The attractive force which holds various constituents (atoms, ions, etc) together in different chemical species to maintain a mutual atomic and a definite but specific geometrical shape, is called a chemical bond. During the formation of the chemical bond, energy is released, i.e., it is an exothermic process and occurs with the decrease in free energy of the system.
The combining power (or capacity of mutual combination) of an element is called its valency. Valency of noble gases, previously called inert gases [i.e., Helium (He), Neon (Ne), Argon (ar), Krypton (Kr), Xenon (Xe) and Radon (Rn( is zero. They generally do not take part in chemical bonding because they have completely filled or very stable outermost shell, ie., 8 electrons (octet) in their outermost shel (except He atom, which has 2 electrons in its outermost shell).
In other words, noble gases have no tendency to lose, gain or share electrons and hence, they exist in free or monoatomic form. Valency of an element is equal to the number of electrons lost or gained during the formation of a compound. It generally depends upon the number of valence electrons.
If an atom has 1, 2, 3, or 4 valence electrons, then its valency = number of valence electrons
If an atom has 5, 6, 7 or 8 valence electrons, then its valency = 8- number of valence electrons.
e.g, in Na (2, 8, 1) there is one valence electron, so it loses the valence electron to achieve octet in valence shell and forms Na+(2, 8).
Electronic theory of Chemical Bonding (Octet Rule)
Kossel and Lewis in 1916 developed a theory of chemical combination between atoms known as electronic theory of chemical bonding which was based on inertness of noble gases.
According to this theory, “atoms combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet or stable configuration just like noble gases in their valence shells”. This is known as octet rule.
Types of Chemical Bonds
Bonding is broadly divided into two classes
i) Chemical Bonding (Ionic, covalent and coordinate bond0
ii) Physical Bonding (hydrogen and Van der Waals bonding)
Ionic Bond or Electrovalent Bond
The bond formed, as a result of the electrostatic attraction between the positive and negative ions, is termed as electrovalent bond. The transfer of electrons takes place in such a way so that each combining element achieves complete octet (or duplet in case of H and He).
The compound formed by the transfer of electrons from a metal to a non-metal atom is known as ionic or electrovalent compound.
e.g., the formation of sodium chloride from sodium and chlorine.
- The sodium atom has one electron in its valence shell so it loses its one electron to attain octet in its outer shell and forms Na+ ion.
- Similarly, Cl atom has 7 electrons in its valence shell so it gains one electron from sodium to attain octet in its outer shell and forms Cl- ion.
- The Na+ and Cl- ions bind together by electrostatic forces of attraction
Other examples of ionic compounds are MgCl2, Cao, NH4Cl, NaOH, etc.
- The electrovalency is equal to the number of unit charge (s) present on the ion(s). Thus, in the above example electrovalency of Na and Cl is 1.
Characteristics of Ionic or Electrovalent Compounds
- Ionic compound are generally crystalline solids, i.e., have definite shape and are somewhat hard and brittle because of the long distance orders of their constituent particles.
- They have high melting and boiling points because of strong electrostatic forces of attraction.
- These are non-volatile and have high density.
- These are soluble in polar solvent like water because of high dielectric constant of water and insoluble in non-polar solvents like benzene, acetone, kerosene, petrol, etc. because these are covalent in nature.
- They can conduct electricity in their aqueous state and in the molten state due to the presence of mobile ions i.e, in these states they behave like an electrolyte. However, these are unable to conduct electricity in solid state because ions are held together by strong electrostatic forces of attraction i.e., ions are not free.
- Ionic compounds undergo very fast reactions in solution.
The bond formed due to sharing of electrons between two similar atoms or different atoms having almost similar electronegativity, is called the covalent bond. The sharing occurs in such a way that the formed molecules achieve a permanent electronic structure just like noble gases.
The compounds that are the result of sharing of electrons are called covalent compounds.
For example, the formation of Cl2, molecule. Each Cl atom has one electron short for the octet configuration. Both Cl atom attains the outer shell octet by sharing of a pair of electrons. Each Cl atom contributes one electron to the shared pair.
Single, Double and Triple Covalent Bonds
When two atoms share one electron pair, the covalent bond between them is called a single covalent bond.
When Two atoms share two pairs of electrons, the covalent bond between them is called a double covalent bond
When the combining atoms share three pairs of electrons, the covalent bond between them is called a triple covalent bond
What is Covalency?
In a covalent compound, covalency of an atom is the number of electrons which are supplied by that atom for sharing. e.g., in H2 molecules, covalency of each hydrogen is 1, in N2 molecules, covalency of each nitrogen atom is 3 and in CH4 covalency of C is 4 and of H is 1.
Characteristics of Covalent Compounds
- In general, covalent compounds are gases or liquids. Some of the compounds are soft solids with a few exceptions like a diamond, silica (SiO2), carborundum (SiC), etc.
- They have relatively low melting and boiling points because of weaker intermolecular forces of attraction.
- These are insoluble in water but soluble in non-polar solvents like benzene, carbon tetrachloride, etc. However some of them like sugar, alcohol are miscible with water due to hydron bonding.
- Covalent compounds are bad conductors of electricity in molten as well as in solid state. However, graphite can conduct electricity as it possesses free electrons.
- The covalent bond is rigid and directional, therefore covalent molecules is rigid and directional, therefore, covalent molecules have specific shapes.
- The reactions of the covalent compound are slow and complex.
Types of Covalent Bond
On the basis of polarity, the covalent bond can be of two types.
- Non-polar Covalent Bond: When a covalent bond is formed between two similar atoms, e.g., H2, O2, Cl2, N2, or F2, the shared pair of electrons is equally attracted by two atoms and situated exactly between two identical nuclei. The bond so formed is called the non-polar covalent bond.
- Polar Covalent Bond: In case of a heteronuclear molecule like HF, the shared pair of electrons between two atoms gets displaced more towards fluorine, since electronegativity of F is greater than H. The resultant bond is polar covalent bond.
Coordinate Bond or Coordinate Covalent Bond or Dative Bond
It is a special type of covalent bond in which both the shared electrons are donated by only one of the atoms. The atom supplying the elctrons pair is called donar atom and that receiving the electrons pair is called acceptor atom.
This bond is represented by placing an arrow pointing towards the acceptor. e.g., Formation of hydronium ion (H3O-).
Characteristics of Compounds having coordinate BOnds
- These compounds exist in all the three states i.e., solid, liquid and gas under ordinary conditions.
- The melting and boiling points of these compounds are higher than that of covalent compounds are higher than that of ionic compounds.
- Like covalent compounds, these are poor conductors of electricity.
- They are sparingly soluble in water but readily soluble in organic solvents.
- Like covalent compounds the reactions of these compounds are slow
The attractive forces between a hydrogen atom (which is already covalently to a strongly electronegative atom i.e., N, O or F) and an electronegative atom is known as a hydrogen bond. Thus, it is formed between the hydrogen atom of one molecule and an electronegative atom of another molecule.
These bonds are present in inorganic molecules like water as well as inorganic molecules like DNA.
Van der Waals’ Forces
These are the sum of attractive or repulsive interaction between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules. These are relatively weak as compared to normal chemical bond and can be of the following three types.
- Forces between two permanent dipoles (Keesom forces)
- Forces between a permanent dipole and a corresponding induced dipole (Debye forces).
- Forces between two instantaneously induced dipoles (London or dispersion of forces). London forces dominate the interaction of non-polar molecules.