Let us consider the occurrence of ionic bonds using the example of the formation of sodium chloride NaCl. The sodium and chlorine atoms from which this compound was formed differ sharply in electronegativity: for the sodium atom it is 0.9, for the chlorine atom it is 3.0. These are atoms with incomplete external electronic levels. To form a stable octet shell of the outer energy level it is easier for the sodium atom to give up 1 electron, and for the chlorine atom to accept 1 electron according to the scheme
Na – e - = Na +
that is electron shell sodium atom turns into a stable shell of the noble gas atom Ne – 1s 2 2s 2 2p 6 (this is the sodium ion Na +), and the shell of the Cl atom turns into the shell of the noble gas atom Ar - 1s 2 2s 2 2p 6 3s 2 3p 6 (this chloride ion Cl -). Electrostatic attraction forces arise between the Na + and Cl - ions, resulting in the formation of the NaCl compound.
The chemical bond between ions carried out by electrostatic attraction is called electrovalent or ionic bond. Compounds that are formed by the interaction of ions are called heteropolar or ionic.
Ionic compounds form two elements that are sharply different in electronegativity, for example, atoms of elements of the main subgroups of groups I and II with elements of the main subgroups VI and VII groups. There are relatively few ionic compounds.
Sodium chloride NaCl molecules exist only in a vapor state. In the solid (crystalline) state, ionic compounds consist of regularly arranged positive and negative ions. In this case there are no molecules.
A covalent bond is more general type chemical bond. The bond theory explains the emergence of an ionic bond from covalent extreme one-way polarization (displacement) of a common electron pair, when the latter becomes the possession of one of the connecting atoms in the NaCl molecule
In the given example, the maximum one-sided polarization is produced by the chlorine atom, which exhibits non-metallic properties (electronegativity χ with l = 3.0). The molecular electron cloud (electron pair) is completely shifted towards the chlorine atom. This is equivalent to the transfer of an electron from a sodium atom to a chlorine atom.
Obviously, polar covalent bond can be defined as a type of covalent bond that has undergone only minor one-way polarization (the bonding electron cloud has shifted to an atom with higher relative electronegativity). It is intermediate between ionic and non-polar covalent bonds.
Thus, in the mechanism of occurrence of non-polar covalent, polar covalent and ionic bonds there are no fundamental difference. They differ only in the degree of polarization (displacement) of common electron pairs.
The polarity of a bond can be predicted by comparing the relative electronegativity values of elemental atoms. The greater the difference in the relative electronegativities of bonded atoms (we denote it by Δχ), the more pronounced the polarity. Extremely high valueΔχ in the CsF compound (4.0 – 0.86 = 3.14). So, chemical bond between atoms ionic if Δχ ≈ 2; at Δχ = 0 - this bond is non-polar covalent; in intermediate cases - polar covalent. In reality, bonds are not 100% ionic. Therefore, they talk about the degree or fraction of ionicity of a bond. It is determined empirically. It turns out that even in a compound such as CsF, the ionic bond is only 89% expressed.
Ionic bond in contrast to covalent bonds, it is characterized lack of direction in space and unsaturation . The non-directionality of the bond is determined by the fact that each ion, which is like a charged ball, can attract an ion opposite sign in any direction. The interaction of ions of the opposite sign does not lead to compensation of force fields: their ability to attract ions of the opposite sign remains in other directions (unsaturation).
Ionic bond is a chemical bond between ions carried out by electrostatic attraction. Ions are formed by completely displacing an electron pair to one of the atoms. This type of bond is formed if the difference in electronegativity between the atoms is large, i.e. ∆Χ ≥ 2.1, ex: NaCl, for Na 0.9, for Cl 3.1, ∆Χ=3.1-0.9=2.2≥2.1 => ionic bond
If 1.4< ∆Χ < 2.1, то связь ионно-ковалентная, имеет свойства ионной связи.
An ionic bond is an extreme case of a covalent polar bond (complete donation of electrons by one of the atoms occurs). To typical connections with ionic type bonds include halides alkali metals(NaCl)
Mechanism of ionic bond formation –
3s1 Na 0 - e à Na+
3s23p5 Cl o + eà Cl-
Na ∙ + ∙ Cl ∙∙ àNa+[ ∙∙ Cl ∙∙ ]−
Thus, there is no fundamental difference in the mechanism of formation of a nonpolar covalent bond, a polar covalent bond, and an ionic bond. They differ only in the degree of polarization (the displacement of common electron pairs). The nature of chemical bonds is the same. The polarity of a bond can be predicted based on the electronegativity values of the element's atoms. The greater the difference in electrical negativity between bonded atoms, the more pronounced the polarity.
Thus, if the electronegativities of atoms differ very much (for example, atoms of alkali metals and halogens), then when they come closer, the valence electrons of one atom completely transfer to the second atom. As a result of this transition, both atoms become ions and take on electronic structure the nearest noble gas. For example, when sodium and chlorine atoms interact, they turn into Na+ and Cl- ions, between which electrostatic attraction occurs. Molecules in which exist in pure form ionic bond, found in the vapor state of a substance. Ionic crystals are composed of endless rows of alternating positive and negative ions bound by electrostatic forces. When ionic crystals dissolve or melt, positive and negative ions pass into the solution or melt.
It should be noted that ionic bonds are very strong, so to destroy ionic crystals it is necessary to expend a lot of energy. This explains the fact that ionic compounds have high temperatures melting.
Unlike a covalent bond, an ionic bond does not have the properties of saturation and directionality. The reason for this is that the electric field created by the ions has spherical symmetry and acts equally on all ions. Therefore, the number of ions surrounding a given ion and their spatial arrangement are determined only by the magnitude of the ion charges and their sizes.