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Physical state
Ionic compounds do not exist in the form of independent molecules but exist in the form of ions in crystalline solids, according to X-ray studies. These ions are arranged to form the crystal lattice in definite geometric pattern. For example, in NaCl crystal, each Na+ ion is surrounded by six Cl- ions and each Cl- ion is surrounded by six Na+ ions. The geometric arrangement of different ionic compounds differ, depending upon the size of the ions and the magnitude of the charges on the ions.
High melting and boiling points
Ionic compounds have high melting and boiling points. A large amount of energy is required to break the crystal lattice, because the ions are held together by strong electrostatic forces of attraction. The variation in melting point depends upon the charges on the ions and ionic radii. The closer the ions in crystal, larger will be the electrostatic forces of attraction and consequently higher will be the melting point. For example in case of sodium halides, the melting points decrease from NaF to NaI as:
NaF (1270K), NaCl (1073K), NaBr (1023K) and NaI (924K).Solubility
Ionic compounds are generally soluble in water and other polar solvents having high dielectric constants. This is due to larger electrostatic interactions between polar solvents and ions. However ionic compounds are insoluble in non-polar solvents.
Electrical conductivity
In solution or in their molten states, ions become free to move. On applying an electric field,the ions get drifted towards towards their respective electrodes.Thus, ionic compounds are good conductors of electricity in these states. However, these do not conduct in their solid state because of strong electrostatic forces between the ions, that does not make them mobile.
Ionic reactions
The ionic compounds split up into oppositely charged ions when dissolved in water. The chemical reactions of ionic compounds are characteristic of the constituent ions and are known as 'ionic reactions'. Such reactions occur almost instantaneously.For example:
1.Neutralization reactions
Crystalline character
In ionic-compounds each ion is surrounded by oppositely charged ions uniformly distributed all around the ion,thus resulting in a three dimensional solid aggregate called an ionic crystal.
Problem
3. Give the Lewis structures and empirical formulae for the ionic compound formed between the following pairs of elements :
(i) Ba,Cl (ii) Na,S (iii) Mg,N (iv) Na, P.Solution
From the charges on the ions, the number of positive and negative ions in the compound can be found out. This gives the empirical formula (E.F.)
Lewis formula:
