General Properties Of Ionic And Covalent Bonds

GENERAL PROPERTIES OF IONIC AND COVALENT BONDS

The different between ionic and covalent binding gives rise to difference in physical and chemical properties. An ionic compound is composed of positive and negative ions –– the ion pairs of opposite charges being held together by electrostatic attraction; the ions arrange themselves in a regular geometrical pattern in the crystal of an ionic compound. X-ray analysis, for example, reveals that the structure of sodium chloride crystal consists of two interlocking face centred cubic lattices, one of sodium ions and the other of chloride ions –– each sodium unit in the crystal is surrounded by six chlorine units, and each chlorine by six sodium units.

An ionic compound does not exist as a definite entity as molecules. There is no evidence, for example, of molecules of sodium chloride as structural units in the lattice of sodium chloride. All truly ionic compounds, e.g., common salt, posses ionic lattice. Owing to the restrictive force in the crystal the ions are not free to move under the influence of an applied electric field. But when an ionic compound is dissolved in water (or a suitable polar solvent which diminishes the attraction between the ions) or melted, the ions acquire mobility and can move in opposite directions in an electric field, i.e., ionic compound is polar and can conduct electricity in the fused state or in solution in a suitable in a suitable solvent.

A purely covalent compound is a non-polar substance and does not conduct an electric current. Anhydrous hydrogen chloride is a non-conductor, but its aqueous solution conducts electricity as a result of chemical reaction which leads to the formation of ions:

HCl + H2O Cl′ + H3O+ (oxonium ion)

The ion-pairs tend to adhere to one another in electrovalent compounds and very powerful forces hold the crystals together. Considerable energy is necessary to break the crystal lattice and separate the ions. The electrovalent compounds are, therefore, solid that are not easily vaporised. They have high melting points and high boiling points. Unlike the electrovalent compounds covalent compounds exist as single molecules which have relatively little attraction to one another –– the only force between them being the weak van der Waals forces; the covalent compounds are therefore either gases, liquids or solids that are easily vaporised. Since the force of attraction between the molecules is not large, covalent compounds melt and boil at comparatively low temperature.

A covalent bond is rigid and directional (i.e., directed in space), and a covalent compound has its atoms held in definite relative positions so that the different arrangements of the atoms are not easily changed, i.e., a covalent compound may exhibit isomerism. Butane and isobutane for example, are two distinct isomeric compounds. The ionic bond, on the other hand, is non-rigid and non-directional, i.e., the charged particles are free to arrange and pack themselves in chosen ionic lattices.

A polar liquid (with a high dielectric constant) such as water, alcohol and liquid ammonia diminishes the force of attraction between the ions of an ionic compound; it will, therefore, generally be a good solvent for ionic compounds. But a non-polar liquid (with a low dielectric constant), e.g. organic solvents such as hydrocarbons and carbon tetrachlorde, is not good enough to overcome the inter-ionic attraction, and therefore will be a very poor solvent for ionic compounds.