Chemical Bonding

The force of attraction between atoms or ions is called chemical bond. Formation of chemical bonds involved electrons and nuclei and mainly energy changes. Bond formation is exothermic and bond breaking is endothermic.

Electronic Theory of Valency

This was proposed by Kossel and Lewis. This theory explains how and why the bonds are formed. Valence electrons are responsible for bonding process.

Inert gases have ns2np6 configuration but, Helium has 1s2. Thus, all inert gases have octet and helium has duplet configuration. Noble gases are chemically inert and will not take part in bonding because they are stable due to octet configuration in the valence shell.

Atoms of all other elements contain less than 8 electrons in valence shell. These elements are chemically reactive and take part in chemical reactions to become stable by attaining octet configuration. Attaining octet configuration in the valence shell is called octet rule or octet theory.

Some elements may become stable by attaining duplet configuration. For example - H, Li, Be. Octet configuration can be achieved by loosing or gaining or mutual sharing of electrons.

Valency: It is the combining capacity of an element i.e., number of bonds formed by the element.

Valence of an element = group number or (8 - group number)

Ionic Bond

Ionic bond was explained by Kossel. The strong electrostatic force of attraction between oppositely charged ions which are formed by the transfer of the electrons is called Ionic bond. Ionic bond is formed between different atoms with different electronegativities.

Ionic bond is generally formed between electropositive and electronegative element or less electronegative and more electronegative elements.

Lattice Energy

The amount of energy released when the oppositely charged gaseous ions combine to form one mole of solid ionic crystal (or) The amount of energy absorbed to separate one mole of solid ionic crystal into oppositely charged gaseous ions is called lattice energy.

Lattice energy is inversely proportional to the sum of radii of cation and anion.

Properties of Ionic Compounds

1. Physical state: They exist as crystalline solids due to close packing structure and strong interionic attractions.

2. Melting and boiling points: Ionic compounds have high MPs and BPs due to strong inter-ionic attractions.

3. Electrical conductance: Ionic compounds are good conductors in fused or aqueous state due to presence of ions and free flow of ions.

4. Ionic bond is non-directional in nature: As the ionic bond is non directional in nature. Ionic compounds do not exhibit space isomerism.

5. Reactions of Ionic compounds: Reactions in between Ionic compounds are very fast in aqueous solution because they does not involve any reshuffling of bonds. In aqueous solution, ions are free and they are just exchanged in reaction.

6. Solubility: Ionic compounds dissolve in polar solvents like H2O due to ion-dipole interactions. Ionic compounds are generally insoluble in non-polar solvents.

Covalent Bond

It was proposed by Lewis. The bond formed by sharing of electron pair is called covalent bond. Covalent bond can be formed between same atoms or different atoms.

Covalency: It is the number of electrons contributed by an atom or the number of covalent bonds formed by an atom.

Properties of Covalent Compounds

  • They exist as either gases or liquids due to weak Vander Waal's forces.
  • MPs and BPs are very low due to weak Vander Waals forces in between the molecules.
  • Electrical conductance: They are bad conductors as they do not contain ions.
  • Due to directional nature of covalent bond, covalent compounds exhibit Isomerism.
  • The reactions in between covalent compounds are slow because they involve breaking and making of bonds.
  •  Solubility: They are soluble in non-polar solvents like CCl4, chloroform, C6H6 and insoluble in polar solvents like water.

Valence Bond Theory

The basis of VBT is Schrodinger's wave equation (wave mechanics). It explains shapes of covalent molecules and strength of covalent bonds.