The chemical bond formed between two or more atoms as a result of the transfer of one or more electrons from electropositive to electronegative atom is called electrovalent bond or ionic bond or polar bond.
Note: Electrovalent bond is not possible between similar atoms. This type of bonding requires two atoms of different nature, one atom should have the tendency to lose electron or electrons, i.e., electropositive in nature and the other atom should have the tendency to accept electron or electrons, i.e., electronegative in nature.
Example of electrovalent bond
Potassium chloride : The free potassium atom has one valency electron (electronic configuration 2,8,8,1), i.e., 4s¹ whereas, the chlorine atom has seven valency electrons (electronic configuration 2,8,7), i.e., 3s² 3p⁵. In forming an ionic bond, the potassium atom loses its valency electron which is accepted by chlorine atom. As a result potassium achieves nobel gas configuration of argon (2, 8, 8) and become a positive ion (K⁺). Chlorine achieves nobel gas configuration of argon (2, 8, 8) and acquires a negative charge (Cl⁻). The attraction between potassium ion and chloride ion is an ionic bond.
It is important to recognize that clean ionic bonding – in which one atom or molecule completely transfers an electron to another cannot exist: all ionic compounds have some degree of covalent bonding, or electron sharing. Thus, the term "ionic bonding" is given when the ionic character is greater than the covalent character – that is, a bond in which a large electronegativity difference exists between the two atoms, causing the bonding to be more polar (ionic) than in covalent bonding where electrons are shared more equally. Bonds with partially ionic and partially covalent character are called polar covalent bonds.
General characteristics of ionic compounds.
(i) Crystalline nature: Ionic compounds are usually crystalline in nature. The constituent units in an ionic crystal are ions and not molecules.
(ii) Melting and boiling points: Due to strong electrostatic forces of attraction, the ions are held tightly in their positions in the crystal lattice. A large amount of energy is needed to dislodge the ions from their positions. Thus, ionic compounds possess high melting and boiling points.
(iii)Hard and brittle: Ionic compounds are hard in nature. The hardness is due to strong forces of attraction between oppositely charged ions which keep them in their alloted positions. The brittleness of the crystals is due to movement of a layer of a crystal on the other layer by application of external force when like ions come infront of each other. The forces of repulsion come into play. The breaking of crystal occurs on account of these forces of these forces or repulsion.
(iv)Solubility: Ionic compounds are fairly soluble in polar solvents and insoluble in polar solvents. The polar solvents have high values of dielectric constants. Water, the solvent, is one of the best solvents as it has a high value of dielectric constant. Due to high value of dielectric constant, the electrostatic force of attraction between the ions decreases and these ions get separated and ultimately solvated by the molecules of the solvent. The non-polar solvents have very low value of dielectric constant and are not capable of dissolving electrovalent compounds.
(v)Electrical conductivity: Ionic solids do not conduct electricity. The reason is that the ions, on account of electrostatic forces of attraction, remain intact occupying fixed positions in the crystal lattice. The ions, thus, do not move where electric current is applied.
(vi)Space isomerism: The electrovalent bond is non-rigid and non-dimensional. Thus, the electrovalent compounds do not show space isimerism or stereo-isomerism.
(vii)Isomorphism: Compounds having same electronic structures are isomorphous to each other. For example, sodium fluride and magnesium oxide are isomorphous to each other.
Na⁺F⁻ Mg²⁺O²⁻
(2,8) (2,8) (2,8) (2,8)
Potassium sulphide, potassium chloride and calcium chloride are isomorphous to each other.
K⁺ S²⁻ K⁺ Cl⁻ Ca²⁺ Cl⁻
(2,8,8) (2,8,8) (2,8,8) (2,8,8) (2,8,8) (2,8,8)
(viii)Ionic reactions: Ionic compounds furnish ions in solution. The chemical reactions are due to the presence of these ions. Such reactions are fast. For example, SO₄²⁻ ions present in Na₂SO₄ solution, from white precipitate of BaSO as soon as BaCl solution is added to it.
Na₂SO₄ 2Na⁺+ SO₄²⁻
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