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
