Properties of Ionic Compounds

Introduction:

A chemical compound in which ions are held together in a lattice structure by ionic bonds is called Ionic compounds. Normally, the positively charged portion have metal cations and the negatively charged portion is an anion or polyatomic ion.

Ions can be single atoms, or more complex groups. But an ion must have a positive or negative charge. So, in an ionic bond, one must exhibit a positive charge and the other negative one.

Chemical compounds are not strictly ionic. The most electronegative/electropositive pairs like caesium fluoride have a degree of covalency. Similarly, covalent compounds have charge separations.

Characteristics of Ionic Compounds

All atoms are electrically neutral. This is because all atoms have the same number of electrons and protons. Each electron has a negative charge of  -1 unit and each proton has a positive charge of +1 unit. Thus, the negative charge of electrons is canceled by positive charge of protons. This is the normal state of the atoms of any element.
Except for the atoms of noble gases, all elements' atoms are not stable. Here, "stable" implies a complete octet or a duplet structure. In an octet or duplet, the last shell or orbit of an atom possesses either 8 or 2 electrons respectively. When an atoms shows octet or duplet structure, it is assumed to be completely stable, and it does not take part in chemical reactions and hence, does not form compounds. All noble gases have octet or duplet electronic configuration.
Therefore the elements' atoms, except noble gases, always attempt to attain an octet or a duplet. In order to do so, they either lose, gain or share electrons. When an atom has lesser than 4 electrons in its valence shell, it tends to lose these valence electrons so as to become stable. When an atom has more than 4 electrons in its valence shell, it tends to gain 3, 2 or 1 electrons to attain stability.
Generally, metals (alkali metals and alkaline earth metals) tend to lose electrons, and nonmetals (group 17 elements mainly) tend to gain these electrons. Thus, metal atoms become positively charged ions and non metal atoms become negatively charged ions, and as unlike charges attract each other, so these ions get attracted by strong electrostatic forces and form a compound called an ionic compound, whose chemical bond is called an ionic or electrovalent chemical bond.

Ionic compounds have strong electrostatic bonds present between particles. They normally have very high melting and boiling points. They exhibit good electrical conductivity when present in molten or in aqueous solution. As ionic inorganic compounds are solids at room temperature and form crystals.

Solubility

Following the phrase, "like dissolves like", ionic compounds easily dissolve in polar solvents, and those which ionize with water and ionic liquids. They are appreciably soluble in other polar solvents like alcohols, acetone and di-methyl sulfoxide. Ionic compounds do not tend to dissolve in nonpolar solvents like diethyl ether or petrol.

The oppositely charged ions in the solid ionic lattice are surrounded by opposite pole of the polar molecule which try to pull them out of the lattice to the liquid. When the force applied is more than the electrostatic attraction of the lattice, the ions gets into the liquid and dissolves in it.

Electricity Conductivity

Solid ionic compounds are mot able to conduct electricity as there are no mobile ions or electrons present in the lattice. When the ionic compunds are dissolved in a liquid or molten form, they can conduct electricity with the mobile ions.

Physical state:-

Ionic compounds exist in the form of crystalline solids (at room temperature).

This is because their constituent particles are ions, and the strong attractive forces between the oppositely charged ions hold them closer and lock them in fixed positions in the crystal structure of the solid.

Lattice structure:-

The lattice of a substance can be considered as the structure of its crystal which is visible under a microscope.
The constituent particles being oppositely charges ions, the electrostatic forces between the oppositely charged ions cause the ions to be arranged closely, in a geometric patter in the lattice. For example, in Sodium Chloride (NaCl), the ions are arranged in a cubic lattice such that each sodium ion is surrounded by 6 chlorine ions and each chlorine ion is surrounded by six sodium ions. Thus, ionic compounds do not show a molecular arrangement.

Let us look at the reasons why ionic compounds have high melting points:

The melting points of ionic compounds are usually high. Crystals are formed by almost all ionic compounds. Salts readily form crystals by forming stacking groups with the small amounts of electrical negative and positive charges. The crystals thus formed are very big molecules with large amounts of positive and negative charges that are stuck together. A massive amount of energy is required for breaking the opposite charged ions apart. This breaking apart of the oppositely charged ions requires additional energy, which is supplied when the compound is heated. At increased temperatures, the energy supplied to the compound is more for breaking it up forming oppositely charged positive and negative ions, which further melts. Hence, during laboratory experiments, the melting of ionic compounds may not achieved by heating on a Bunsen burner, since the melting points of these ionic compounds are very high.
Ionic compounds due to the high electro negativity have high melting points and the strong intermolecular forces between results in an increase in the melting points of ionic compounds. The ionic bonding between the oppositely charged negative and positive ions involved in the formation of an ionic compound is very strong, which makes the compound very stable and hence requires high temperatures for the bonds to break apart and further melting of the ionic compound.

Examples of Ionic Compounds:

Table salt NaCl,

Zinc chloride ZnCl,

Copper Floride CuF₂,

Sodium hydroxide NaOH,

Potassium hydroxide KOH,

Potassium permanganate K₂Cr₂O₇