chemical compounds

Aug 26, 2022, 16:45 IST

Type of chemical compounds based on Chemical bond

The force of attraction between atoms in a molecule or ion is called chemical bond. The modern concept believes that the electrons are responsible for chemical combination.read chemistry formulas for effective revision.

Types of chemical bonds

Depending upon the mode of acquiring the stable electronic configuration the chemical bonds can be classified as :

i) Ionic bond or electrovalent bond.

ii) Covalent bond.

iii) Coordinate bond or dative bond.

1. Ionic bond or Ionic Compounds

The electrostatic force of attraction between oppositely charged ions is called ionic bond.It is formed by the complete transfer of one or more electrons from one atom (metal) to another atom (non-metal). e.g., formation of ionic bond in NaCl.

Ionic bond is also called as electrovalent bond or polar bond.

The compounds containing ionic bonds are called ionic, electrovalent or polar compounds.

Ionic bond is a non-directional bond.

The number of electrons lost or gained by an atom represent the electrovalency of an atom.

Factors Favorable for the formation of ionic compounds

a. Factor favourable for cation formation:

i. Large atomic size:

Large atoms form cations easily compared to small atom i.e., form ionic compounds easily, due to decrease in force of attraction of the nucleus on the valence electrons.e.g : compounds of Cs are more ionic than the compounds of other alkali metals due to large size and low IP.

ii. Low ionization potential (IP):

Ionization energy of cation forming atom should be low, for the formation of ionic compounds easily.e.g., potassium forms ionic compounds easily than sodium, because it has low ionization energy. K, IP = 495.57 kJ mol-1 Na, IP = 519.82 kJ mol-1

iii. Low charge on the ion:

Ions of lower positive charges are easily produced compared to the ions of higher charges. An atom that has to lose more electrons to get stable configuration does not readily form cation or ionic compound or Ionic bond.

iv. Inert gas configuration:

Those elements, which give cations with noble gas configuration (ns2 np6), form readily ionic compounds.The other elements, which give cations with 18 electrons in the outermost orbit known as pseudo inert gas configuration or nickel group configuration also form ionic compound but not as readily as the elements with noble gas configuration.e.g., (i) out of the two cations Zn2+ (2, 8, 18) and Ca2+ (2, 8, 8), calcium ion (Ca2+) gives ionic compound readily than Zn2+ due to larger size and more stability.

b)Factors favourable for anion formation:

i)Small atomic size:

Small atoms hold the electrons, gained by them strongly and hence form anions as well as ionic compounds easily than the large anions.e.g., the order of ionic nature in halides is F- > Br > I-

ii)High electronegativity and electron affinity:

Higher the electronegativity and higher electron affinity of the atoms, greater will be the ease of formation of anions as well as ionic compounds.e.g., The order of ease of formation of anions is F- > O2- > N3-. In addition to the factors discussed above other influencing factors are also known for the formation of an ionic bond which are given below.

i)Over all decrease in energy:

The summation of three energies should be negative i.e., energy is released.Ionization potential + electron affinity + lattice energy = negative value. ii)Difference in electronegativity:Greater the difference of electronegativity between two atoms, higher will be the possibility of ionic bond formation.Highly electropositive elements of groups IA and IIA combine with highly electronegative elements of VIA and VIIA groups, to form ionic or electrovalent compounds.

e.g., halides, oxides, sulphides, nitrides and hydrides of alkali metals and alkaline earth metals are generally ionic.The atoms of transition metals can also form ionic bond but do not acquire inert gas configuration always.

iii)High Lattice energy:

Higher the value of lattice energy of the ionic compound, the greater will be the stability of the compound and hence greater will be the ease of its formation.The value of the lattice energy increase with increase of charge on the ions due to greater force of attraction between them.

Properties of Ionic compounds:

The important properties of ionic compounds are ____

i.These compounds usually exists in the solid state.

ii.The crystalline ionic compounds have well defined crystal structure or crystal lattice.

iii.Ionic compounds have high melting points and boiling points due to powerful electrostatic force between ions.e.g., m.pt of NaCl is 8030C.

iv.They are generally soluble in polar solvents, having high value of dielectric constant but insoluble in non-polar solvents due to low dielectric constant. The solubility of ionic compounds decreases with increase in covalent character of ionic compounds. The solubility of these compound is governed by ___

a) Lattice energy: Larger the lattice energy, the lesser is the solubility.e.g., sulphates and phosphates of Ba & Sr are insoluble in water due to high lattice energy.

b) Heat of hydration: Larger the heat of hydration, the more is the solubility of ionic compound.e.g., AlCl3, though covalent in nature is soluble in water due to high value of heat of hydration.

v.In solid state they do not conduct electricity since there is no free movement of electrons but in molten state and in solution they are good conductor of electricity due to presence of charged ions in them.

vi.Ionic compounds undergo ionic reactions which have very high reaction rates. i.e., quite fast e.g., NaCl-(aq) + Ag+ NO3(aq)  AgCl(s)  + NaNO3(g) white ppt.

vii.They do not exhibit isomerism due to non-directional nature of the polar bonds present in these compounds.

2. Covalent Bond and covalent compounds

This concept of covalent bond was given by G.N. Lewis and improved by Langmuir, in 1919, by suggesting that atoms may also combine by mutual, sharing of electrons in their outermost shells and attain noble gas configuration.

A chemical bond which is formed by mutual sharing of two or more electrons between two atoms of same or different elements, is called covalent bond.

A covalent bond, on the basis of sharing of two or more electrons can be devided into three types.

a)Single covalent bond:

A covalent bond which is formed by sharing of two electrons (i.e., 1 electron pair) is called single bond or single covalent bond. It is represented by a single line between two atoms.

Properties of covalent compounds

i)Under normal conditions of temperature and pressure, these compounds exist as gases or liquids of low boiling points, due to weak van der waal’s forces exist between discrete molecules. But, sulphur, phosphorus, iodine are exist as soft solids.

ii)They have low melting points and boiling points because weak van der waal’s forces of attraction operate between the molecules.

Example. H2, O2, N2, CH4 are gases and H2O, CHCl3, C6H6 are low boiling liquids.

iii)The crystal structure of covalent compounds differs from that of ionic compounds. They usually consist of molecules rather than ions.

iv)In general, they are insoluble in polar solvents like H2O alcohol and soluble in non-polar solvents like CCl4 & benzene, because this is based on the principle “like dissolves like”.

e.g.,Polar covalent substances like glucose, fructose etc are soluble in polar solvents and non polar substances like iodine, camphor etc are soluble in non-polar solvents like benzene and carbon tetra chloride etc.

v)In, general, covalent substances are bad conductors of electricity. Substances which have polar character like HCl, in solution, can conduct electricity.Covalent solids having giant molecules, are bad conductors since they do not contain charged ions or free electrons. Therefore, they are non-electrolytes.

The graphite can conduct electricity due to delocalization of  - electrons in a layer but not from one layer to another.

vi)Rate of reaction:

Covalent substances undergo molecular reactions, they have low reaction rate than ionic compounds, because they involve, breaking of covalent bonds of reactants and making of new bonds. Hence these reactions are generally slow and rarely proceed to completion.

vii)Isomerism:

The covalent compounds exhibit isomerism due to directional nature of a covalent bond. e.g.a covalent substance with molecular formula, C2H6O exhibits two isomers i.e., ethyl alcohol (C2H5OH) and ether (CH3OCH3).

Comparison between ionic and covalent compounds:

Coordinate bond or Coordination compounds

It may be defined as, “ a covalent bond in which both electrons of the shared pair are contributed by one of the two atoms”. It is represented by an arrow () pointing its head towards the acceptor atoms. Atom / ion / molecule donating electron pair is called donor or Lewis base. Atoms / ion / molecule accepting electron pair is called acceptor or Lewis acid. Compounds having this linkage are known as coordinate compounds. Since this bond has some polar character. As the coordinate bond is a combination of one electrovalent bond and one covalent bond, it is also known as dative or semipolar or co-ionic bond.