron fluorine BF are formed of the exchange type (boron and fluorine atoms provide one electron each). After the formation of three covalent bonds at the boron atom, the free p-sublevel remains on the outer electron shell, due to which the boron fluoride molecule can act as an electron acceptor.
The electrons in the nitrogen atom are distributed in the following order: 1s 2 2s 2 2p 3 .
Three electrons from the N and H atoms participate in the formation of the nitrogen-hydrogen bond. After this, nitrogen has two more s-type electrons, which it can provide for the formation of bonds by the donor-acceptor mechanism.
In the reaction of boron trifluoride and ammonia, the NH 3 molecule plays the role of an electron donor, and the BF 3 molecule plays the role of an acceptor. A pair of nitrogen electrons occupies the free orbital of boron fluoride and the chemical compound NH 3 BF 3 is formed .
Another example of a donor-acceptor bond formation mechanism is the preparation of a beryllium fluoride polymer.
Schematically, the reaction is as follows:
BeF 2 + BeF 2 + ... + BeF 2 -> (BeF 2 ) n
The electrons in the Be atom are located like this - 1s 2 2s 2 , and in the F atom - 1s 2 2s 2 2p 5 .
Two beryllium-fluorine bonds in the beryllium fluoride molecule are covalent exchange type (two p-electrons from two fluorine atoms and two s-sublevel electrons of the beryllium atom are involved).
Between a pair of atoms of beryllium (Be) and fluorine (F), two more covalent bonds are formed by the donor-acceptor mechanism. In a beryllium fluoride polymer, a fluorine atom is an electron donor, a beryllium atom is their acceptor having a vacant orbital.
Donor-acceptor mechanism for the formation of covalent bonds between organic molecules
When a bond is formed by the mechanism under consideration between molecules of an organic nature, more complex compounds are formed - complexes. Any organic compound with a covalent bond contains both occupied (non-binding and binding) and empty orbitals (loosening and non-binding). The possibility of donor – acceptor complex formation is determined by the degree of complex stability, which depends on the bond strength.
Consider an example - the reaction of the interaction of a molecule of methylamine with hydrochloric acid with the formation of methylammonium chloride. In the methylamine molecule, all bonds are covalent, formed by the exchange mechanism — two HN bonds and one N-CH 3 bond. After bonding with hydrogen and a methyl group, the nitrogen atom has another pair of s-type electrons. As a donor, he provides this electron pair for a hydrogen atom (acceptor), which has a free orbital.
Donor-acceptor mechanism without chemical bonding
Not in all cases of donor-acceptor interaction the socialization of the electron pair and the formation of a bond take place. Some organic compounds can be combined with each other by overlapping the filled donor orbital with the empty acceptor orbital. Charge transfer occurs - electrons are delocalized between the acceptor and the donor, located very close to each other. Complexes with charge transfer (CTC) are formed.
Such an interaction is characteristic of pi systems, whose orbitals easily overlap, and electrons are easily polarized. The role of donors may be metallocenes, unsaturated amino compounds, and TDAE (tetrakis (dimethylamino) ethylene). Acceptors are often fullerenes, quinodimethanes having acceptor substituents.
Charge transfer can be either partial or complete. Complete charge transfer occurs upon photoexcitation of the molecule. In this case, a complex is formed that can be observed spectrally.
Regardless of the completeness of charge transfer, such complexes are unstable. To increase the strength and life time of such a state, a bridge group is additionally introduced. As a result of this, donor-acceptor systems are successfully used in solar energy conversion devices.
In some organic molecules, a bond according to the donor – acceptor mechanism is formed inside the molecule between the donor and acceptor group. This type of interaction is called the transannular effect, which is characteristic, for example, for atranes (organoelement compounds with N-> B, N-> Si bonds).
Semipolar communication, or Dative communication formation mechanism
In addition to the exchange and donor-acceptor mechanisms, there is a third mechanism - the dative (other names are semipolar, semipolar or coordination). A donor atom gives a pair of electrons to the free orbital of a neutral atom, which needs two electrons to complete the external level. A peculiar transition of electron density from the acceptor to the donor occurs. In this case, the donor becomes positively charged (cation), and the acceptor becomes negatively charged (anion).
The chemical bond itself is formed due to the bonding shell (overlapping of two paired electrons of one of the atoms with the external free orbital of the other) and the electrostatic attraction that arises between the cation and the anion. Thus, covalent and ionic types are combined in a semipolar bond. A semi-polar bond is characteristic of d-elements, which in different compounds can play the roles of both an acceptor and a donor. In most cases, it is found in complex and organic substances.
Examples of native communication
The simplest example is a chlorine molecule. One Cl atom gives up a pair of electrons to another chlorine atom, which has a free d-orbital. In this case, one Cl atom is positively charged, the other is negatively charged, and electrostatic attraction arises between them. Due to the large length, the native bond has lower strength in comparison with the covalent exchange and donor-acceptor types, but its presence increases the strength of the chlorine molecule. That is why the Cl 2 molecule is stronger than F 2 (the fluorine atom has no d-orbitals, the fluorine-fluorine bond is only covalent exchange).
The carbon monoxide molecule CO (carbon monoxide) is formed by three CO bonds. Since oxygen and carbon atoms at the external level have two single electrons, two covalent exchange bonds are formed between them. After that, a vacant orbital remains at the carbon atom, and two pairs of electrons at the external level at the O atom. Therefore, in the molecule of carbon monoxide (II) there is a third bond - a semipolar bond, which is formed due to two valence paired oxygen electrons and a free orbital of carbon.
Consider a more complex example - the formation of this type of bond on the example of the interaction of dimethyl ether (3-- 3 ) with aluminum chloride AlCl 3 . The oxygen atom in dimethyl ether is linked by two covalent bonds with methyl groups. After that, he still has two more electrons on the p-sublevel, which he gives to the acceptor atom (aluminum) and becomes a positive cation. In this case, the acceptor atom acquires a negative charge (turns into an anion). The cation and anion electrostatically interact with each other.
The value of donor-acceptor bonds
The mechanism of the formation of a donor-acceptor bond is important in human life and is widespread in chemical compounds of both organic and inorganic nature, which is confirmed by the above examples. Ammonium chloride, which contains ammonium cation, is successfully used in household, medicine and industrial production of fertilizers. The hydroxonium ion plays a major role in the dissolution of acids in water. Carbon monoxide is used in industry (for example, in the production of fertilizers, laser systems) and is of great importance in the physiological systems of the human body.